Patent Publication Number: US-8127095-B1

Title: Restore mechanism for a multi-class file system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related to the field of computer systems and, more particularly, to data storage systems. 
     2. Description of the Related Art 
     In data storage environments such as corporate LANs, total storage needs are increasing and storage costs are an increasing part of the IT budget. More and/or higher capacity storage devices may be added, but this solution is expensive and difficult to manage, and does not address the root of the problem. There is a limit to storage capacity no matter how much storage capacity is added. This solution tends to provide a constant cost per byte for storage, as it tends not to take advantage of lower cost-per-byte storage devices. A high percentage of data stored in a storage environment may be infrequently accessed, or never accessed at all after a certain time. Lower cost-per-byte for storage may be realized using methods that move at least some of this infrequently accessed data off more expensive storage devices and on to the less expensive storage devices. 
     Hierarchical Storage Management (HSM) is a data storage solution that provides access to vast amounts of storage space while reducing the administrative and storage costs associated with data storage. HSM systems may move files along a hierarchy of storage devices that may be ranked in terms of cost per megabyte of storage, speed of storage and retrieval, and overall capacity limits. Files are migrated along the hierarchy to less expensive forms of storage based on rules tied to the frequency of data access. 
     In HSM systems, data access response time and storage costs typically determine the appropriate combination of storage devices used. A typical three tier HSM architecture may include hard drives as primary storage, rewritable optical as secondary storage, and tape as tertiary storage. Alternatively, hard drives may be used for secondary storage, and WORM (Write Once, Read Many) optical may be used as tertiary storage. 
     Rather than making copies of files as in a backup system, HSM migrates files to other forms of storage, freeing hard disk space. Events such as crossing a storage threshold and/or reaching a certain file “age” may activate the migration process. As files are migrated off primary storage, HSM leaves stubs to the files on the hard drive(s). These stubs point to the location of the file on the alternative storage, and are used in automatic file retrieval and user access. The stub remains within the file system of the primary storage, but the file itself is migrated “offline” out of the file system onto the alternative storage (e.g. tape). 
     In HSM, when a file that has been migrated to a lower rank of storage, such as tape, is accessed by an application, the stub may be used to retrieve and restore the file from the lower rank of storage. The file appears to be accessed by the application from its initial storage location, and demigration of the file back into the file system is performed automatically by the HSM system using the stub. While on the surface this demigration may appear transparent to the user, in practice the process of accessing and restoring the file from offline storage (e.g. tape) may introduce a noticeable time delay (seconds, minutes, or even hours) to the user when compared to accessing files stored on primary storage. Thus, accessing offloaded data in an HSM system is typically non-transparent to the application or user because of the difference in access time. In addition, since HSM introduces a substantial time lag to access offloaded data, HSM systems typically only offload low access (essentially, no access) data. 
     A file system may be defined as a collection of files and file system metadata (e.g., directories and inodes) that, when set into a logical hierarchy, make up an organized, structured set of information. File systems may be mounted from a local system or remote system. File system software may include the system or application-level software that may be used to create, manage, and access file systems. 
     File system metadata may be defined as information file system software maintains on files stored in the file system. File system metadata may include definitions and descriptions of the data it references. Generally, file system metadata for a file includes path information for the file as seen from the application side and corresponding file system location information (e.g. device:block number(s)). File system metadata may itself be stored on a logical or physical device within a file system. 
     File systems may use data structures such as inodes to store file system metadata. An inode may be defined as a data structure holding information about files in a file system (e.g. a Unix file system). There is an inode for each file, and a file is uniquely identified by the file system on which it resides and its inode number on that system. An inode may include at least some of, but is not limited to, the following information: the device where the inode resides, locking information, mode and type of file, the number of links to the file, the owner&#39;s user and group IDs, the number of bytes in the file, access and modification times, the time the inode itself was last modified and the addresses of the file&#39;s blocks on disk (and/or pointers to indirect blocks that reference the file blocks). 
     As data increases and disks get cheaper and faster than tape, the cost of tape media and drives is becoming larger relative to the cost of the online storage. Also, since data tends to increase faster than the speed of tape drives, backup windows are being squeezed. Conventional backup mechanisms may perform incremental backups and then offline apply the incremental backups to the last full backup to make a synthetic full backup. This may help reduce the backup window but does not reduce the total storage media or number of tape drives needed. 
     Conventional restore mechanisms typically restore a full backup, then one or more incremental backups. In backup systems using cumulative incremental backups (data that has not changed is backed up in a “full” backup, then data that has not changed since the full backup is backed up in incremental backups), the incremental backups grow over time. This may consume considerable amounts of storage media, as well as a considerable amount of time in performing backups and restores. Conventionally, incremental backups are typically performed as file-level incremental backups. When a file or portion of a file is modified, the entire file is backed up during an incremental backup. 
     SUMMARY 
     Embodiments of a system and method for performing optimized restores in a multi-class file system are described. Embodiments of a restore mechanism in combination with a multi-class file system implemented by an embodiment of the multi-class storage mechanism may be used to preferably reduce the restore window. One embodiment may enable a partial restore of the data in a multi-class file system to be performed, and an application or applications to be started before all of the data has been restored. 
     A multi-class file system including a hierarchy of storage classes may be implemented on one or more storage devices. In one embodiment, each storage class includes one or more storage devices assigned to the storage class according to one or more characteristics of the storage class, e.g. performance characteristics. In one embodiment, the storage classes may be ordered in the hierarchy of storage classes according to performance characteristics from a highest storage class including one or more high-performance storage devices to a lowest storage class comprising one or more low-performance storage devices. 
     More actively accessed files may be assigned and/or migrated to higher storage classes and less actively accessed files may be assigned and/or migrated to lower storage classes in the hierarchy by a multi-class storage mechanism according to a set of user-defined policies. In one embodiment, file system metadata may be modified to indicate assignment or migration of the files. In one embodiment, the file system software may migrate portions of a file to different storage classes according to the user-defined policies. In one embodiment, files migrated to a lowest storage class may be compressed. 
     If necessary, a backup of the storage classes in the hierarchy of storage classes may be restored. The backup may include full backups of each of the storage classes and incremental backups of one or more higher storage classes. In one embodiment, the full backups may be image backups of storage device(s) in the storage class. File system metadata and the higher storage class or classes including the more actively accessed data may be restored first. Restoring a higher storage class may include restoring the full backup of the storage class and then applying one or more incremental backups of the storage class. 
     The multi-class file system may then be brought online. An application may be granted access to the restored more actively accessed data on the higher storage classes after completion of the restore of the file system metadata and the higher storage classes. 
     The lower storage class(es) including the less actively accessed data may then be restored. In one embodiment, access to the less actively accessed data may be blocked until the less actively accessed data are restored. In one embodiment, the file system metadata for the less actively accessed data may be modified to block access to the less actively accessed data. If an application tries to access data that has not been restored, the access may be delayed until the requested data are restored. In one embodiment, file system software may track the restore of blocks of the less actively accessed data and may grant the application(s) access to the blocks of the less actively accessed data as they are restored. In one embodiment, if the file system software receives an access request to one or more blocks of the less actively accessed data that have not been restored, the file system software may request that the restore mechanism restore the requested one or more blocks out-of-order to reduce the delay for the application to access the data. 
     In one embodiment, if the file system software receives an access request to one or more blocks of the less actively accessed data that have not been restored, the file system software may obtain an estimated time until restore of the requested one or more blocks from the restore mechanism. The file system software may then notify the requesting application of the estimated time until restore of the requested blocks. In one embodiment, the application may notify a user of the estimated time until restore of the requested data. The application may then generate another access request for the requested data after the estimated time has elapsed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  illustrates file system software including a multi-class storage mechanism implementing a multi-class file system according to one embodiment. 
         FIG. 2  illustrates file system software including a multi-class storage mechanism implementing a multi-class file system and assigning and migrating data within the file system according to one embodiment. 
         FIG. 3  illustrates a multi-class file system based on a multi-device feature of File System software according to one embodiment. 
         FIG. 4  illustrates a multi-class file system based on multiple independent file systems under a virtualization layer according to one embodiment. 
         FIG. 5  illustrates a multi-class file system in a network storage environment according to one embodiment. 
         FIG. 6  is a flowchart of a method for assigning and migrating files or portions of files in a multi-class file system according to one embodiment. 
         FIGS. 7A through 7E  illustrate the migration process in an exemplary multi-class file system with three storage classes according to one embodiment. 
         FIGS. 8A through 8D  illustrate an exemplary metadata structure for data that may be modified during assignment and migration of data in a multi-class file system according to one embodiment. 
         FIG. 9  illustrates a backup mechanism for an exemplary multi-class file system on a host system in a network storage environment according to one embodiment. 
         FIG. 10  illustrates a backup mechanism for an exemplary multi-class file system with three storage classes according to one embodiment. 
         FIGS. 11A-11D  illustrate an exemplary migration and backup schedule for an exemplary multi-class file system according to one embodiment. 
         FIG. 12  is a flowchart of a method for performing backups of a multi-class file system according to one embodiment. 
         FIG. 13  illustrates a restore mechanism for an exemplary multi-class file system on a host system in a network storage environment according to one embodiment. 
         FIGS. 14A and 14B  illustrate a restore mechanism for an exemplary multi-class file system with three storage classes, previously described in  FIG. 5 , according to one embodiment. 
         FIG. 15  is a flowchart of a method for performing restores of a multi-class file system according to one embodiment. 
     
    
    
     While the invention is described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the invention is not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of a system and method for implementing a file system across different classes of storage in a storage system are described. Embodiments may define multiple classes of storage and automatically and transparently migrate data between the storage classes within the same file system to meet the usage needs of the business. This data may be migrated as files or portions of files, and may include, but is not limited to, application data and/or file system metadata. Embodiments may be used to transparently adapt the cost of storage to the usage patterns of data on the storage, thereby reducing the amount that customers need to spend for their storage. In embodiments, storage devices may be classified into two or more different classes of storage to implement a multi-class file system. Embodiments may provide a multi-class storage mechanism for managing and implementing sets of user-defined policies for assigning and migrating data within the multi-class file system. 
       FIG. 1  illustrates file system software including a multi-class storage mechanism implementing a multi-class file system according to one embodiment. File system software  102  may include, but is not limited to, an embodiment of the multi-class storage mechanism  104  and software that provides “traditional” File System functionality  106 . File System functionality  106  may be provided by any of various File System implementations including, but not limited to, disk-based file systems such as Veritas&#39; VxFS and UNIX File System (UFS), and networked file system such as Network File System (NFS) and AT&amp;T&#39;s Remote File Share (RFS). 
     File system software  102  may implement, on one or more storage devices, a multi-class file system  120 . In one embodiment, the software that provides the File System functionality  106  may be used to implement the multi-class file system  120 , while the multi-class storage mechanism  104  manages the policies and performs assignment and migration of data within the multi-class file system  120  according to the policies. The multi-class file system  120  may include one or more physical and/or logical storage devices organized in a hierarchy of two or more storage classes  122  according to one or more characteristics of the storage devices and/or data requirements of the user(s) of the file system. The multi-class file system  120  is made available to the application(s)  100  as one file system. The storage classes  122  of the multi-class file system  120  are transparent to the application(s)  100 . 
     In embodiments, the multi-class storage mechanism  104  may make an automatic and transparent determination on where to initially place a file (i.e., in which storage class  122 ) according to the policies. The multi-class storage mechanism  104  may later relocate or migrate the file (or a portion of the file) to another storage class  122  according to the policies. A migrated file remains online within the file system, even if the file is moved to the lowest storage class  122 ; migration does not move files to offline storage. 
     In a multi-class file system  120 , the same file system has multiple storage classes  122 . Each storage class  122  is part of a single overall file system. As far as the user or application is concerned, the files are stored in the same logical storage location within the file system (e.g. a directory or folder). It is transparent from the application and/or user&#39;s perspective whether a file is on one storage class  122  or another, or if parts of a file are on different storage classes  122 . From the application and/or user&#39;s perspective, the file appears to be in the same location in the file system where the user put it. Migrated data remains in the file system no matter which storage class it is migrated to. Migrated data remains active or online in the file system, and does not have to be restored from offline storage media in order to be accessed. 
