Abstract:
A shared storage distributed file system is presented that provides users and applications with transparent access to shared data stored on network attached storage devices by utilizing layering techniques to inherit file management functionality from existing file systems. The present invention stores meta-data for the shared data as real-data in a standard, non-modified, client-server distributed file system, such as NFS. In effect, the standard client-server file system acts as a meta-data server. The name space consisting of inode files stored as real-data on the meta-data server acts as the name space for the shared data. Similarly, file attributes of the inode files are utilized as the file attributes of the shared data. By utilizing an existing client-server system as the meta-data server, development time and complexity are greatly reduced, while speed advances in the underlying client-server system may be incorporated without alteration of the present invention. A method for communicating with network attached storage devices over layered file systems is also presented.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. patent application Ser. No. 10/738,371, filed Dec. 16, 2003, which is a continuation application of U.S. patent application Ser. No. 09/045,340 filed Mar. 20, 1998, which is now U.S. Pat. No. 6,697,846. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates general to computer file systems. More specifically, the present invention involves a distributed file system based on two technologies: shared storage and file system layering. 
       BACKGROUND OF THE INVENTION 
       [0003]    File Systems The term “file system” refers to the system designed to provide computer applications with access to data stored on storage devices in a logical, coherent way. File systems generally hide the details of how data is stored on a storage device from the application program. For instance, data on a storage device is generally block accessible, in that data is addressed with the smallest granularity of a block; with multiple blocks forming an extent. The size of the particular block depends upon the actual device involved. Application programs generally request data from file systems byte by byte. Consequently, file systems are responsible for seamlessly mapping between application program memory space and the storage device address space. 
         [0004]    Application programs store and retrieve data from files as contiguous, randomly accessible segments of bytes. Users are responsible for organizing data stored in these files, since file systems are generally not concerned with the content of each file. With a byte-addressable address space, users may read and write data at any offset within a file. Users can grow files by writing data to the end of a file. The size of the file increases by the amount of data written. Conversely, users can truncate files by reducing the file size to a particular length. 
         [0005]    To maximize storage efficiency, file systems place “holes” in areas within files that contain no data. Holes act as space holders between allocated sections of user data. File systems must manage holes, though no data is allocated to the holes until users write data to the location. When a user reads from a hole, the file system fills the user buffer with zeros. 
         [0006]    A hole can either occupy space within an allocated block or occupy space of entire blocks. File systems manage block aligned holes in a manner similar to real-data blocks, yet no blocks are allocated. File systems manage holes internal to allocated blocks simply by zeroing the space of the hole. 
         [0007]    In addition, file systems are generally responsible for maintaining a disk cache. Caching is a technique to speed up data requests from application programs by saving frequently accessed data in solid-state memory for quick recall by the file system without having to physically retrieve the data from the storage device. Caching is also useful during file writes; file system may write user data to cache memory and complete the request before the data is actually written disk storage. 
         [0008]    Additionally, file systems maintain information indicating which data blocks are available to be allocated to files. File systems modify these free lists during file allocation and de-allocation. Most modern file systems manage free lists by means of bitmap tables. File systems set bits to signify blocks that are allocated to files. 
         [0009]    File systems present data to application programs as files—contiguous, randomly accessible segments of bytes. These files, called regular files, are presented to application programs through directory files which form a tree-like hierarchy of files and subdirectories containing more files. The complete directory structure is called the file system name space. Link files are a third type of file used to provide multiple file names per physical file. 
         [0010]    File systems are required to map this application level interface to the often non-contiguous data blocks stored on the storage device. Generally, information required to map a particular file or directory to the physical locations of the storage device is stored by the file system in an inode within a data block. Inodes contain information, called attributes, about a particular file, such as file type, ownership information, access permissions and times, and file size. Inodes also contain a list of pointers which address data blocks. These pointers may address single data blocks or address an extent of several consecutive blocks. The addressed data blocks contain either actual data or a list of other pointers. With the information specified by these pointers, the contents of a file can be read or written by an application program. When an application program writes to a file, data blocks may be allocated by the file system. Such allocation modifies the inode. 
         [0011]    The terms meta-data and real-data classify file system structure data and user data, respectively. In other words, real-data is data that users store in regular files. Other terms for real-data include user data and file data. File systems create meta-data to store layout information, such as inodes and free block bitmap tables. Meta-data is not directly visible to users. Meta-data requires a fraction of the amount of storage space that real-data occupies and has significant locality of reference. As a result, meta-data caching drastically influences file system performance. 
