Patent Publication Number: US-7225211-B1

Title: Multi-class storage mechanism

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 that file system software maintains on files stored in the file system. File system metadata may include, but is not limited to, definitions and descriptions of the data it references. File system metadata may include one or more of, but is not limited to, inodes, directories, mapping information in the form of indirect blocks, superblocks, etc. 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). 
   SUMMARY 
   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. 
   In one embodiment, file system software may include, but is not limited to, an embodiment of the multi-class storage mechanism and software that provides “traditional” File System functionality. In one embodiment, the software that provides the File System functionality may be used to implement the multi-class file system, while the multi-class storage mechanism manages the policies and performs assignment and migration of data within the multi-class file system according to the policies. The multi-class file system 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 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. The multi-class file system is made available to application(s) as one file system. The storage classes of the multi-class file system are transparent to the application(s). 
   The multi-class storage mechanism may store files and/or portions of files in storage classes of the multi-class file system according to user-defined policies. The multi-class storage mechanism may make an automatic and transparent determination on where to initially place a file (i.e., in which storage class) according to the policies. The multi-class storage mechanism may also migrate files and/or portions of files between storage classes in the multi-class file system according to the policies. Business-related attributes of the data such as importance, value, or ownership may be considered in decisions to assign and/or migrate the data. 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) that access the multi-class file system. 
   In one embodiment, file system metadata may be used to track the storage and migration of data in the multi-class file system. When a file or portion(s) of a file is stored in or migrate 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. 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. 
   In one embodiment, a storage class may be added to a multi-class file system and data may be automatically and transparently moved to the new storage class. For example, one or more new, faster storage devices may be added to the multi-class file system as a new, highest class of storage. In one embodiment, storage classes may be grown. For example, 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. 

   
     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 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. 
   

   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 0–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. 
   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.