     In one embodiment, the number of storage classes  122  in a multi-class file system  120  is configurable—there may be 2 to N user-defined storage classes  122  in a multi-class file system  120 . As an example, a three-class file system may be implemented, and a first portion of the data (e.g. 5%) may be classified as most active data and may be placed in a highest storage class, a second portion of the data (e.g. 15%) may be classified as less active data and may be placed in a middle storage class, and a third portion of the data (e.g. 80%) may be classified as least active (or inactive) data and may be placed in a lowest storage class (without offloading the data to tape or other media). Thus, embodiments may preferably reduce cost of overall storage without significantly impacting performance, robustness, etc. 
     In embodiments, there may be multiple devices in the same storage class  122 . A storage class  122  may be based, for example, on cost/performance characteristics of the storage devices. A storage class  122  may, for example, include five different devices, which may or may not be the same make and model, but all the devices are considered in the same storage class  122 . If a file is put on a storage class  122 , it may be stored to any of the devices in the storage class  122  (or alternatively across two or more devices in the storage class  122 ). In one embodiment, portions of a file may be stored or migrated to devices in two or more storage classes  122 . For example, one or more active records of a database may be stored in a higher storage class  122  including high-performance storage devices, while less or inactive records of the database may be stored in or migrated to lower storage classes  122  including lower-performance storage devices. 
     In embodiments, storage classes  122  may be defined (e.g. by a user or administrator) based on one or more characteristics including, but not limited to, performance and cost of the storage. Storage devices do not have to be exactly similar in characteristics to be placed in the same storage class  122 , but it may be preferable to place storage devices that are similar in one or more primary characteristics together in the same storage class  122 . For example, storage devices may be grouped into storage classes  122  according to performance characteristics, with the fastest storage devices placed in a first (highest) storage class  122 , and slower storage devices placed in lower storage classes  122 . For example, two or more fast mirrored storage devices (which may be different makes or models) may be grouped in a first storage class, two or more RAID-5 devices may (which may be different makes or models) be grouped in a second storage class, even slower storage devices (which may be different makes or models) may be placed in a third storage class, and so on. 
     Embodiments may allow a user to define as many storage classes  122  as desired or required and to separate the storage classes  122  according to any characteristics the user desires or requires. In one embodiment, a user may further segment a storage class  122  into two or more subclasses of storage as desired or required. As examples, a storage class  122  may be segmented into subclasses for two or more departments of an enterprise to separate the departments&#39; data onto different storage devices within the storage class, or into subclasses containing frequently-read data and less-frequently-read data, or into subclasses containing frequently-written data and frequently-read data, or into subclasses containing frequently-written data and less-frequently-written data. In this embodiment, the policies maintained and implemented by the multi-class storage mechanism  104  may specify, at the storage class and/or subclass level, storage classes and/or subclasses of a storage class to which certain files or portions of files are to be assigned and/or migrated. 
     Embodiments may provide business-oriented hierarchical management of online storage. Rather than simply migrating data to lower classes of storage based on how recently or frequently the data are accessed, embodiments may make migration decisions within the multi-class file system  120  based on one or more of other business-oriented factors. Embodiments may organize and migrate data in a multi-class file system  120  using policies oriented towards the business value of the data. The business value may be based, for example, on the user, application, department, etc that originated, owns, or most frequently accesses the data, time criticality of the data, and/or time of the data in a business or application cycle. Embodiments may implement classes of storage and place data in the storage classes  122  according to the business-oriented policies. In embodiments, files or portions of files may be placed in a storage class  122  and/or migrated to different storage classes  122  according to usage parameters and/or business-oriented parameters such as how valuable and/or important the data is, as determined by the predefined policies for evaluating the parameters. Embodiments may apply policies about the value, importance, and usage of the data, or even other factors such as ownership of the data (e.g. what department or user owns and/or accesses the data), within the multi-class file system  120  to preferably provide a lower underlying hardware cost with essentially the same responsiveness. 
     Policies used by the multi-class storage mechanism  104  may be based on file type, users, location, department, application, importance, value, or other factors in addition to file size and usage. The factors that policies may evaluate may be factors that are important to a business such as how valuable, critical, or important the data is, and how critical the response time is. For example, the policies may specify that a set of business-critical data that may be infrequently accessed should remain in a highest storage class, and/or that another set of data that is not as critical may be stored or migrated to a lower storage class regardless of the data&#39;s frequency of access. The data in the lower storage classes or even the lowest storage class, still within the same file system, may be accessed without having to be restored to the file system from offline storage, and may be migrated to higher storage classes transparently to the application and/or user if the policies, applied by the multi-class storage mechanism  104 , indicate that migration of the files is necessary or desired. 
     Implementation of the policies may be performed at allocation time and/or during use. The policies may be implemented within the File System software  102 , eliminating a level of interruption. 
     Embodiments of the multi-class storage mechanism  104  may migrate data within the multi-class file system  120 , no matter how busy the data is or whether the data is being used for read or write, if the policies indicate that the data is to be migrated. The multi-class storage mechanism  104  makes policy-based decisions about the importance, value, and/or usage of data and migrates the data to a storage class  122  with appropriate characteristics. The data remains online as part of the same file system, and the migration of the data is transparent to the user. Data, including application data and file system metadata, may be moved up or down in the multi-class file system  120  transparently to the user. 
     In one embodiment, the multi-class storage mechanism  104  may place or relocate parts of a file in different storage classes  122  according to the policies. For example, one part of a file that is frequently accessed may be placed in a higher storage class composed of faster storage devices, and another part of the file that is less frequently accessed may be migrated to a lower storage class composed of slower, less expensive storage devices. Data, including application data and file system metadata, remains active in the same file system regardless of which storage class  122  the files or parts of files containing the data are stored in or migrated to. 
     In one embodiment, file system metadata may be used to track the storage and migration of data in the multi-class file system  120 . When a file or portion(s) of a file is stored in or migrated to a storage class, the file system metadata may be modified to indicate the location of the file or portion(s) of the file in the multi-class file system  120 . From the application or user&#39;s perspective, the path to the file does not change when the metadata is modified to reflect migration of a file or portion of a file. The file system metadata may be stored and handled separately from other data or alternatively may be handled together with other data in a storage class  122  (typically, but not necessarily, at the highest storage class). 
     In one embodiment, to migrate a file or portion of a file, a range of blocks of the file may be locked while the blocks are being migrated. In one embodiment, the File System software  102  obtains a lock on the range of blocks, reads the data into system memory, writes the data out to the new storage class location, changes the mapping in the metadata, and unlocks the range of blocks. In other embodiments, ranges of blocks may be copied from one storage class location to another without being read into and written out of system memory. The range of blocks may be locked only for the time the data is being migrated. The rest of the blocks in the file remain unlocked during the migration of the range of blocks. In one embodiment, the number of blocks in a range that are locked and migrated at one time may be tuned for performance. The effects of the migration are transparent to any user of the data. 
     In one embodiment, to migrate a file or portion of a file, the file or portion of a file being migrated may not be locked during the transfer of the data. Instead, the data may be migrated to its new location, and the file or portion of a file may be locked only while updating the metadata. In one embodiment, an intelligent controller, array, or other mechanism may be used to migrate the data outside the host without going through system memory, without locking the file or portion of a file while the data is being transferred, and locking the file or portion of a file only while updating the metadata. In these embodiments, the file system software may need to check when it locks the file or portion of a file for the metadata update that no writes happened while the blocks were being copied. These embodiments lock the file or portion of a file for a shorter time than locking during the migration, so are less disruptive to file accesses. In one embodiment, if the file or portion of a file is being written, then a migration up for the written blocks may be performed by allocating a new set of blocks for the data being written and then directly writing them to the new disk location. 
     In one embodiment, a storage class  122  may be added to a multi-class file system  120  and data in files or portions of files, including application data and file system metadata, may be automatically and transparently moved to the new storage class. For example, if new, faster storage devices are available, one or more of the storage devices may be added to the multi-class file system  120  as a new, highest class of storage, and files or portions of files may be automatically migrated to the new, highest storage according to one or more policies. The same may be performed for intermediate or lowest classes of storage. 
     The capacity of storage classes may be increased or decreased if necessary or desired. In one embodiment, a storage class may be monitored for free space and automatically grown if necessary. In one embodiment, a storage device within a storage class may be grown to increase the capacity of the storage class. For example, a storage device with a total of 4 gigabytes of disk space may have 2 gigabytes of the disk space allocated to a storage class in a multi-class file system. If necessary or desired, free disk space on the storage device may be added to the storage class to grow the storage class. Data may be automatically and transparently moved to the newly allocated disk space on the storage device according to the policies. In one embodiment, one or more storage devices may be added to a storage class and data may be automatically and transparently moved to the new storage device according to the policies. 
     In one embodiment, the migration of data in a multi-class file system may be controlled by the type(s) of application data and/or file system metadata in the multi-class file system. In other words, data may be segregated in the hierarchy of storage classes according to its type. In one embodiment, file system metadata may be segregated into different storage classes based on the type of metadata and migrated between storage classes as can application data. 
       FIGS. 2 through 8D  illustrate means for implementing a multi-class file system comprising a hierarchy of storage classes on a plurality of storage devices and means for assigning and migrating data to different storage classes in the hierarchy of storage classes according to a set of policies for the multi-class file system. 
       FIG. 2  illustrates file system software including a multi-class storage mechanism implementing a multi-class file system and assigning and migrating data within the file system according to one embodiment. File system software  102  may include, but is not limited to, an embodiment of the multi-class storage mechanism  104  and software that provides “traditional” File System functionality  106 . File System functionality  106  may be provided by any of various File System implementations including, but not limited to, disk-based file systems such as Veritas&#39; VxFS and UNIX File System (UFS), and networked file system such as Network File System (NFS) and AT&amp;T&#39;s Remote File Share (RFS). 
     File system software  102  may implement, on one or more storage devices, a multi-class file system  120 . In one embodiment, the software that provides the File System functionality  106  may be used to implement the multi-class file system  120 , while the multi-class storage mechanism  104  manages the policies and performs assignment and migration of data within the multi-class file system  120  according to the policies. The multi-class file system  120  may include one or more physical and/or logical storage devices organized in a hierarchy of two or more storage classes according to one or more characteristics of the storage devices and/or data requirements of the user(s) of the file system. In this example, the multi-class file system  120  has N classes—a first storage class  110 , a second storage class  112 , and so on to an Nth storage class  114 . The multi-class file system  120  is made available to the application(s)  100  as one file system. The storage class structure of the multi-class file system  120  is transparent to the application(s)  100 . 
     In one embodiment, each storage class of the multi-class file system  120  may have different characteristics such as cost and/or geometry. In one embodiment, the storage classes may form a hierarchy, with the most expensive and typically fastest storage devices at the top, and the least expensive (and typically slower) storage devices at the bottom. The different storage classes may include heterogeneous storage devices to help achieve cost differences. For example, first storage class  110  may include one or more fast mirrored storage devices, second storage class  112  may include one or more RAID-5 storage devices, and a lowest storage class may include one or more read/write optical storage devices. In one embodiment, the amount of total storage at each storage class in the hierarchy may increase at each lower storage class of the hierarchy, though this is not required. 
     Note that, in some implementations, other criteria than cost and performance may be used to organize the storage devices into a hierarchy of storage classes. For example, storage capacity may be used as a criterion to select storage devices for a lower storage class. As another example, ownership or usage of the storage devices may be used as a criterion to place storage devices in storage classes. In this example, a storage device may be placed in a storage class based on the department or application that uses the storage device rather than on cost or performance characteristics of the storage device, and the policies may be implemented such that data belonging to that department or application is assigned to that storage device. 
     Multi-class storage mechanism  104  may store files, or portions of files, in storage classes of the multi-class file system  120  according to one or more user-defined policies geared to the business needs of the enterprise. Multi-class storage mechanism  104  may also migrate files and/or portions of files between storage classes in the multi-class file system  120  according to one or more user-defined policies. For example, as indicated at  130 , files and/or portions of files may be migrated from first storage class  110  to second storage class  112 , or from second storage class  112  to first storage class  110 , according to the policies. As indicated at  132 , files and/or portions of file may be migrated from second storage class  112  to lower storage classes in the multi-class file system  120  (e.g. the Nth storage class  114 ). The policies may be configured to move data up or down in the hierarchy of storage classes. In one embodiment, when data is reclassified to a lower or lowest storage class, the data may be compressed to further lower the cost of storage. Note that migrated files and/or portions of files remain in the same file system and are not migrated offline when migrated to any of the storage classes, even the lowest storage class. Also note that migration is performed transparently to the application(s)  100 . 