         [0012]    Meta-data consistency is to vital file system integrity. Corruption of meta-data may result in the complete destruction of the file system. Corruption of real-data may have bad consequences to users but will not effect the integrity of the whole file system. 
         [0013]    Distributed Files Systems 
         [0014]    File systems can generally be divided into two separate types. Local file systems allow computers to access files and data stored on locally attached storage devices. While local files systems have advanced significantly over the years, such file systems have limited usefulness when data needs to be shared between multiple computers. Distributed files systems have been developed in order to make shared data available to multiple computer systems over a computer network. Distributed file systems provide users and applications with transparent access to files and data from any computer connected to the file system. Distributed file system performance cannot equal local file system performance due to resource sharing and lack of data locality. 
         [0015]    Traditional distributed file systems are based on client-server architectures. Server computers store shared data on locally attached storage devices, called server-attached devices. Clients send file system requests to server computers via networks. Early distributed file systems, such as Sun Microsystems Network File System (NFS), use a central server to store real and meta-data for the file system. These central servers locally maintain meta-data and transport only real-data to clients. The central server design is simple yet efficient, since all meta-data remains local to the server. Like local file systems, central servers only need to manage meta-data consistency between main memory and storage devices. In fact, central server distributed file systems often use local file systems to manage and store meta-data for the file system. In this regard, the only job of the central server file system is to transport real-data between client and server. 
         [0016]    As the need grew for greater parallelism and enhanced availability, distributed file system designs evolved from central servers to multiple server configurations. As with central servers, multiple servers, also known as distributed servers, store all file system data on devices connected to server computers. Since multiple servers cooperatively manage the file system, servers may share meta-data between computers. The complexity of these designs increases an order of magnitude, since distributed system integrity requires strong meta-data consistency between servers. Such systems cannot use local file systems to store data. As a result, server software must manage, store, and transport meta-data between servers. Two examples of distributed server file systems are the Andrew File System from Carnegie Mellon University and the Sprite File System from the University of California at Berkeley. 
         [0017]    Distributed server file systems have further evolved into designs where clients and servers are often difficult to distinguish. In these systems, clients manage, store, and transport real-data and meta-data between servers and other clients. Coda from Carnegie Mellon University and the xFS File System from the University of California at Berkeley are two examples of merged client-server designs. 
         [0018]    One aspect of client-server file system designs that has remained unchanged among central server, distributed server, and merged client-server designs is the local attachment of storage devices to computers. Unfortunately, this architecture has performance and availability weaknesses. With devices attached to computers, a computer failure renders data stored on the storage device inaccessible. Although redundant devices on separate computers can be added to improve availability, such a technique adds complexity and cost to the system. 
         [0019]    Furthermore, the architecture limits performance when clients access data stored on remote devices. The data-path between client and storage device includes a server computer. This server adds overheads caused by server workload and overheads relating to storage device interface to network interface protocol translations. Server computers designed to support large workloads are very expensive. 
         [0020]    Shared Storage Distributed Files Systems 
         [0021]    Distributed file system designs that use shared storage, or shared disk, technologies have followed a slightly different evolution path. Instead of storing data on storage devices connected locally to computers, shared storage designs store data on devices shared between client computers. Shared storage systems have a short data-path between clients and devices. 
         [0022]    These distributed system require arbitration for the storage devices and consistency management of any data cached on the clients. Consistency mechanisms are either centrally located or distributed within the system. The consistency mechanisms may include software running on computers, hardware mechanisms attached to the networks, or a combination of both. 
         [0023]    Two distinct file system designs utilize shared storage technology. The first case uses private file managers, in which client computers independently access meta-data and real-data directly from the storage devices. Private file manager schemes do not require dedicated file servers, since all necessary data is taken directly from the shared storage devices. With private file manager designs, each client views storage as locally attached. Clients only service local file requests. No direct communication is needed between clients. Such systems are often derived from modified local file systems. Examples of such systems include the Cray Research Shared File System, the Digital VAXcluster, and the Global File System from the University of Minnesota. 
         [0024]    As a result of their designs, clients utilizing private file manages remain independent from the failures and bottlenecks of other clients. Similarly, client resources such as memory, CPUs, and bus bandwidth are not spent servicing requests from other clients. However, private file manager designs do have several disadvantages. First, the designs can only support a primitive form of caching. Clients may only access data cached locally in memory or stored on the shared devices; data cached in the memory of other clients is not accessible. The second disadvantage deals with complications encountered during recovery. Since clients are not aware of other clients, clients must indirectly determine data corruption caused by other client failures. 
         [0025]    The second type of shared storage distributed file system design utilizes file manager server computers. These file servers manage file system directory structures and meta-data on non-shared storage devices. Clients make requests to the servers, the servers determine the location of real-data on shared devices by calling and examining meta-data from the non-shared storage device. Once the location is determined, the servers either initiate transfers between clients and storage devices or inform clients how to invoke the transfer. Servers must maintain and store meta-data, manage real-data, and control transfers between clients and storage devices. These shared storage designs suffer from many of the same difficulties as client-server architectures based upon server-attached disks. The server design is complex, since servers need to provide a great deal of functionality. Servers that fail or become overworked tend to disrupt file system operation. Since this form of distributed file system differs considerably from other shared storage designs, these designs can be classified as shared file manager, shared storage systems. The HPSS/SIOF project at Livermore National Laboratories is an example that uses a shared file manager to facilitate transfers between storage servers and clients. 
         [0026]    I/O Interfaces 
         [0027]    I/O interfaces transport data between computers and devices as well as among computers. Traditionally, interfaces fall into two categories: channels and networks. Computers generally communicate with storage devices via channel interfaces. Channels predictably transfer data with low-latency and high-bandwidth performance; however, channels span short distances and provide low connectivity. High-performance requirements often dictate that hardware mechanisms control channel operations. 
         [0028]    Computers communicate with other computers through networks. Networks are interfaces with more flexibility than channels. Software controls substantial network operations, providing networks with flexibility but low performance. 
         [0029]    Recent interface trends combine channel and network technologies into single interfaces capable of supporting multiple protocols. For instance, Fibre Channel (FC) is an emerging ANSI serial interface that supports channel and network operations. Fibre Channel supports traditional network protocols like Transmission Control Protocol/Internet Protocol (TCP/IP); Fibre Channel supports traditional channel protocols such as Small Computer System Interface (SCSI-3). Combined interfaces allow shared storage file systems to have high connectivity, connect long distances, and operating in unpredictable environments. A new term for I/O interfaces that support shared storage is storage area network (SAN). Shared storage devices that connect to SANs are also referred to as network attached storage (NAS) devices. The term NAS device refers to extent addressable storage systems connected to a network. 
         [0030]    File System Layering 
         [0031]    File system designers can construct complete file systems by layering, or stacking, partial designs on top of existing systems. The new designs reuse existing services by inheriting functionality of lower levels. For instance, NFS is a central-server architecture that utilizes an existing local file system to store and retrieve data on a storage device attached locally to the server. By layering NFS on top of local file systems, NFS software is free from the complexities of name space, file attribute, and storage management. NFS software consists of simple caching and transport functions. As a result, NFS benefits from performance and recovery improvements made to local file systems. 
         [0032]    Other examples of file system layering include adding quota support to existing file system, strengthening consistency of cached data in an existing distributed file system, and a file system layer that compresses or encrypts files for a file system without such support. 
         [0033]    Installable File System Interfaces 
         [0034]    Most modern operating systems include installable file system interfaces to support multiple file system types within a single computer. In UNIX, the Virtual File System (VFS) interface is an object-oriented interface that supports various file system types within a single operating system. VFS occupies the level between the user/system call interface and installed file systems. Each installed file system provides the UNIX kernel with functions associated with VFS and vnode operations. VFS functions operate on whole file systems and perform tasks such as mounting, unmounting, and reading status. Vnode operations manipulate individual files. Vnode operations include opening, closing, creating, removing, reading, writing, and renaming files. 
         [0035]    Vnode structures are the objects upon which vnode functions operate. A vnode is the VFS virtual equivalent of an inode. VFS creates and passes vnodes to file system vnode functions. Each vnode includes a pointer, called v_data, for file systems to attach private structures such as inodes. 
         [0036]    While several UNIX implementations incorporate VFS, the interfaces differ slightly between platforms. Several non-UNIX operating systems, such as Microsoft Windows NT, have interfaces similar to VFS. Installable file system interfaces such as VFS allow multiple file system types within an operating system. Each system is capable of making calls to other file systems though the virtual file system interface. For instance, an NFS server may be implemented to access a local file system through VFS. In this manner, the server software does not need to be specifically coded for the local file system type; new file systems may be added to an operating system without reconfiguring NFS. 