     While  FIG. 2  shows data migrating down from a higher storage class down to a next lower storage class, and data migrating up from a lower storage class to a next higher storage class, note that data may be migrated from any storage class to any other storage class in the hierarchy of storage classes according to the policies. For example, when migrating data down in the hierarchy of storage classes, some data may be migrated from the highest storage class to the next lower storage class, and other data may be migrated directly to the lowest storage class according to the policies. Similarly, when migrating data up in the hierarchy of storage classes, some data may be migrated from the lowest storage class to the next higher storage class, and other data may be migrated directly to the highest storage class according to the policies. 
     In one embodiment, the multi-class storage mechanism  104  may store new files in the multi-class file system at the highest or higher storage classes of the hierarchy. In one embodiment, as data ages (data including files and portions of files, e.g. a set of records in a database), the data may be moved down the hierarchy to lower-cost storage on lower storage classes to reduce the cost of storing the data. In one embodiment, data may be written into fast storage (e.g. fast mirrored storage) and then batched to slower storage such as RAID-5 storage, for example on a nightly schedule. Note that business-related attributes of the data such as importance, value, or ownership may be considered in decisions to assign and/or migrate the data. For example, a user may define the policies so that a file or portion of a file considered critical to business operations, and thus requiring fastest possible access times, may be kept in a highest storage class although access information may indicate that the file or portion of the file be migrated to a lower storage class. As another example, a user may define the policies so that a file or files, or portions of files, belonging to a certain group within the business may be kept in a higher storage class although access information may indicate that the file(s) or portions of file(s) be migrated to a lower storage class. 
     In one embodiment, when multi-class storage mechanism  104  migrates a file or portion of a file down to a lower storage class or up to a higher storage class in the multi-class file system, file system metadata  108  (e.g., inode information) is modified to reflect the move because the file or portion of a file is being moved to a different storage device. Note that file path information exposed to the application(s)  100  is not modified; the move is transparent to the application(s)  100 . Also note that the moved data remains “on-line” in the multi-class file system  120 , even when moved to the lowest storage class. The data is not moved “offline” and replaced by a stub (e.g. as in HSMs). Thus, the metadata (e.g., inode of the file) is modified to indicate the new location (e.g. device:block number(s) or device:extent(s)) of the data. If an application  100  requests the data, the data is still on-line in the file system, so it can be accessed directly without having to restore the data from offline storage. Thus, the structure of the multi-class file system  120  is transparent to application(s)  100  accessing the data stored in the multi-class file system  120 . Regardless of where a file, block, or extent is stored in the multi-class file system  120 , the file appears the same to the application(s)  100 . 
     In one embodiment, the policies may be implemented to consider business criteria (e.g. value) and/or usage patterns of the data in making assignment and migration decisions. In one embodiment, the policies for classifying and migrating data in the hierarchy may be tailored to business needs to lower the total cost of the storage farm without compromising on performance or availability. There are many possible combinations of data usage and business criteria that may be implemented in the policies for use by the multi-class storage mechanism  104  in classifying data as to its initial storage class in multi-class file system  120  and in reclassifying and migrating data down to a lower storage class or up to a higher storage class in multi-class file system  120 . The policies may make decisions according to the application, user, and/or business segment (e.g., department, division, etc) accessing or associated with the data, access patterns for the data, a specified policy (e.g. files in a particular directory or of a particular business group or employee could be assigned to high, medium, or low access classes), and/or any other desired parameter or parameters. 
     In one embodiment, multi-class storage mechanism  104  may track access of data, including application data and file system metadata, in files or portions of files, automatically downgrade at least a portion of the data on a storage class (e.g. at a time interval specified by a user or administrator) that has not been accessed for a given time interval, and migrate the downgraded data, in files or portions of files, to a lower storage class. In one embodiment, multi-class storage mechanism  104  may also be configured to migrate data on a storage class that has been accessed to a higher storage class. In one embodiment, a policy or policies may be operationally driven (e.g. to migrate data up to a higher class to do end-of-period processing, then back down when the processing is done). 
     One embodiment may break files down into smaller ranges of blocks or extents and keep statistics on the ranges (access times, usage characteristics, etc) to help make policy-based automatic decisions. The ranges of the files may then be separated on different classes according to the statistics as determined by the policies. For example, databases may have large files with high-priority and low-priority records mixed in. The files may be broken down into ranges that include high-priority or low-priority records, and statistics may be kept on the individual ranges. As another example, DB2 databases may have partitioned data based on certain indexes or fields in the database. Combinations of partitions may be created, and the DB2 database may be partitioned among classes of storage. This may be used, for example, in scaling clustering solutions. 
     In one embodiment, the partitioning technique used to implement a multi-class file system may be a multi-device feature of a File System (represented as File System functionality  106  in  FIG. 2 ).  FIG. 3  illustrates a multi-class file system based on a multi-device feature of File System software according to one embodiment. File system software  142  may include, but is not limited to, an embodiment of the multi-class storage mechanism  144  and software that provides “traditional” File System functionality  146 . File System functionality  146  may be provided by any of various File System implementations including, but not limited to, disk-based file systems such as Veritas&#39; VxFS and UNIX File System (UFS), and networked file system such as Network File System (NFS) and AT&amp;T&#39;s Remote File Share (RFS). 
     Each storage device in the multi-class file system  160  may be a logical or physical storage device. Typically, a physical device will be assigned to a particular class, but it is possible that two or more logical devices on one physical device may be allocated to different storage classes. The storage devices may be segmented into N storage classes in the multi-class file system  160  (in this example, three storage classes) by File System functionality  146 . In this example, the first storage class includes storage device  150 , the second storage class includes storage devices  152 A and  152 B, and the third storage class includes storage devices  154 A,  154 B, and  154 C. Note that a storage class may include any number of individual storage devices. 
     The storage devices may be divided into storage classes by File System functionality  146  according to characteristics of the storage devices. In one embodiment, the storage classes may be arranged from a first storage class of highest-performing (and typically most expensive) storage devices, to a lowest storage class of poorest-performing (and typically least expensive) storage devices. There may be more storage devices per storage class in lower storage classes than in higher storage classes, but this is not required. The storage devices may be heterogeneous; that is, there may be different vendors and/or models of storage devices in the different storage classes, and there may be different vendors or models of storage devices within a storage class. In other words, storage devices may be grouped in a storage class and/or divided into different storage classes according to performance characteristics rather than make and model. 
     In one embodiment of the multi-device implementation, the file system software  142  may address each device as a range of blocks (e.g. from O-n). In one embodiment, the multi-class file system may address a particular block in the file system as a device:block number, or alternatively device:extent, if extents are used. The device:block number may be associated with a path or directory in the file system metadata. The multi-class storage mechanism  144  tracks which storage devices are assigned to which storage classes. If the multi-class file system  144  moves a file or portion of a file to another storage device on another storage class, the file or portion of a file remains active in the file system, and the metadata is modified to reflect the move. The device:block number is changed, but the path or directory information remains the same. Thus, the move is transparent to an application that accesses the file. 
     A multi-class file system  160  based on a multi-device feature of a File System may be expanded. For example, storage devices may be added to the storage classes and/or additional storage classes may be added using the File System functionality  146 , and the multi-class storage mechanism  144  may store or migrate files or portions of files to the new storage device(s) and/or storage class(es) automatically and transparently to the application(s) that access the multi-class file system  160 . As another example, storage allocated to a storage class on a storage device within the storage class may be expanded to increase the size of the storage class. 
     In another embodiment, the partitioning technique used to implement a multi-class file system may be multiple independent file systems under a virtualization layer, as illustrated in  FIG. 4 . File system software  142  may include, but is not limited to, an embodiment of the multi-class storage mechanism  204 , software that provides “traditional” File System functionality  206 , and a virtualization layer  208 . File System functionality  206  may be provided by any of various File System implementations including, but not limited to, disk-based file systems such as Veritas&#39; VxFS and UNIX File System (UFS), and networked file system such as Network File System (NFS) and AT&amp;T&#39;s Remote File Share (RFS). 
     File System functionality  206  may implement a plurality of separate file systems (e.g., file systems  212 A,  212 B, and  212 C). Each file system  212  may be implemented to represent a class in a multi-class file system  210 . For example, file system  212 A may be a highest storage class, file system  212 B may be a middle storage class, and file system  212 C may be a lowest storage class. Each file system  212  may be implemented on a separate storage device, or alternatively one or more of the file systems  212  may logically extend across two or more storage devices. Each file system  212  may have its own file system metadata. Virtualization layer  208  may be used to map these file systems  212  to a single file system and present the single file system to one or more application(s). Virtualization layer  208  maintains some form of metadata for the single file system that maps data paths as used by application(s) to data locations in the file systems  212 . 
     Multi-class storage mechanism  204  maintains and implements the user-defined policies of the multi-class file system  210 . Multi-class file system  210  may initially assign files or portions of files to storage classes (i.e., file systems) based on the policies, and may monitor file usage and migrate files or portions of files up or down in the storage classes according to the policies (in this example, the three storage classes represented by file system  212 A, file system  212 B, and file system  212 C). To assign or migrate a file or a portion of a file, multi-class file system  210  may interact with File System functionality  206  to actually move the file from one file system  212  to another (including modifying the metadata on the affected file systems  212 ), and with virtualization layer  208  to modify the metadata for the single file system presented to the application(s) to reflect the move. Alternatively, File System functionality  206  may interact with virtualization layer  208  to modify the metadata for the single file system presented to the application(s) to reflect the move. In the metadata for the single file system presented to the application(s), the path information is not modified when a file or portion of a file is migrated. Further, the file or portion of a file remains within the single file system presented to the applications(s) by the virtualization layer  208 , and within the file systems  212 , and is not moved offline. Thus, the multi-class file system  210 , and migration of data within the multi-class file system  210 , remains transparent to the application(s). All the application(s) see, or need to see, is the single file system presented by the virtualization layer  208 . 
     In yet another embodiment, the partitioning technique used to implement a multi-class file system may be block ranges with different characteristics. The block ranges may be implemented by either the software that provides file system functionality (e.g., Veritas&#39; VxFS) or in the underlying volume manager or disk subsystem(s). In this embodiment, sets of characteristics may be assigned to different ranges of blocks. In the multi-device implementation, the file system software may address each device as a block range (e.g. from 0-n), and addresses one particular block in the file system as device:block number. In the block range implementation, the device may be made the high-order bits of the block number, and the concatenated block numbers, separated into ranges representing storage classes, may be used to implement the multi-class file system. Thus, a block range may extend across one or more devices. In this implementation, one virtual device may be presented to the application with distinctly different characteristics associated with block ranges of the virtual device, with the file system software understanding what block ranges of the file system have those characteristics. 
     The multi-class storage mechanism maintains and implements the user-defined policies of a multi-class file system implemented using block ranges as storage classes. The multi-class file system may initially assign files or portions of files to storage classes (block ranges) based on the policies, and may monitor file usage and migrate files or portions of files up or down in the storage classes according to the policies. To assign or migrate a file or a portion of a file, the multi-class file system may interact with the software that provides File System functionality to actually move the file from one block range to another (including modifying the metadata to reflect the move). In the metadata, the path information is not modified when a file or portion of a file is migrated. Further, the file or portion of a file remains within the file system presented to the applications(s), and is not moved offline. Thus, the multi-class file system, and migration of data within the multi-class file system, remains transparent to the application(s). 
     Note that other techniques to partition storage devices into storage classes than those described herein may be used in other embodiments. Embodiments of the multi-class storage mechanism may function transparently to the particular underlying partitioning technique used to organize storage devices or file systems into a multi-class file system. 
       FIG. 5  illustrates a multi-class file system in a network storage environment according to one embodiment. Host system  250  may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, workstation, network computer, or other suitable device. Host system  250  may include at least one processor  252 . The processor  252  may be coupled to a memory  254 . Memory  254  is representative of various types of possible memory media, also referred to as “computer readable media.” Hard disk storage, floppy disk storage, removable disk storage, flash memory and random access memory (RAM) are examples of memory media. The terms “memory” and “memory medium” may include an installation medium, e.g., a CD-ROM or floppy disk, a computer system memory such as DRAM, SRAM, EDO RAM, SDRAM, DDR SDRAM, Rambus RAM, etc., or a non-volatile memory such as a magnetic media, e.g., a hard drive or optical storage. The memory medium may include other types of memory as well, or combinations thereof. Host system  250  may couple, for example over a wired or wireless network or networks, to one or more other devices, e.g. client system  290 , via one or more wired or wireless network interfaces. Host system  250  may couple over a network  262  to two or more storage devices, in this example storage devices  264 ,  266 ,  268 ,  270  and  272 . Network  262  may, for example, be a Storage Area Network (SAN), a LAN with Network-Attached Storage (NAS), or any network capable of coupling storage devices to a host system  250 . The storage devices may include any of one or more types of storage devices including, but not limited to, storage systems such as RAID (Redundant Array of Independent Disks) systems, disk arrays, JBODs (Just a Bunch Of Disks, used to refer to disk cabinets that do not have a built-in RAID controller), tape devices, and optical storage devices. 