       SUMMARY OF THE INVENTION 
       [0037]    The present invention is a shared storage distributed file system that provides users and applications with transparent access to shared data stored on network attached storage devices. The file system uses layering techniques to inherit file management functionality from existing systems. Meta-data in the present invention is stored and shared among multiple computers by storing the meta-data as real-data in regular files of a standard, non-modified, client-server distributed file system. In effect, the standard client-server file system serves as the meta-data file system (MFS) for the present invention. 
         [0038]    Real-data is stored on network attached storage devices attached to a storage area network. SFS benefits from direct network device attachment, since NAS devices off-load time-consuming data transfers from server computers. Furthermore, client computers operating under the present invention store file system meta-data on a meta-data file system. Using this meta-data, clients manage real-data stored on the network attached storage devices. The meta-data file systems also maintain the present file system name space and file attributes. 
         [0039]    By utilizing an existing client-server system as a meta-data file system, the present invention is able to utilize the small-file access speed, consistency, caching, and file locking that is built into modern client-server file systems. Not only is development work reduced, but implementation is also simplified. Furthermore, future advances in client-server architectures are able to be incorporated easily and quickly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a representational drawing of a network environment utilizing a file system of the present invention. 
           [0041]      FIG. 2  is a representational drawing of the network environment of  FIG. 1 , showing additional details of the client element. 
           [0042]      FIG. 3  is a flow chart showing the basic structure of meta-data file consistency management of the present invention. 
           [0043]      FIG. 4  is a representational drawing of a directory structure that the present invention stores in the name space of the meta-data file system. 
           [0044]      FIG. 5  is a representational drawing of an inode file data layout of the present invention. 
           [0045]      FIG. 6  is a flow chart showing the basic structure of the file creation process of the present invention. 
           [0046]      FIG. 7  is a flow chart showing the basic structure of reading and writing file data of the present invention. 
           [0047]      FIG. 8  is a flow chart showing the basic structure of the file removal process of the present invention. 
           [0048]      FIG. 9  is a flow chart showing the basic structure of retrieving an inode file of the present invention. 
           [0049]      FIG. 10  is a flow chart showing the basic structure of updating an inode of the present invention. 
           [0050]      FIG. 11  is a flow chart showing the basic structure of storage block allocation of the present invention. 
           [0051]      FIG. 12  is a flow chart showing the basic structure of file truncation of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0052]    The present invention is a distributed file system that provides users and applications with transparent access to shared data found on storage devices attached directly to a network. This access is provided by utilizing existing, non-modified, client-server distributed file systems for meta-data storage. The client-server file system also provides file attribute and name space management. For purposes of this application, the present invention will be referred to as the Shared File System, or SFS. 
         [0053]    Referring to  FIG. 1 , a network environment  100  is shown that utilizes a file system of the present invention. In the figure, network environment  100  has both a local area network (LAN)  102  and a storage area network (SAN)  104 . The storage area network  104  is represented as a subset of the local area network  102  to illustrate that SAN  104  often exists merely as part of the LAN  102 . For instance, Fibre Channel is an interface standard that can simultaneously support both local area network  102  and storage area network  104  traffic. However, it is conceivable and within the scope of this invention for the SAN  104  to be separate from the LAN  102 , utilizing different interface protocols and different physical links than LAN  102 . Example interfaces that could be used by SAN  104  include Fibre Channel, High Performance Parallel Interface (HiPPI), Intelligent Peripheral Interface (IPI-2) and Small Computer System Interconnect version 2 (SCSI-2). These SAN interfaces may utilize different protocols including SCSI-3 and IPI-3. Interfaces suitable for LAN  102  are Ethernet, Fibre Channel, and Asynchronous Transfer Mode (ATM). Examples of LAN protocols are Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) on Internet Protocol (IP). 
         [0054]    Attached to the LAN  102  are SFS clients  106  and a meta-data file system (MFS) server  108 . The MFS server  108  has direct access to a local storage device  112 . Attached to the SAN  104  are SFS clients  106  and network attached storage (NAS) devices  110 . For the purposes of this invention, NAS devices  110  are considered to include all extent addressable storage systems connected to a network. Example NAS devices  110  include single disk drives, striped disks, disk arrays, solid-state storage devices, tape drives, tape robots, and even computers with locally-attached disks running special software that make disk storage extent addressable. These devices  110  communicate with the SFS clients  106  through SAN  104 . 
         [0055]    SFS is currently implemented in the Silicon Graphics IRIX 6.2 operating system under the Virtual File System (VFS) interface. SFS use the Sun Microsystems Network File System (NFS) as the meta-data file system. SFS stores real-data on Fibre Channel network attached SCSI devices. Fibre Channel may be used to communicate between MFS clients and servers, though an Ethernet LAN suffices. While these implementation details specify an existing, preferred embodiment, alternatives to one or all of these protocols would be obvious to one skilled in the art and are within the scope of the present invention. For instance, it would be obvious to one skilled in the art to utilize a Microsoft Server Message Block (SMB) based distributed file system or the Distributed File System (DFS) (a Distributed Computing Environment, or DCE, application based on the Andrew File System) as the meta-data file system. 
         [0056]    Under the present invention, the MFS server  108  is actually operating an existing, prior art distributed file system, such as NFS. The meta-data requested by client  106  is like any other regular file for which the MFS server  108  is responsible. For instance, an SFS inode file that contains the block addresses of the real-data stored on the NAS devices  110  is simply a regular real-data file for the MFS server  108 . SFS client software operating on clients  106  is responsible for storing the SFS meta-data in MFS regular files. Because of the structure of the SFS system, clients  106  are able to use the MFS directory structure and file attributes with very little modification. 
         [0057]    The utilization of an unaltered distributed file system as a meta-data file system allows SFS to maintain meta-data consistency using the distributed file systems built-in file locking mechanisms. In addition, since most distributed file systems have good small file performance and failure recovery processes, such systems are ideal candidates for handling meta-data. The availability of the MFS to the network environment  100  can be enhanced using redundant servers  108 , and devices  112 , which is well known in prior art distributed file systems. Finally, this structure for handling meta-data files eliminates much of the complex and tedious tasks of directory and file attribute manipulation, caching, and consistency that are inherent in other techniques for shared storage file systems. 
         [0058]    Multiple File Systems 
         [0059]    In order for the SFS system to utilize an existing client-server file system as the MFS  108 , it is necessary for the client-server system to be operating and accessible to the SFS software running on client  106 . One method of accomplishing this is to implement SFS on a system allowing for multiple file systems to coexist. In the preferred embodiment, SFS is implement through the use of the UNIX Virtual File System interface (VFS).  FIG. 2  illustrates the utilization of the VFS interface  122  on SFS client  106 . User application  120  is a program running in user space on client  106 . When the application  120  needs to store or retrieve file data stored on an SFS file system, the application  120  makes the request to the operating system. The request is received by the kernel-level, Virtual File System (VFS) interface  122 , which routes the request to SFS software  124 . 
         [0060]    In order to access file data on NAS device  110 , SFS software  124  must receive the inode file (meta-data) for the file from the meta-data file system  132 , which is comprised of the MFS client  126  and the MFS server  108 . To obtain this meta-data, SFS software  124  makes a request for the file containing the meta-data through VFS  122 . The VFS  122  routes this request to the MFS client (NFS client) software  126 . The MFS client software  126  forwards the request to the MFS server  108  through network device drivers  128  and the local area network  102 . The MFS server  108  then retrieves the requested file from local storage  122 , and returns the file to the requesting MFS client software  126 . MFS server  108  may also be utilizing the Virtual File System interface to coordinate communication between the request from the MFS client  126  and a local file system controlling data on local storage  112 . 
         [0061]    The meta-data file received by MFS client software  126  is then returned to SFS software  124  via the VFS interface  122 . When the SFS software  124  receives the inode file, SFS software  124  then transfers the real-data through the NAS device drivers  130  of the client  106 . These drivers  130  access one or more of the NAS devices  110  connected to the storage area network  104 . The NAS device drivers  130  may consist of multiple layers of drivers. One layer may control single NAS devices  110  and a higher layer may group single devices into logical NAS devices  110 . 
         [0062]    As seen in  FIG. 2 , the meta-data path of the present invention is considerably longer than the real-data path. However, meta-data may be cached on the SFS client  106  or in the main memory of the MFS server  108  through standard caching and consistency mechanisms built into the MFS client-server architecture. The SFS software  124  may cache real-data in SFS client  106  main memory, though SFS software  124  may bypass caching for large requests with no locality. SFS software  124  maintains real-data consistency by comparing the time data is cached with the modification time of the inode file retrieved from MFS server  108 . If these times differ, the cached data is invalid. 
         [0063]    In functioning in this manner, the application program  120 , the VFS interface  122 , the MFS client software  126  and MFS server  108 , the device drivers  128 ,  130 , and the storage devices  110 ,  112  all operate without alteration from the previously known prior art. In other words, the present invention allows the implementation of a shared storage distributed files system merely by operating SFS software  124  on SFS clients  106 . 