     Host system  250  may include, in memory  254 , file system software  256 . File system software  256  may include, but is not limited to, an embodiment of the multi-class storage mechanism  260  and software that provides “traditional” File System functionality  258 . File System functionality  258  may be provided by any of various File System implementations including, but not limited to, disk-based file systems such as Veritas&#39; VxFS and UNIX File System (UFS), and networked file system such as Network File System (NFS) and AT&amp;T&#39;s Remote File Share (RFS). In one embodiment, file system software  256  may also include a virtualization layer (not shown) that may be used to map file systems on the storage devices to a single file system and to present the single file system to one or more application(s). 
     File System functionality  258  of file system software  256  may implement, on one or more of the storage devices, a multi-class file system  280  (in this example, on storage devices  268 ,  270 , and  272 ). One of the methods for implementing a multi-class file system previously described may be used. In one embodiment, the software that provides the File System functionality  258  may be used to implement the multi-class file system  280 , while the multi-class storage mechanism  260  manages the policies and directs assignment and migration of data within the multi-class file system  280  according to the user-defined policies. 
     The multi-class file system  280  may include one or more physical and/or logical storage devices organized in a hierarchy of two or more storage classes according to one or more characteristics of the storage devices and/or data requirements of the user(s) of the file system. For example, the multi-class file system  280  may have three classes—a first storage class on storage device  268 , a second storage class on storage device  270 , and a third storage class on storage device  272 . As another example, storage device  268  may be a first storage class, and storage devices  270  and  272  may both be included in a second storage class. The multi-class file system  280  is made available to the application(s)  292  on client system  290  as one file system. The structure of the multi-class file system, and assignment and migration of data within the multi-class file system, are transparent to the application(s)  292 . 
     In one embodiment, each storage class of the multi-class file system  280  may have different characteristics such as cost and/or geometry. In one embodiment, the storage classes may form a hierarchy, with the most expensive and typically fastest storage devices in the top or first storage class, and the least expensive (and typically slower) storage devices in the bottom or last storage class. 
       FIG. 6  is a flowchart of a method for assigning and migrating files or portions of files in a multi-class file system according to one embodiment. As indicated at  300 , a multi-class file system including a hierarchy of storage classes may be implemented on one or more storage devices. In one embodiment, a multi-device feature of file system software (e.g. Veritas&#39; VxFS) may be used to implement the multi-class file system. Different architectures for the underlying structure of the multi-class file system were previously described. In one embodiment, each storage class includes one or more storage devices assigned to the storage class according to one or more characteristics of the storage class, e.g. performance characteristics. In one embodiment, the storage classes may be ordered in the hierarchy of storage classes according to performance characteristics from a highest storage class including one or more high-performance storage devices to a lowest storage class comprising one or more low-performance storage devices. 
     In one embodiment, a multi-class storage mechanism of the file system software may initially place files in the storage classes in the hierarchy of storage classes according to a user-defined policy for assigning new files to an initial storage class in the hierarchy of storage classes. 
     As indicated at  302 , the multi-class storage mechanism of the file system software may monitor access of files stored in the hierarchy of storage classes to generate access information for the files. As indicated at  304 , the multi-class storage mechanism software may apply the access information and other information about the file (e.g. ownership of the file, business value or importance of the file, etc.) to a set of user-defined policies for initially assigning and migrating files in the hierarchy of storage classes. 
     As indicated at  306 , the multi-class storage mechanism of the file system software may migrate one or more of the files to different storage classes in the hierarchy of storage classes according to the set of user-defined policies when applied to the access or other file information. In one embodiment, less-frequently-accessed files may be migrated to lower storage classes including lower-performing storage devices, and more-frequently-accessed files may be migrated to higher storage classes including higher-performing storage devices. In one embodiment, files migrated to one or more storage classes in the hierarchy of storage classes, for example to the lowest class, may be compressed. In one embodiment, the multi-class storage mechanism software may modify file system metadata when a file is assigned or migrated to indicate the storage class for the file. In embodiments, migrating files to different storage classes may be performed transparently to an application that accesses the files in the hierarchy of storage classes. Note that files migrated to a lowest storage class remain online in the file system, i.e. the data is still present in the file system and is not moved offline. 
     In one embodiment, the multi-class storage mechanism software may assign or migrate a portion of a file (e.g. a range of blocks or sectors) to one storage class in the hierarchy of storage classes while assigning or leaving the remainder of the file on a different storage class according to the user-defined policies for assigning or migrating files. 
     In one embodiment, a new storage class may be added to the hierarchy of storage classes, and the multi-class storage mechanism software may transparently migrate one or more of the files or portions of the files from one or more others of the storage classes to the new storage class according to the policies. Storage classes may be increased or decreased in capacity if necessary or desired. In one embodiment, one or more storage devices may be added to a storage class and data may be automatically and transparently moved to the new storage device according to the policies. In one embodiment, a storage device within a storage class may be grown to increase the capacity of the storage class. If necessary or desired, free disk space on a storage device may be added to the storage class to grow the storage class. Data may be automatically and transparently moved to the newly allocated disk space on the storage device according to the policies. 
       FIGS. 7A through 7E  illustrate the migration process in an exemplary multi-class file system with three storage classes according to one embodiment. In this example, file system software may include a multi-class storage mechanism as previously described. The file system software may, as previously described, implement a multi-class file system  240  including a hierarchy of storage classes on one or more storage devices; in this example there are three storage classes  220 ,  222 , and  224 , each of which may include one or more storage devices. Note that other multi-class file systems may have different numbers of storage classes. Also note that the term “data” as used in this example indicates a file or a portion of a file (e.g., a range of blocks or one or more extents). 
     The multi-class storage mechanism may maintain a set of policies for data assignment and migration within the multi-class file system, may monitor data accesses within the multi-class file system, and may cause data being monitored to be assigned to storage classes and/or migrated to different storage classes within the multi-class file system according to the set of policies. In this example, first storage class  220  may be mirrored on one or more high speed disks, and actively written data may be assigned or migrated there according to the policies. Data that have not been written to for some period (e.g. in the last month) are migrated to the second storage class  222  according to the policies, and data that have not been written to for some period (e.g. in the last quarter) are migrated to the third storage class  224  according to the policies. The second storage class  222  and third storage class  224  may each include one or more storage devices, which may be less expensive and slower than the storage device(s) of the first storage class (e.g. RAID-5 array(s)); the third storage class  224  may include one or more less expensive, and slower, storage devices than the storage device(s) of the second storage class  222 . If data in the second storage class  222  or third storage class  224  is written to, it may be migrated back to the first storage class  220  according to the policies. 
       FIGS. 8A through 8D  illustrate an exemplary metadata structure for data  230  of  FIGS. 7A-7D  that may be modified by the file system software during assignment and migration of data  230  in multi-class file system  240  according to one embodiment. As the migration process is described using  FIGS. 7A-7E ,  FIGS. 8A-8D  will be referenced to indicate modifications in the metadata to reflect the migration(s). 
     As illustrated in  FIG. 7A , data  230  may initially be written to first storage class  220  of multi-class file system  240 . Data  230  may be a file or a portion of a file (e.g. one or more records of a database file, a range of blocks, or one or more extents of a file). The data may have been generated by some application using the multi-class file system  240 . From the application&#39;s perspective, the data  230  was created at some logical location in the file system as seen by the application indicated to the application by a path. For example, a path to data  230  may look something like “MyDrive/MyFolder/MyFile” Note that this example is exemplary and is not intended to be limiting; other methods for indicating a path to data in a file system are possible. 
       FIG. 8A  illustrates metadata for data  230  when assigned to first storage class  220 . When data  230  is assigned to the first storage class  220 , the data  230  is written to a block range on a storage device in the first storage class  220 . In this illustration, the path as seen by the application is shown on the left, and the location information for the data  230  in the multi-class file system  240  as used by the file system software is shown on the right. The location information for the data  230  in the multi-class file system  240  indicates that the data  230  is located at “First Storage Class:Device:Block Number”. Note that this example is exemplary and is not intended to be limiting; other methods for storing location information for data in a multi-class file system are possible. For example, an alternative is to simply indicate the device and block number, as the storage class may be determined from the device identifier. Another alternative is to concatenate the device identifier and block number (as is done in embodiments where the underlying implementation of the multi-class file system uses block ranges as storage classes, as previously described). Also note that the metadata may include other information not shown, such as the number of blocks the data  230  occupies starting at the indicated block number. 
     As illustrated in  FIG. 7B , at some point migration  232  of data  230  from first storage class  220  to second storage class  222  may occur. For example, the multi-class storage mechanism may monitor (read and/or write) access of data on first storage class  220 , and periodically migrate data that has not been accessed for a period of time (e.g. one month) to second storage class  222  according to the policies. In this example, after data  230  is initially assigned to first storage class  220 , the multi-class storage mechanism may monitor access of data  230  and, if the data has not been accessed for a period of time as specified by the policies, may migrate data  230  to second storage class  222 . The migration  232  does not move data  230  offline; data  230  remains online within the file system. Note that the policies may include other criteria than simply access criteria to determine when and if data is to be migrated. For example, the policy may indicate that data  230  is critical to business operations, or is owned by a particular user or application, and is therefore not to be migrated. As another example, the policy may specify that all actively accessed data is to be stored on first storage class  220  and migrated to second storage class  222  after a month of inactivity except data that belongs to a particular non-critical application, which is to be migrated to second storage class  222  after one week of inactivity. Using the policies, other criteria, including business-related criteria, than simply data access criteria may be applied by the multi-class storage mechanism in making assignment and migration decisions. 
       FIG. 8B  illustrates the metadata for data  230  after migration  232  to second storage class  222 . Note that the location information for the data  230  in the multi-class file system  240  used by the file system software now indicates that data  230  is stored starting at a block number on a device of the second storage class  222 . Also note that the path as seen by the application has not changed; from the application&#39;s perspective, migration  232  is transparent. The migration  232  does not move data  230  offline; data  230  remains online within the file system. 
     As illustrated in  FIG. 7C , at some point migration  234  of data  230  from second storage class  222  to the third storage class  224  may occur. For example, the multi-class storage mechanism may monitor (read and/or write) access of data on the second storage class  222 , and periodically migrate data that has not been accessed for a period of time (e.g. three months) to third storage class  224  according to the policies. In this example, after data  230  has migrated to second storage class  222 , the multi-class storage mechanism may monitor access of data  230  and, if the data has not been accessed for a period of time as specified by the policies, may migrate data  230  to third storage class  224 . The migration  234  does not move data  230  offline; data  230  remains online within the file system. Note that the policies may include other criteria than simply access criteria to determine when and if data is to be migrated. Using the policies, other criteria, including business-related criteria, than simply data access criteria may be applied by the multi-class storage mechanism in making assignment and migration decisions. 
       FIG. 8C  illustrates the metadata for data  230  after migration  234  to third storage class  224 . Note that the location information for the data  230  in the multi-class file system  240  used by the file system software now indicates that data  230  is stored starting at a block number on a device of the third storage class  224 . Also note that the path as seen by the application has not changed; from the application&#39;s perspective, migration  234  is transparent. The migration  234  does not move data  230  offline; data  230  remains online within the file system. 
     While  FIGS. 7A-7C  show data migrating from a higher storage class down to a next lower storage class, note that data may be migrated from any storage class to any other storage class in the hierarchy of storage classes according to the policies. For example, when migrating data down in the hierarchy of storage classes, some data may be migrated from the highest storage class to the next lower storage class, and other data may be migrated directly to the lowest storage class according to the policies. 
       FIGS. 7D and 7E  illustrate an exemplary method of handling of an access of data  230  on third storage class  224  according to one embodiment. At some point after migration  234  of data  230  to third storage class  224 , an application may generate an access request (e.g. a read or write request) for the data  230 . In  FIG. 7D , the file system software may provide data  230  to the application to satisfy the access request, as indicated at  236 . The policies may indicate to the multi-class storage mechanism that data stored on third storage class  224  is to be migrated to first storage class  220  if the data is accessed. In  FIG. 7E , the multi-class storage mechanism may migrate  238  data  230  from third storage class  224  to first storage class  220  in response to the access of data  230  by the application if so indicated by the policies. The migration  238  does not have to restore data  230  from offline storage back into the file system; data  230  was online within the file system prior to the migration  238 . 
     Note that, alternatively, data  230  may be migrated  238  to first storage class  220  prior to providing the data  230  to the application as indicated at  236 . Also note that the policies used by the multi-class storage mechanism to make migration decisions may indicate some other action, which may be determined using other criteria than simply data access criteria, than migration to first storage class  220  upon access of data  230  is to be performed. For example, the policies may indicate that, if the access is a write access, data  230  is to be migrated to first storage class  220 , but if the access is a read access, data  230  is to remain on third storage class  224 . As another example, the policies may indicate that an access of data  230  by a particular application (or user) does not require data  230  to be migrated, but access by another application (or user) does require data  230  to be migrated. 
     While  FIGS. 7D and 7E  show data migrating up from a lowest storage class to a the highest storage class, note that data may be migrated from any storage class to any other storage class in the hierarchy of storage classes according to the policies. For example, when migrating data up in the hierarchy of storage classes, some data may be migrated from the lowest storage class to the next higher storage class, and other data may be migrated directly to the highest storage class according to the policies. 
       FIG. 8D  illustrates the metadata for data  230  after migration  238  to first storage class  220  from third storage class  224 . Note that the location information for the data  230  in the multi-class file system  240  used by the file system software now indicates that data  230  is stored starting at a block number on a device of the first storage class  220 . Note that the device is not necessarily the same device as in the metadata of  FIG. 8A , and the block number is not necessarily the same block number as in the metadata of  FIG. 8A . Also note that the path as seen by the application has not changed; from the application&#39;s perspective, migration  238  is transparent. 
     One embodiment may include one or more storage classes of writeable storage and one or more storage classes that are not writeable (read-only). Note that, in one embodiment, storage devices within a storage class may be segmented into subclasses of writeable and read-only storage devices. Files may be migrated down to a read-only storage class, but the read-only storage class is only written to during migration. Thus, there may be one or more read-only storage classes in a multi-class file system. To write to one of the files in a read-only storage class, the file may first be migrated up to a writeable storage class to be written to. The file may remain in the higher storage class for a period until the multi-class storage mechanism determines that the file may be migrated back to the read-only storage class, or alternatively may be immediately migrated back to the read-only storage class, according to the policies. 
     In one embodiment, it may not be necessary to migrate an entire file because the file mapping in the file system may be implemented so that the blocks or sectors of the file that have not been modified may remain on a read-only storage class and the blocks or sectors that are to be modified are migrated to a writeable storage class according to the policies. Thus, parts of files as well as entire files may be migrated, and different blocks or sectors of a file may concurrently reside on different storage classes. To write to a file on a read-only storage class, blocks, sectors or records (in a database) that need to be modified may be migrated to a writeable storage class while the rest of the file stays on the read-only storage class. 
     The ability to migrate a portion of a file may be an important feature for databases, as an example. Once completed, a transaction in a database is typically not modified again. All transactions that have not been completed yet may reside in a writeable storage class. Periodically, completed transactions may be migrated to a read-only storage class (the database application may still need to access the transactions, so keeping them available is preferable). Inactive or completed records in a database may be migrated down to a read-only storage class while records that are still active are maintained in a writeable storage class according to the policies. 
     In one embodiment, a multi-class file system may include one or more levels of general storage classes, with one or more storage classes in each general storage class. A general storage class, like a storage class, may be based, for example, on cost/performance characteristics of the storage devices. Two or more storage classes in a general storage class may be considered at the same level in the hierarchy of storage classes. Data may be stored on or migrated to different classes of storage within a more general storage class according to the policies. For example, a general read-only storage class may include a storage class for actively accessed read-only data and a storage class for non- (or rarely-) accessed read-only data, the latter of which may be compressed. As another example, a general writeable storage class may include two or more classes based on criteria such as frequency of access. Note that other criteria than frequency of access may be applied to segment data within storage classes in a general storage class. As an example, importance of the data may be used as a criterion; some data within a general storage class may be deemed more important or critical than other data, and thus may be segmented from the less important data by storing the different data on different storage classes within the general storage class. 
     Using Compression in a Multi-Class File System 
     Embodiments may use compression to achieve further data storage efficiency in a multi-class file system. In a multi-class file system, there may be one or more classes of storage that contain files or blocks that are not actively written. By compressing these files (or blocks), a significant amount of disk space may be saved. One embodiment may compress all files or ranges of blocks migrated to a lower or lowest storage class. Rather than compressing all the files or blocks in a storage class, another embodiment may gather usage statistics for files or blocks in a storage class or storage classes and compress files (or, alternatively, ranges of blocks) in the storage class that are less frequently accessed. 
     Since it is known that the files or block ranges to be compressed are not being actively written, the file system may choose a compression algorithm that provides better space savings at the expense of not being able to do efficient in-place updates. In one embodiment, data in older (lower) storage classes may be compressed. 
     One embodiment may keep statistics on file usage and compress less-used (below some usage threshold) files and/or ranges of blocks. This may save disk space without the processing cost of decompressing when a file is read. One embodiment may perform the compression when migrating to a lower storage class for inactive files, as the file system has had time to gather statistics on the file access patterns. This may be used to implement only changing data on the lower storage classes at fixed times. 
     In one embodiment, when migrating data in files or portions of files to a lower storage class, the data may be compressed. One of the characteristics of files that may be examined by a policy-driven multi-class storage mechanism is files that have not been written to for some period. A typical problem with compression in file systems is that, when a file is to be compressed, it is not known in advance exactly how much compression will be achieved. A file is essentially random-access; if one byte or block in the middle of a compressed file is changed (typically by decompressing the file, changing the byte, and recompressing the file), the change to the byte or block may potentially change the compression size. For an uncompressed file, rewriting one block does not change the file size. If a compressed file is changed, recompressing the file might not leave enough space for the file, as the recompressed file may be larger than the original compressed files. 
     In one embodiment, files that have not been modified for a period (e.g. a month) may be migrated to a lower storage class. Files from that storage class may be migrated to an even lower storage class if the files are not modified for another, longer period (e.g. six months). The files may be compressed when moving to the even lower storage class based on the polices with confidence that the files are not likely to be uncompressed and modified (the files have not been modified for the longer period, so are not likely to be modified), achieving further cost savings. Using a policy such as this, processing resources are not wasted compressing files that are likely to be uncompressed in the future, and further the problem of not having space for recompressed files described above may be avoided. 
     Embodiments may implement compression and decompression as part of migration from one storage class to another. In one embodiment, different compression algorithms may be used depending on the type of data being migrated. One embodiment may use a compression algorithm or algorithms that decompresses very fast, with a high a level of compression. Compression may not need to be particularly fast since compression is performed on files that are not likely to be accessed; decompression is performed when access to the file is desired or required, so decompression may have time constraints that compression does not. 
     Backup Mechanism for a Multi-Class File System 
     Embodiments of the multi-class storage mechanism and the multi-class file system may be used in combination with a backup mechanism to reduce one or more of, but not limited to, total media used, the number of tape drives (or other offline storage devices) needed, the backup window, and restore time. 
     Embodiments of the multi-class storage mechanism used in combination with a backup mechanism may provide the ability to schedule backups to save backup media storage space as well as backup (and restore) time. In one embodiment, a full backup of all the storage classes, including one or more higher, writeable storage classes and one or more lower, read-only storage classes, may be performed periodically, and incremental backups may be performed on the writeable storage class(es) between the full backups of the writeable storage classes. For example, data in read-only storage class may be periodically backed up in an image-based full backup while the system remains online Periodically, incremental backups of the writeable storage class(es) may be performed. Thus, the read-only data is segregated into its own full backup. During restore, the writeable storage classes may be restored first, the applications may be started up, and then the inactive data (read-only) data may be restored while the system is up. Thus, both the backup window and the restore window may be shorter than in conventional backup and restore mechanisms. Further, in one embodiment, there may be less overall work when doing backups and restores, as there is no need to do a split mirror of storage devices in read-only storage classes. 
     By organizing storage into a hierarchy of storage classes including one or more writeable storage classes and one or more read-only storage classes, embodiments may reduce backup media (e.g. tape) costs, as well as scanning of file system costs, since the backup and restore does not have to walk through the entire file tree. 
       FIGS. 9-12  illustrate means for migrating aged data and performing backups of the storage classes in a hierarchy of storage classes of a multi-class file system.  FIG. 9  illustrates a backup mechanism for an exemplary multi-class file system on a host system in a network storage environment according to one embodiment. Host system  370  may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, workstation, network computer, or other suitable device. Host system  370  may include at least one processor  372 . The processor  372  may be coupled to a memory  374 . Memory  374  is representative of various types of possible memory media, also referred to as “computer readable media.” Hard disk storage, floppy disk storage, removable disk storage, flash memory and random access memory (RAM) are examples of memory media. The terms “memory” and “memory medium” may include an installation medium, e.g., a CD-ROM or floppy disk, a computer system memory such as DRAM, SRAM, EDO RAM, SDRAM, DDR SDRAM, Rambus RAM, etc., or a non-volatile memory such as a magnetic media, e.g., a hard drive or optical storage. The memory medium may include other types of memory as well, or combinations thereof. Host system  370  may couple, for example over a wired or wireless network or networks, to one or more other devices, e.g. client systems, storage devices and backup devices, via one or more wired or wireless network interfaces. The network may, for example, be a Storage Area Network (SAN), a LAN with Network-Attached Storage (NAS), or any network capable of coupling devices to a host system  370 . The storage devices may include any of one or more types of storage devices including, but not limited to, storage systems such as RAID (Redundant Array of Independent Disks) systems, disk arrays, JBODs (Just a Bunch Of Disks, used to refer to disk cabinets that do not have a built-in RAID controller), tape devices, and optical storage devices. 
     Host system  370  may include, in memory  374 , file system software  352 . File system software  352  may include, but is not limited to, an embodiment of the multi-class storage mechanism and software that provides “traditional” File System functionality. File System functionality may be provided by any of various File System software implementations including, but not limited to, Veritas&#39; VxFS software or NFS software. In one embodiment, the software that provides the File System functionality may be used to implement the multi-class file system  380  including a hierarchy of storage classes  382 , while the multi-class storage mechanism of the file system software manages the policies and directs assignment and migration of data within the multi-class file system  380  according to the user-defined policies. 
     Host system  370  may also include, in memory  374 , a backup mechanism  350  that may interact with file system software  352  and with the multi-class file system to perform backups of the storage classes  382  of multi-class file system  380 . Backup mechanism  350  may periodically perform full backups of each of the storage classes  382 . In one embodiment, the full backups may be image-based backups of the storage devices in the storage classes  382 . In one embodiment, the lower storage classes are backed up less frequently than the higher storage classes. 
     In one embodiment, one or more of the lower storage classes may be read-only storage classes, and data stored on a higher, writeable storage class that has not been written for a period (e.g. one month) may be periodically migrated (e.g. once a month) to a lower, read-only storage class. Data stored on read-only storage class that has not been accessed for a period (e.g. three months) may be periodically migrated (e.g. every three months) to a lower read-only storage class. In one embodiment, data on a read-only storage class that needs to be written to may first be migrated to a higher, writeable storage class. Thus, the backup mechanism  350  may periodically perform image-based full backups of the lower, read-only storage classes at longer intervals than backups of the higher, writeable storage classes. In one embodiment, the migration schedule for a storage class may coincide with the backup schedule of the storage class. 
     For example, in a multi-class file system containing a writeable storage class and two read-only storage classes, the writeable storage class may have a periodic (e.g. weekly) image-based full backup performed, along with incremental backups (e.g. performed daily). Periodically, e.g. monthly, data that have been inactive on the writeable storage class for at least that period (e.g. one month), may be migrated to the first read-only storage class. After the periodic (e.g. monthly) migration of inactive data from the writeable storage class to the first read-only storage class, a monthly image-based full backup of the first read-only storage class may be performed. At a longer period (e.g. every three months), data that have been inactive on the first read-only storage class for at least the longer period (e.g. three months) may be migrated to the second read-only storage class. After this migration, an image-based backup of the second read-only storage class may be performed. Note that these migrations leave the data active within the file system and do not move the data offline. 