         [0064]    Meta-Data Consistency 
         [0065]    SFS clients  106  manage meta-data file consistency using file locking mechanisms built into MFS  132 . Both read-locks and write-locks can be placed on files in MFS  132 . MFS  132  grants read-locks when a file is not locked or is locked with other read-locks; MFS  132  grants write-locks when a file is not already locked.  FIG. 3  illustrates how SFS software  124  utilizes MFS  132  file locking mechanisms when accessing meta-data files. At step  140 , SFS  124  decides whether the meta-data is to be modified. If SFS  124  intends to modify the meta-data, SFS  124  submits to MFS  132  a write-lock request for the meta-data file at step  150 . If SFS  124  intends to access without modifying the meta-data, SFS  124  submits to MFS  132  a read-lock request for the meta-data file at step  142 . SFS  124  must wait for the lock requested in steps  142  or  150  to be granted by MFS  132  before proceeding to steps  144  or  152 . By requiring write-lock on an SFS meta-data file before the file can be modified, it is impossible to modify a meta-data file that is currently being utilized by another client  106 . Similarly, the requirement of obtaining a read-lock before use prevents the use of a meta-data file that is currently being modified by a different client. 
         [0066]    At steps  144  and  152 , SFS  124  submits to MFS  132  read requests for the meta-data file. At step  146 , SFS  124  uses the meta-data without modification. After step  146 , the process continues to step  148 . On the modify branch at step  154 , SFS  124  modifies the meta-data. This branch continues to step  156 , where SFS  124  submits to MFS  132  write requests of the modified meta-data file. The process ends at step  148  where SFS  124  unlocks the meta-data file. 
         [0067]    SFS Meta-Data 
         [0068]    SFS  124  stores meta-data files in an MFS  132  directory structure shown in  FIG. 4 . These files are not directly accessible to user applications  120 , but rather are utilized by the SFS software  124  to store information about the NAS devices  110  and the real-data file stored on these devices  110 . This information includes system wide information, such as superfile  164 , the remove directory  166 , and segment files  170 ; as well as inode files  180  which contain information on the various files in the file system. Most of the meta-data is contained in the superfile  164 , the segment files  170 , and the inode files  180 . Table 1 lists the specific contents of these three main types of meta-data files. The remove directory  166  is used only for temporary storage of files prior to deletion. 
         [0069]    SFS  124  utilizes the name space and directory structure of the inode files  180  on MFS  132  to maintain the name space and directory structures for the real-data files stored on NAS devices  110 . By allowing application programs  120  to view the file locations and directory structure of the inode files  180  on MFS, there is no need for SFS to maintain a separate file structure. SFS software  124  also does not need to manage directory and link files. SFS  124  forwards, without modification, directory and link file requests between the user application  120  and MFS  132 . 
         [0070]    The circled areas  160  in  FIG. 4  enclose user visible files and directories and thereby show the name space for SFS  124 . In this Figure, inode files f 1  through f 6  ( 180 ) in subdirectory tree are SFS inode files stored as MFS  132  regular files. Directories d 1  and d 2  ( 172 ) are standard MFS  132  directories. The SFS file system is mounted on the directory called root  162 . The name space subdirectory tree  160  rooted at root/mount is also visible to users. For instance, users access file f 4  with the path root/d 2 /f 4 . SFS  124  translates the user path into root/mount/d 2 /f 4  in order to access the f 4  inode file stored on MFS  132 .  FIG. 4  also shows that the other meta-data files such as the superfile  164 , the remove directory  166 , and the segments directory  168  are not visible to user applications  120 . 
         [0071]    Superfile 
         [0072]    As was shown in  FIG. 1 , each network environment  100  utilizing the SFS file system consists of one or more shared NAS devices  110  attached to the SAN  104 . Several NAS storage devices  110  may form a logical volume to comprise a single, logical device. SFS  124  stores information about logical NAS devices  110  in a meta-data file called a superfile  164 . This device information includes the device name, number, and capacity. 
         [0073]    The superfile  164  also contains the file system block size. This block size is a multiple of the least common multiple of all client computers  106  page sizes. Suppose some clients  106  have 4096 byte page size and other have 16384 byte page sizes. The file system block size needs to be at least 16384 bytes but could be 32768 bytes or 65536 bytes. The choice of block size is a tradeoff between performance and storage efficiency. Larger block sizes require less meta-data transfer and reduce external fragmentation, but reduce storage efficiency since partially filled, large blocks waste more space than partially filled, small blocks. 