     In one embodiment, if data on one of the read-only storage classes needs to be modified, the data are first migrated to the writeable storage class. This ensures that the read-only storage classes include only data that is not actively written, and that the modified data is on the writeable storage class where it will be more frequently backed up. In one embodiment, when data are migrated from one of the read-only storage classes to the writeable storage class, or if data on one of the read-only storage classes is deleted (e.g. if the file containing the data is deleted), the data blocks on the read-only storage class that contain the data are left untouched; the migration or deletion is indicated by changing the metadata to indicate the new location (or deletion) of the data. Thus, until the next periodic migration of data to the read-only storage class (which may potentially overwrite the data blocks), deleted data on the read-only storage class may be recovered by simply modifying the metadata rather than restoring the data from offline storage. In one embodiment, file block information from the most recent image backup may be used to recover and restore the block map. 
     Using this method of backing up a multi-class file system may result in reduced requirements for backup media (e.g. tapes) and potentially backup devices (e.g. tape drives). In one embodiment, files migrated to one or more lower storage classes  382  may be compressed, resulting in further savings of backup media. 
     Note that, while this example shows backup mechanism  350  and file system software  352  in memory  374  on the same host system  370 , backup mechanism  350  and file system software  352  may reside on different systems. For example, backup mechanism  350  may reside on a different host system coupled to host system  370 . 
       FIG. 10  illustrates a backup mechanism for an exemplary multi-class file system with three storage classes according to one embodiment. In one embodiment, a multi-class file system  380  may be set up with a hierarchy of storage classes. The storage classes may be created using any of the mechanisms previously described. In this example, file system software  352  may include a multi-class storage mechanism as previously described. File System functionality of file system software  352  may, as previously described, implement a multi-class file system  380  on one or more storage devices; in this example, there are three storage classes. Backup mechanism  350  may interact with the multi-class file system  380  to perform backups of the three storage classes in this example according to the following description. Note that a similar method may be applied in multi-class file systems with different numbers of storage classes. Also note that, while the term “file” is used in this example, the description also applies to portions of files (e.g. ranges of blocks or extents). 
     In this example, a first class of data storage (first storage class  354 ) is mirrored on the highest speed disks and contains all the data actively being written. The second storage class  356  is a read-only storage class that contains all data that have not been written in the last month, and the third storage class  358  is a read-only storage class that contains all data that have not been written in the last quarter. The second and third storage classes may include less expensive storage devices than the first storage class  354 . In one embodiment, each class of storage may be monitored for free space and automatically grown as needed. In one embodiment, if data in a read-only storage class needs to be written to, it is first migrated to a writeable storage class. In one embodiment, data may be compressed when migrated to the read-only storage classes. 
     If a multi-device file system is being used for the multi-class file system  380 , then, in one embodiment, all the metadata may be stored on the first storage class  354  and the block mappings for files on the second and third classes may be kept stable. To keep the block mappings stable, if files are removed or migrated back to the first storage class  354 , then the blocks are simply left unused in the second or third storage class and not written over. This allows an image-based backup of the first storage class  354  to be performed, and for the image-based backup of the first storage class  354  to be used in conjunction with an earlier image-based backup of the second and third storage classes without worrying about block aliasing. In one embodiment, the block mappings in the second and third storage classes may be stably maintained, and a write lock mechanism may be used on the lower (in this example, second and third) storage classes, along with a timestamp that indicates the last time the write lock was enabled. Using a write lock mechanism may allow a backup mechanism to verify the fidelity of those storage classes. 
     As an example, suppose that there are three storage classes, and that all three storage classes may be modified. Metadata is on a first storage class, and is backed up every week. In week  1 , a file that resides on the third class is located on blocks  1 - 10 . If the file is written in week  2 , the file may be moved to the second storage class and located on blocks  101 - 110 . If the image-based backup of week  2  is combined with the image-based backup of the third storage class from week  1 , the file would point to the wrong blocks. This is the problem of block aliasing, or mistracking the identity of block(s). To ensure that storage classes backed up at the same time are static in terms of block mapping, in one embodiment, between the times that storage classes are being migrated to, file reorganization, extensions, or modifications are not allowed to occur within the storage classes. It may be permitted to remove a file; however, writing to the space vacated by the file is not allowed until the next time a file is migrated down to the device. In this embodiment, the images of the different storage classes may be backed up to get a reliable image of the file system. Thus, in one embodiment, when data is migrated down (or up) to a storage class, a new image backup of the storage class(es) may be generated. In this embodiment, by making one or more lower storage classes read-only and keeping the block mappings static if possible, it may be known that the blocks on the image backups match what the metadata says they are. 
     One embodiment may include one or more storage classes of writeable storage and one or more storage classes that are not writeable (read-only). Note that, in one embodiment, storage devices within a storage class may be segmented into subclasses of writeable and read-only storage devices. Files may be migrated down to a read-only storage class, but the read-only storage class is only written to during migration. To write to one of the files in a read-only storage class, the file may first be migrated up to a writeable storage class to be written to. The file may remain in the higher storage class for a period until a the multi-class storage mechanism implementing the policies determines that the file is to be migrated back to the read-only storage class, or alternatively may be immediately migrated back to the read-only storage class according to the policies. 
     In one embodiment, it may not be necessary to migrate an entire file because the file mapping in the file system may be implemented such that the blocks or sectors of the file that have not been modified remain on the read-only storage class and the blocks or sectors that are to be modified are migrated to the writeable storage class. Thus, parts of files as well as entire files may be migrated, and different blocks or sectors of a file may concurrently reside on different storage classes. To write to a file on a read-only storage class, blocks, sectors or records (in a database) that need to be modified may be migrated to a writeable storage class while the rest of the file stays on the read-only storage class. 
     The ability to migrate a portion of a file may be an important feature for databases, as an example. Once completed, a transaction in a database is typically not modified again. All transactions that have not been completed yet may reside in a writeable storage class. Periodically, completed transactions may be migrated to a read-only storage class. The database application may still need to access the transactions, so the transaction records are kept online and available (files migrated within the multi-class file system remain online in the same file system). Inactive or completed records in a database may be migrated down to a read-only storage class (remaining online in the file system) while records that are still active are stored in a writeable storage class. 
     Thus, embodiments of the multi-class storage mechanisms may know that data on a read-only storage class is not changing. For backups, it may be known that a large percentage of data (e.g. 80%) has not changed, so the active data (e.g. 20%) may be backed up more often (e.g. every week), and the inactive data (e.g. 80%) may be backed up less often (e.g. every month). Thus, the total amount of storage media, e.g. tape, necessary for backup may be preferably reduced by a considerable amount, as well as reducing backup time. 
       FIGS. 11A-11D  illustrate an exemplary migration and backup schedule for an exemplary multi-class file system according to one embodiment. In this example, first storage class  354  may be a writeable storage class, and the second and third storage classes  356  and  358  may be read-only storage classes. The read-only storage classes may have write locks for preventing data to be written to the storage classes (unless the write lock is disabled) and a time stamp for determining the last time the read-only storage classes were written to. Note that this is an exemplary multi-class file system and is not intended to be limiting; a multi-class file system may be configured with one or more writeable storage classes and one or more read-only storage classes, only writeable storage classes, or even only read-only storage classes. 
     In  FIG. 11A , data  386  are initially written to the first storage class  354 . Periodically (e.g., once a week) a full backup  360  of the first storage class  354  is performed. The full backup  360  may be an image backup. Incremental backups  362  of the first storage class  354  may be performed between full backups  360 , for example every day. The incremental backups  362  may be file-based backups. 
     In  FIG. 11B , periodically (e.g., once a month), the write lock is disabled on the second storage class  356 , and data  390  in the first storage class  354  that has not been written to for at least that period (e.g. for at least a month) is migrated from the first storage class  354  to the second storage class  356 , as indicated at  388 . Note that migrated data  390  remains online in the file system. After migration, the write lock is re-enabled for the second storage class  356 . A full backup of the second storage class  356  may be generated by the backup mechanism after the data  390  are migrated. Full backup  364  may be an image backup. In addition, a full backup of the first storage class  354  may be performed after migration  388 . In one embodiment, data  390  may be compressed when migrated to second storage class  356 . 
     In  FIG. 11C , periodically, (e.g. once a quarter), the write lock is disabled on the third storage class  358 , and data  394  in the second storage class  356  that has not been written to for at least that period (e.g. for at least three months) is migrated from the second storage class  356  to the third storage class  358 , as indicated at  392 . Note that migrated data  390  remains online in the file system. After migration, the write lock is re-enabled for the third storage class  358 . A full backup of the third storage class  358  may be generated by the backup mechanism after the data  394  are migrated. Full backup  358  may be an image backup. In addition, a full backup of the first storage class  354  may be performed after migration  392 . In one embodiment, data  394  may be compressed when migrated to third storage class  358 . 
     With this schedule, the third storage class  358  may only be backed up at longer intervals, for example every three months. The second storage class  356  may be backed up at shorter intervals than the third storage class  358 , for example once a month. The backups of the read-only storage classes may be performed in conjunction with the migration of aged data to the read-only storage classes. In one embodiment, the full backups of the read-only storage classes may be done online without using split mirrors, since the storage classes are write locked and stable. The first storage class  354  and the metadata may have a full backup  360  more often, for example weekly, along with a set of incremental backups  362  (which may be file-based backups) performed, for example, daily. In one embodiment, the full backups of all storage classes may all be image-based, which is typically faster than file-based backup. Thus, there is no need for a synthesized full backup. 
       FIGS. 11A-11C  illustrate migrating aged data from a first storage class to a second class, backing up the second storage class, migrating aged data from the second class to a third storage class, and backing up the third storage class. In an alternative migration and backup schedule, aged data may first be migrated from a second storage class to a third storage class, and then the third storage class may be backed up. After completion of the migration to the third storage class, aged data may be migrated from the first storage class to the second storage class, and then the second storage class may be backed up. Finally, the first storage class may be backed up after migrating aged data to the second storage class. This migration and backup schedule empties the second storage class of aged data before migrating in the new aged data from the first storage class. Further, files are not backed up from the second storage class and then migrated to the third storage class to be backed up again. Similarly, files are not backed up from the first storage class and then migrated to the second storage class to be backed up again. Thus, for this migration and backup schedule, the order of  FIGS. 11A-11C  may be reversed. 
       FIG. 11D  illustrates migrating data to be modified from a read-only storage class to a writeable storage class. In this example, data  396  on third storage class  358  may need to be modified. Data  396  is migrated from the read-only third storage class  358  to the writeable first storage class  354  prior to modification, and may then be modified on the writeable first storage class  354 . 
     In one embodiment, a read-only storage class remains stable between periodic migrations of aged data. When data that needs to be modified is migrated from a read-only storage class to a writeable storage class, the data blocks in the read-only storage class may not be overwritten until the next migration of aged data to the read-only storage class. The data is copied to the writeable storage class, and the metadata for the data is modified to indicate the new location for the data in multi-class file system  380 . Similarly, when data is deleted from a read-only storage class, the data blocks in the read-only storage class may not be overwritten until the next migration of aged data to the read-only storage class. The metadata for the data is modified to indicate the data has been deleted. Thus, if data that was deleted since the last periodic migration needs to be restored to a read-only storage class, instead of restoring the data from a backup of the storage class, a block map may be used (e.g. a block map from a full backup performed after the last migration of aged data) to restore the metadata to indicate that the data is on the read-only storage class at the block locations indicated by the block map from the full backup. 
     To restore the entire multi-class file system  380 , the last full backup of each storage class may be restored, and then the incremental backups of the first storage class  362  may be applied. In one embodiment, a full backup of the first storage class  360  may first be applied, then any necessary incremental backups of the first storage class  362 , followed by the full backups of the second and third storage classes. In one embodiment, the multi-class file system  380  may be brought back online and made available to application(s) after the full and incremental backups of the first storage class  354  are restored and before the full backups of the other storage classes have been (fully) restored. 
     Consider as an example, a multi-class file system where 10% of the data is active and stored according to the polices on a first storage class, 20% of the data was written in the last quarter and is stored according to the policies on a second storage class, and 70% of the data has been unwritten for the last quarter and is stored according to the policies on a third storage class. In this example, each week, 10% of the data may be backed up offline, so the backup window may be reduced by a factor of 10. Split mirrors for only 10% of the data are required, reducing the cost of extra disks for backup. In addition, cost to preserve offsite copies of the media is reduced. For 52 weeks of full backups, 52 times as much tape as there is disk would be needed. Using an embodiment of the backup mechanism, 4*.0.7 plus 12*0.2 plus 52*0.1, or 9.4 times as much tape as disk, may be needed. This saves about 80% of tape media cost and, since the weekly backups are much smaller, there may be savings on drives as well. 
     Embodiments of a backup mechanism for multi-class file systems may enable an optimized restores of data to the multi-class file systems. To do a restore using the above example, the image backup of the active 10% may be restored to the first storage class. Then, the incremental backups (which may be file based) may be applied. At this point, all the active data has been restored. The application(s) that access the data may then be started and may access the active data while the second and third storage classes are being restored. In effect, operations have been restarted after restoring only about 10% of the data. In one embodiment, to handle read requests for data on the second or third storage classes, either the volume manager or the file system software may block the request until the data has been restored. In one embodiment, a snapshot mechanism of a volume manager (e.g., Veritas&#39; Volume Manager) may be used to do this and, since the restore is image based, the restore mechanism may skip ahead to restore requested blocks at the expense of slowing down the completion of the restore slightly. 
     One embodiment may provide the ability to perform device-oriented or device-level backup rather than strictly file-level backups. Data, even within files, that is not being written may be segregated from actively written data. An image backup of the devices within the read-only storage classes may be performed periodically, e.g. every one, three, or six months. More active data on storage devices within the writeable storage classes may be backed up more often, e.g. every month. The most active data on storage devices within a highest storage class may be backed up even more often. Data is segregated by storage class, so in one embodiment image backups of storage devices in different storage classes may be performed. Image backups tend to be faster than file-based or other backups, and only data that is needed for the backups of the particular storage classes may be backed up in the image backups. 
     In one embodiment of the backup mechanism, incremental backups may back up just the data blocks or records within a file that have been changed, and not the entire file. Unmodified portions of a file remain on devices within the read-only storage classes, while modified portions of the file are stored on devices within writeable storage classes. The backup mechanism may perform incremental backups of just the writeable storage classes, and thus may only back up the portions of the file that have been modified and not the entire file. 
     In one embodiment, the storage classes within the multi-device file system may themselves be layered, and the backup/restore mechanism may be applied within a storage class. For example, a read-only storage class may be broken into levels of actively accessed read-only data and non- or rarely-accessed read-only data, the latter of which may be compressed. Each of these levels may be stored on different subclasses of storage within the more general read-only storage class. The actively accessed read-only data may be backed up more frequently than the non- or rarely-accessed data. This segmentation may also be applied to other classes of storage, e.g. a writeable storage class may be segmented into two or more subclasses based on criteria such as frequency of access. Note that other criteria than frequency of access may be applied within a storage class to create subclasses or divisions within the storage class. As an example, importance of the data may be used as a criterion; some data within a storage class may be deemed more important or critical than other data, and thus may be segmented from the less important data (e.g. by storing the different data on different storage devices within the storage class) and a backup schedule may be geared towards selectively backing up the qualitatively different data based on the criteria used to segment the data. 
     Note that embodiments may perform individual file backups and restores in addition to image backups and restores. For serious or widespread damage, the entire multi-class file system may be restored. 
       FIG. 12  is a flowchart of a method for performing backups of a multi-class file system according to one embodiment. As indicated at  400 , data may be assigned and migrated in a multi-class file system including a plurality of storage classes. The multi-class file system may be implemented on one or more storage devices, e.g. by File System functionality of the file system software. In one embodiment, a multi-device feature of the file system software (e.g. Veritas&#39; VxFS) may be used to implement the multi-class file system. Different mechanisms for implementing the underlying hierarchy of the multi-class file system were previously described. In one embodiment, each storage class includes one or more storage devices assigned to the storage class according to one or more characteristics of the storage class, e.g. performance characteristics. In one embodiment, the storage classes may be ordered in a hierarchy according to performance characteristics from a highest storage class including one or more high-performance storage devices to a lowest storage class comprising one or more low-performance storage devices. 
     Recently accessed files may be assigned and/or migrated to higher storage classes and less recently accessed files may be assigned and/or migrated to lower storage classes in the hierarchy by a multi-class storage mechanism according to a set of user-defined policies. In one embodiment, one or more of the higher storage classes may be designated as writeable storage classes for storing data that have been recently modified, and one or more of the lower storage classes may be designated as read-only storage classes for storing “aged” data that have been less recently modified. In one embodiment, file system metadata may be modified to indicate assignment or migration of the files. In one embodiment, the file system software may migrate portions of a file to different storage classes according to the user-defined policies. For example, actively written records of a database file may be stored in a higher storage class of the multi-class file system, with inactive database records migrated to a lower or lowest storage class. In one embodiment, files migrated to lower storage classes may be compressed. 
     As indicated at  402 , the file system software may provide access to the data in the multi-class file system to one or more applications. The applications may write new data to one or more of the storage classes, access data on the writeable storage class(es) for read or write operations, and access data on the read-only storage classes for read operations. If an application needs to access data currently stored on a read-only storage class for a write operation, the data are first migrated to a writeable storage class where the application&#39;s write operations may be performed on the data. 
     As indicated at  404 , at a given time interval (e.g. once a month), data that has not been modified for a given time interval (e.g. one month) may be migrated from a first, writeable, storage class to a second, read-only, storage class of the multi-class file system. In one embodiment, to migrate the data to the second storage class, a write lock of the second storage class may be disabled during the migration and re-enabled after the migration. The data on the second storage class is not modifiable by the application(s) while on the second storage class. To modify data currently stored on the second storage class, the data are first migrated to the first storage class. In one embodiment, data migrated to the second storage class may be compressed. In one embodiment, a backup of the second storage class may be performed after the migration of aged data from the first storage class to the second storage class. 
     The multi-class file system may include a third, read-only storage class. At a given time interval (e.g. once a quarter), data that has not been modified for a given time interval (e.g. three months) may be migrated from the storage class to the third storage class. In one embodiment, to migrate the data to the third storage class, a write lock of the third storage class may be disabled during the migration and re-enabled after the migration. The data on the third storage class is not modifiable by the application(s) while the data is on the third storage class. To modify data currently stored on the third storage class, the data are first migrated to the first storage class. In one embodiment, data migrated to the third storage class may be compressed. In one embodiment, a backup of the third storage class may be performed after the migration of aged data from the second storage class to the third storage class. 
     Note that migrated data remains in the same file system and is not moved offline. Migration is performed by modifying file system metadata, and is transparent to the applications that access the multi-class file system. Path information in the file system metadata is not modified during migration. 
     Since data is not modifiable by the applications while on a read-only storage class, the read-only storage class remains stable between migrations of aged data to the read-only storage classes. When a portion of the data on a read-only storage class is deleted, the data blocks are not erased or overwritten until the next migration of aged data to the read-only storage class. Deleting data on a read-only storage class modifies the file system metadata to indicate the deletion and does not erase or overwrite the data blocks containing the deleted data. Thus, if a portion of the data on a read-only storage class is deleted (e.g. if an application deletes a file, at least part of which is on the read-only storage class), the portion of the data may later be restored, if desired, by modifying the file system metadata to point to the data blocks on the read-only storage class containing the portion of the data without requiring that the portion of the data be restored from a backup. 
     As indicated at  406 , backups of each of the storage classes may be performed at given time intervals using a backup mechanism. In one embodiment, the backups of the read-only storage classes may be performed after migrating aged data to the read-only storage classes. In one embodiment, the backups may be image-based backups of the storage devices in the storage classes. In one embodiment, lower storage classes are backed up less frequently than higher storage classes. For example, in a multi-class file system with three storage classes, a first, writeable storage class may be backed up once a week, a second, read-only storage class once a month (e.g. after migration of aged data from the first storage class), and a third storage class once every three months (e.g. after migration of aged data from the second storage class). The time intervals between the backups may be user-specified according to business requirements. Using this method of backing up a multi-class file system may result in reduced requirements for backup media (e.g. tapes) and potentially backup devices (e.g. tape drives). 
     In one embodiment, one or more of the lower storage classes may be write-locked, and the backups of the write-locked lower storage classes may be performed without using split mirrors of the storage devices in the write-locked lower storage classes. 
     In one embodiment, one or more incremental backups of the higher storage classes including recently modified data may be performed between full backups of the higher storage classes. For example, a full backup of a first, writeable storage class may be performed weekly, with incremental backups performed nightly. In one embodiment, recently modified portions of files may be stored on writeable storage classes while less recently modified portions of the files are stored on or migrated to on read-only storage classes. Incremental backups of the writeable storage classes may thus back up only the recently modified portions of the files, saving backup media space and backup time. 
     Note that the multi-class file system may include one or more writeable storage classes and one or more read-only storage classes. Alternatively, all the storage classes may be writeable, or all the storage classes may be read-only. Progressively less recently modified (or read-accessed, for read-only storage classes) data may be migrated down in the hierarchy of storage classes, at progressively longer time intervals, from a highest storage class to a lowest storage class. Backups of the storage classes may be performed progressively less frequently from the highest storage class to the lowest storage class. Accessed data may be migrated up in the hierarchy of storage classes; for modification, data on read-only storage classes is first migrated to a writeable storage class. 
     Restore Mechanism for Multiple Storage Classes 
     Embodiments of the multi-class storage mechanism and the multi-class file system may be used in combination with a restore mechanism to reduce the restore window. One embodiment may enable a partial restore of the data in a multi-class file system to be performed, and an application or applications to be started before all of the data has been restored. 
       FIGS. 12-14  illustrate means for performing restores of the storage classes in a hierarchy of storage classes of a multi-class file system and means for granting an application access to more actively accessed data on the higher storage classes before completion of the restore of less actively accessed data on the lower storage classes.  FIG. 13  illustrates a restore mechanism for an exemplary multi-class file system on a host system in a network storage environment according to one embodiment. Host system  370  may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, workstation, network computer, or other suitable device. Host system  470  may include at least one processor  472 . The processor  472  may be coupled to a memory  474 . Memory  474  is representative of various types of possible memory media, also referred to as “computer readable media.” Hard disk storage, floppy disk storage, removable disk storage, flash memory and random access memory (RAM) are examples of memory media. The terms “memory” and “memory medium” may include an installation medium, e.g., a CD-ROM or floppy disk, a computer system memory such as DRAM, SRAM, EDO RAM, SDRAM, DDR SDRAM, Rambus RAM, etc., or a non-volatile memory such as a magnetic media, e.g., a hard drive or optical storage. The memory medium may include other types of memory as well, or combinations thereof. Host system  470  may couple, for example over a wired or wireless network or networks, to one or more other devices, e.g. client systems, storage devices and backup devices, via one or more wired or wireless network interfaces. The network may, for example, be a Storage Area Network (SAN), a LAN with Network-Attached Storage (NAS), or any network capable of coupling devices to a host system  470 . The storage devices may include any of one or more types of storage devices including, but not limited to, storage systems such as RAID (Redundant Array of Independent Disks) systems, disk arrays, JBODs (Just a Bunch Of Disks, used to refer to disk cabinets that do not have a built-in RAID controller), tape devices, and optical storage devices. 
     Host system  470  may include, in memory  474 , file system software  452 . File system software  452  may include, but is not limited to, an embodiment of the multi-class storage mechanism and software that provides “traditional” File System functionality. File System functionality may be provided by any of various File System software implementations including, but not limited to, Veritas&#39; VxFS software or NFS software. In one embodiment, the software that provides the File System functionality may be used to implement the multi-class file system  480  including a hierarchy of storage classes  482 , while the multi-class storage mechanism of the file system software manages the policies and directs assignment and migration of data within the multi-class file system  480  according to the user-defined policies. 
     Host system  470  may also include, in memory  474 , a restore mechanism  450  that may interact with file system software  452  and with the multi-class file system to perform restores to the storage classes  482  of multi-class file system  480 . A backup mechanism may periodically perform full backups of each of the storage classes. In one embodiment, the full backups may be image-based backups of the storage devices in the storage classes. In one embodiment, the lower storage classes are backed up less frequently than the higher storage classes. 