         [0074]    Segment Files 
         [0075]    SFS  124  partitions logical NAS devices  110  into multiple segments in order to exploit parallelism in the network environment  100 . Segmentation allows multiple processes to simultaneously allocate and de-allocate file data. Each segment contains multiple data blocks found on NAS device  110 , and has its own allocation table associated with these data blocks. Allocation tables store information about currently unused data blocks that are available to be allocated to file. These allocation tables are implemented via bitmap tables, as is well-known in the prior art. Each of the segment files  170  contains the allocation table associated with that segment. SFS software  124  retrieves and modifies the appropriate segment files  170 , designated by segment file number, during file allocation, file de-allocation, and file system statistic gathering operations. 
         [0076]    Inode File 
         [0077]      FIG. 5  illustrates an inode file  180 . Each inode file  180  maintains information pertaining to a single SFS  124  regular file stored on an NAS device  110 . Since MFS  132  treats inode files  180  as real-data, MFS  132  maintains file attributes for each file, such as file name, ownership, access privileges, access, creation, and modification times, and file size. SFS uses these inode file attributes as the attributes for the associated SFS file. In this manner, SFS  124  is freed from the overhead of maintaining file attributes. File attribute requests received from user application  120  can be forwarded to MFS  132  with little overhead. Responses from MFS  132  can similarly be forwarded back to the inquiring user application  120 . 
         [0078]    Each inode file  180  contains a list of extents that address data blocks storing file real-data. To minimize meta-data space, each extent  184  may address several consecutive device data blocks. To indicate all the necessary addressing information, each extent  184  includes a flag, the segment number of the segment containing the real-data, the block offset into the segment, and the number of blocks within the extent that contain real-data. The flag determines whether or not the extent addresses real-data or a hole in the file. 
         [0079]    Some file attributes are not maintained directly by MFS  132 . As a result, each inode file  180  also contains a fixed-size header  182  for such attributes and any additional information not maintained by MFS  132 , such as the number of extents in the inode. 
         [0080]    SFS  124  cannot determine file size based upon the amount of normal inode data, since a file&#39;s meta-data is typically only a small fraction of the size of the file&#39;s real-data. Rather than store the real file size in the inode header, SFS  124  appends a single byte, the last-byte  188 , to the inode file  180  beyond the end of the list of extents  184 . This last-byte  188  is positioned at an offset that creates an empty space or hole  186  in the inode file  180 . This hole  186  requires almost no storage space on MFS  132 , yet increases the file size of the inode file  180  by the length of the hole  186 . SFS  124  can then translate the inode file size  190  to the real file size  192  by subtracting a constant length from the inode file size  190  maintained by MFS. 
         [0081]    In the preferred embodiment, SFS  124  calculates the file size  192  by subtracting the size of the header  182 , one extent  184 , and one byte from the inode file size  190 . For instance, an empty file has a file size  192  of zero. The inode file  180  of this empty file has a length equal to the size of the header  182 , one extent  186 , and the last-byte  188 . 
         [0082]    SFS  124  supports user level record locking by placing MFS  132  record locks on inode files  180 . SFS  124  forwards user lock requests to MFS  132  with the slight modification to the requested lock record, in that SFS  124  increments the byte offset of the lock record by the size of the inode file header  182 . Since inode file sizes  190 , including holes  186 , are always larger than the real file size  192 , adjusted lock record offsets fall within the real file size  192  range. Other clients  106  requesting record locks at the same file locations will receive a notice, originating from MFS  132  and forwarded by SFS  124 , that the requested records are already locks. 
         [0083]    SFS and the VFS Interface 
         [0084]    As shown in  FIG. 2 , SFS  124  is accessible through the VFS interface  122 . User application  120  requests route through the VFS interface  122  to SFS software  124 . Furthermore, SFS  124  makes meta-data requests through VFS  122  to the meta-data file system client  126 . SFS  124  maintains MFS client  126  vnodes for directories, links, and meta-data files. 
         [0085]    SFS  124  maintains several structures during the course of file operations, including an SFS vnode, an in-core SFS inode, and an MFS vnode. For each open file, the kernel retains pointers to the corresponding SFS vnode. The VFS interface  122  passes this pointer to all SFS vnode routines. 
         [0086]    SFS  124  maintains an in-core inode for each regular file. This inode contains a copy of the inode file header  182 , and some or all of the file extents. The SFS inode also includes a pointer to the MFS vnode of the inode file  180 . SFS  124  routines pass this MFS vnode pointer to MFS  126  vnode routines. Using MFS file locks, SFS  124  maintains consistency between the in-core inode and the inode file  180 . 