     In the multi-class file system  480 , more actively accessed data may be assigned and/or migrated to higher storage classes and less actively accessed data may be assigned and/or migrated to lower storage classes. In a restore, the higher storage class(es) including more actively accessed data may be restored by restore mechanism  450  first. The multi-class file system  480  may then be brought online for access by application(s). The lower storage class(es) may then be restored. In one embodiment, if a request for data that have not been restored is received, the file system software  452  may obtain an estimated time until restore of the requested data from the restore mechanism  450 . The file system software  452  may then notify the application of the estimated time until restore. The application may notify a user of the estimated time, and may generate another request for the data after the estimated time has elapsed. 
     Note that, while this example shows restore mechanism  450  and file system software  452  in memory  474  on the same host system  470 , restore mechanism  450  and file system software  452  may reside on different systems. For example, restore mechanism  450  may reside on a different host system coupled to host system  470 . 
       FIGS. 14A and 14B  illustrate a restore mechanism for an exemplary multi-class file system with three storage classes, such as the system previously described in  FIG. 10 , according to one embodiment. In this example, file system software  452  a may include multi-class storage mechanism as previously described. File System functionality of file system software  452  may, as previously described, implement a multi-class file system  480  on one or more storage devices; in this example there are three storage classes. Restore mechanism  450  may interact with file system software  452  and with the multi-class file system  480  to perform restores of the storage classes (three storage classes, in this example) according to the following description. Note that a similar mechanism may be applied in multi-class file systems with different numbers of storage classes. Also note that, while the term “file” is used in this example, the description also applies to embodiments using ranges of blocks or extents. 
     In this example, a first class of data storage (first storage class  454 ) may be mirrored on the highest speed disks and contains all the files actively being written. The second storage class  456  contains all files that have not been written in the last month, and the third storage class  458  contains all files that have not been written in the last quarter. The second and third storage classes may include less expensive storage devices than the first storage class  454 . 
     The first storage class  454  and the metadata may have a full backup  460  performed, for example, weekly, along with a set of incremental backups  462  (which may be file-based) performed, for example, on a daily basis. The second storage class  456  may have a full backup  464  performed, for example, once a month, and the third storage class  458  may have a full backup performed, for example, once a quarter. In one embodiment, the full backups of all storage classes may be image-based, which is typically faster than file-based backup. To restore the multi-class file system, as illustrated in  FIG. 14A , the last full backup  460  of the first storage class  454  may be restored, and then the incremental backups of the first storage class  462 , which may be file-based, may be applied. As illustrated in  FIG. 14B , the multi-class file system may be brought back online and made available to application(s) (e.g. application  470 ) after the full and incremental backups of the first storage class  454  are restored. Restore mechanism may then proceed with the restore of second storage class  456  and third storage class  458 . As described below, file system software  452  and restore mechanism  450  may provide one or more mechanisms to handle requests to access data that has not yet been restored. 
     A multi-class storage mechanism may group files or portions of files into storage classes of the multi-class file system based on their activity. In one embodiment, files with recent write activity may be placed or migrated to one storage class. The multi-class file system may also have a separate storage class for files that are heavily read accessed. Alternatively, read- and write-accessed files may be segmented within a storage class, e.g. each on separate storage devices within a storage class. Less active or inactive files may be placed or migrated to lower storage classes in the multi-class file system. If the multi-class file system needs to be restored, the restore mechanism may restore the active storage class(es) first along with all the file system metadata. Then the multi-class file system may be made available to one or more applications while the less active storage classes are restored. This preferably reduces the restore window as most of the data does not have to be restored before making the multi-class file system accessible to applications. 
     Embodiments may allow the user to prioritize which storage classes are restored first. In one embodiment, active storage classes (storage classes that include actively accessed writeable or read-only data) may be restored first. There may be further prioritization within the active storage classes. For example, there may be one active storage class for metadata, and one or more other active storage classes for file data. In one embodiment, storage classes with read-only data may be restored first or early in the restore process if necessary or desired. A user may define a hierarchy for importance of the data to be restored based on business needs. One characteristic of data that may be considered is if the data is being written or not. Another characteristic of data that may be considered is if the data is being accessed (not necessarily written; this could be read-only access) or not. Another characteristic is metadata. All of these storage classes may be backed up and/or restored in user-defined priorities. In one embodiment, the file system metadata is backed up and/or restored first, then the most important accessed data (written or read-only). At some point, the application(s) may be started that access the data, and other, less active or inactive data may be restored using one or more of the various mechanisms described herein to handle requests for data that has not been restored. 
     As an example of using an embodiment of the restore mechanism with backups of a multi-class file system, an image-based restore of the most active data and the metadata may be performed. A full backup and potentially one or more incremental backups may be restored to accomplish this. At this point, the multi-class file system may be remounted (all the metadata has been restored), even though the less active data on lower storage classes may not have been restored. One or more applications may then be started. 
     When applications are allowed to begin accessing the multi-class file system before all the storage classes have been restored, there may be no problems as long as the application(s) are accessing active data that has already been restored. However, when a file lookup is performed, the file system software may discover that the file or part of the file is stored on a set of blocks that have not been restored yet. The metadata has been restored, but not the requested data that the metadata points to. There are several different mechanisms for handling this situation, described below. Note that other mechanisms than those described may be used. 
     Since files that have not been (fully) restored may be accessed, one embodiment may provide a mechanism whereby an attempted access to a file that has not been (fully) restored yet will be blocked until the data has been restored. This allows applications to transparently start using the multi-class file system before the restore is complete. In one embodiment, at the device level, within the file system software itself, or at a layer above the file system software, the request may be blocked until the requested data is restored. In one embodiment, the restore process may be requested to restore the needed blocks next, and thus out-of-order, to achieve faster response time. 
     In one embodiment, assuming an image-based backup, the restore mechanism may understand the file system metadata and modify the file system metadata to flag each file in the lower storage classes that are not restored when access to the multi-class file system is allowed to block access to the not-yet-restored files. In one embodiment, this may be done with a callout such as DMAPI. Alternatively, the file system software, multi-class storage mechanism, or a logical volume manager may be set to delay accesses to blocks on the lower storage classes until they have been restored. 
     One embodiment may provide an interface between the restore process and the file system software and/or logical volume manager that allows a data access requested by an application to be prioritized in the restore process. In one embodiment, the file system software or volume manager may track which blocks have been restored so that accesses to already restored blocks may be allowed to go through without significant delay. 
     In one embodiment, an application may be aware of the partial restore process, and the restore mechanism may return an indicator to the application to wait (e.g. for ten minutes) for requested data that is not yet restored to be restored before trying again. One embodiment may return an indication that requested data is not yet available and an estimate of when the data will be available. The application may then decide that it needs data sooner than the indicated and may request that the restore mechanism restore the data sooner if possible. 
     In one embodiment, the restore mechanism may tell the application to try again later, e.g. in five or ten minutes when the requested data is estimated to be restored. This may require cooperation between the application and the restore mechanism working in conjunction with the multi-class storage mechanism and thus requires the application to be aware of the multi-class storage mechanism and the restore mechanism based on backups of the multi-class file system. 
     In this embodiment, an application that was written with knowledge of the multi-class file system and the multi-class storage mechanism, the request may be rejected and the application may retry the request at a later point in time. In one embodiment, the application may be provided with an estimated time when the data is expected to be restored. In one embodiment, service for one or more critical customers and/or functions of an application may be restored first so that the application can begin operations, but when the restore mechanism receives a request from the application for data that is not yet restored, the file system software may inform the application that the data has not been restored yet, and the application may inform the user that the data is not ready and the user should try again later rather than waiting. 
     An example of this is in a flight reservation system. In conventional systems, if the system goes down, every request may be rejected until the entire system is online. In one embodiment, as each part of the database gets restored, requests for that part of the database may be allowed to be processed even though the entire system has not been restored yet. Online, at the counter, the application may immediately notify the user to try again e.g. in five minutes, rather than waiting for five minutes for a response. Thus, options may include, but are not limited to, having the application blocked when it makes a request for not-yet-restored data, telling the application the data is not yet restored yet ad to try again later; or temporarily blocking the request and requesting the restore mechanism to restore the requested data ASAP. 
     Another example may be a credit card application. In a credit card application, customers that use their cards more often (and, thus, whose records reside on a higher, more active, storage class) may be restored first; customers that do not use their cards as often (and, thus, whose records reside on a lower storage class) may be restored later. If a non-frequent customer happens to try to use their card during a restore, the application may tell them to try again in, for example, ten minutes rather than causing them to wait for a response for ten or so minutes. Alternatively, the customer&#39;s record(s) may be restored out-of-order to grant the customer immediate usage of the card. 
     Thus, embodiments may allow the restore of most frequently used (writeable or read-only) data first, and then an application or applications to be restarted before all of the data in the multi-class file system is fully restored. The restore mechanism may then continue restoring other data (whether read-only or writeable) to the multi-class file system. Requests for data that has not yet been restored may be handled using one or more of the techniques described herein. 
       FIG. 15  is a flowchart of a method for performing restores of a multi-class file system according to one embodiment. As indicated at  500 , a multi-class file system including a hierarchy of storage classes may be implemented on one or more storage devices. In one embodiment, a multi-device feature of file system software (e.g. Veritas&#39; VxFS) may be used to implement the multi-class file system. Different architectures for the underlying structure of the multi-class file system were previously described. In one embodiment, each storage class includes one or more storage devices assigned to the storage class according to one or more characteristics of the storage class, e.g. performance characteristics. In one embodiment, the storage classes may be ordered in the hierarchy of storage classes according to performance characteristics from a highest storage class including one or more high-performance storage devices to a lowest storage class comprising one or more low-performance storage devices. 
     More actively accessed files may be assigned and/or migrated to higher storage classes and less actively accessed files may be assigned and/or migrated to lower storage classes in the hierarchy by a multi-class storage mechanism according to a set of user-defined policies. In one embodiment, file system metadata may be modified to indicate assignment or migration of the files. In one embodiment, the multi-class storage mechanism may migrate portions of a file to different storage classes according to the user-defined policies. In one embodiment, files migrated to a lowest storage class may be compressed. 
     A backup of the storage classes in the hierarchy of storage classes may be restored. The backup may have been previously generated using a method such as that described in  FIG. 12 . The backup may include full backups of each of the storage classes and incremental backups of one or more higher storage classes. In one embodiment, the full backups may be image backups of storage device(s) in the storage class. As indicated at  502 , file system metadata and the higher storage class or classes including the more actively accessed data may be restored first. Restoring a higher storage class may include restoring the full backup of the storage class and then applying one or more incremental backups of the higher storage class. 
     The multi-class file system may then be brought online. As indicated at  504 , an application may be granted access to the restored more actively accessed data on the higher storage classes after completion of the restore of the file system metadata and the higher storage classes. 
     As indicated at  506 , the lower storage class(es) including the less actively accessed data may then be restored. In one embodiment, access to the less actively accessed data may be blocked until the less actively accessed data are restored. In one embodiment, the file system metadata for the less actively accessed data may be modified to block access to the less actively accessed data. If an application tries to access data that has not been restored, the access may be delayed until the requested data are restored. In one embodiment, file system software may track the restore of blocks of the less actively accessed data and may grant the application(s) access to the blocks of the less actively accessed data as they are restored. 
     In one embodiment, if the file system software receives an access request to one or more blocks of the less actively accessed data that have not been restored, the file system software may request that the restore mechanism restore the requested one or more blocks out-of-order to reduce the delay for the application to access the data. 
     In one embodiment, if the file system software receives an access request to one or more blocks of the less actively accessed data that have not been restored, the file system software may obtain an estimated time until restore of the requested one or more blocks from the restore mechanism. The file system software may then notify the requesting application of the estimated time until restore of the requested blocks. In one embodiment, the application may notify a user of the estimated time until restore of the requested data. The application may then generate another access request for the requested data after the estimated time has elapsed. 
     Conclusion 
     Various embodiments may further include receiving, sending or storing instructions and/or data implemented in accordance with the foregoing description upon a carrier medium. Generally speaking, a carrier medium may include storage media or memory media such as magnetic or optical media, e.g., disk or CD-ROM, volatile or non-volatile media such as RAM (e.g. SDRAM, DDR, RDRAM, SRAM, etc.), ROM, etc. As well as transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as network and/or a wireless link. 
     The various methods as illustrated in the Figures and described herein represent exemplary embodiments of methods. The methods may be implemented in software, hardware, or a combination thereof. The order of method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. 
     Various modifications and changes may be made as would be obvious to a person skilled in the art having the benefit of this disclosure. It is intended that the invention embrace all such modifications and changes and, accordingly, the above description to be regarded in an illustrative rather than a restrictive sense.