         [0087]    SFS software  124  maintains similar structures for directories and links. Like regular files, directory and link structures include an SFS vnode, an SFS inode, and an MFS vnode. Since SFS  124  redirects directories and links requests to MFS  132 , SFS  124  maintains no extent lists. MFS  132  manages directories and links without SFS  124  intervention. 
         [0088]    Basic File System Operations 
         [0089]    The basic file system operations are creating a file, writing to a file, reading from a file, and removing a file. These operations require other operations such as reading and writing inode files as well as allocating and de-allocating files. 
         [0090]    File Creation 
         [0091]    A file creation operation of the present invention is illustrated in the flow chart shown in  FIG. 6 . The process starts by an application program  120  running on one of the SFS clients  106  desiring to create a new file. The application program  120  submits a create request to the SFS software  124 , as shown in step  200 . At step  202 , the SFS software  124  receives this request, and then summits a request to MFS  132  for a name space  160  search for the file name given by the application program  120 . If, at step  204 , MFS  132  indicates that the file already exists, the operation concludes. However, if the file does not already exist, SFS  124  submits a request to MFS  132  to create the file. At step  208 , MFS  132  creates a regular file for SFS  124 . At step  210 , SFS  124  writes an inode file  180  to this newly created regular file. Since no data has been allocated to the SFS file, the inode file  180  contains no valid extents  184 . The process of writing an inode file to MFS  132  is described in more detail below in connection with  FIG. 10 . The file creation process then completes. 
         [0092]    File Reads and Writes 
         [0093]    Read and write operations are illustrated in the flow chart shown in  FIG. 7 . The process starts by application program  120  desiring to transfer program data between user memory and a file. The application  120  submits either a read or a write request to SFS  124 , as shown in step  220 . At step  222 , the SFS software  124  receives this request, and in turn submits to MFS  132  a lock request for the inode file  180  corresponding to the real-data file. MFS  132  grants the lock when it becomes available. After the inode file is locked, SFS  124  reads the inode file  180  from MFS  132 , as shown in step  224 . Reading an inode file is shown in more detail in  FIG. 9 . 
         [0094]    The request made at step  224  is now seen to be simple requests for regular file data from the client-server file system operating as the MFS  132 . In step  226 , MFS  132  retrieves the requested file from local storage  112  or MFS cache, and MFS  132  delivers the file to client  106 . SFS  124  receives the meta-data for the requested file from MFS  132 , and in step  228  determines how to map the application  120  request to NAS devices  110  disk blocks. 
         [0095]    If the application program  120  submits a read request in step  220 , as determined at step  230 , SFS  124  retrieves data blocks from devices  110  and delivers real-data to the application program  120  at step  232 . Once the step  232  transfer is complete, SFS  124  submits a request to MFS  132  to unlock the inode file  180  at step  242 . 
         [0096]    If the application program  120  submits a write request in step  220 , SFS  124  must decide at step  234  whether additional data blocks stored on NAS devices  110  need to be allocated. If SFS  124  determines that no new data needs to be allocated to the SFS file, SFS  124  at step  240  writes the application  120  data to the devices  110 . At step  242 , SFS  124  completes the operation by submitting a request to MFS  132  to unlock the inode file  180 . 
         [0097]    If, at step  234 , SFS  124  determines data must be allocated to the SFS file, SFS  124  must read and alter one or more segment files  170  stored on MFS  132  at step  236 . This step is shown in more detail in  FIG. 11 . At step  238 , SFS  124  then updates the inode file  180  and saves it to MFS  132 . This latter step is further explained in connection with  FIG. 10 . The process continues to step  240  as explained above. 
         [0098]    File Removal 
         [0099]    A file removal operation of the present invention is illustrated in the flow chart shown in  FIG. 8 . The process starts by an application program  120  desiring to remove an existing file. The application program  120  submits a remove request to the file system of the present invention, as shown in step  250 . 
         [0100]    Removing a file in a file system like NFS requires several state transitions. Given a failure between any state, the file system may become inconsistent. To compensate, SFS  124  modifies the name space  160  of MFS  132  before removing a file. At step  252 , SFS  124  renames the inode file  180  corresponding to the SFS regular file marked for removal. This inode file  180  is moved to the SFS  124  remove directory  166  and renamed to a unique name allocated by the file system. In the preferred embodiment, SFS  124  uses the MFS  132  inode number of the file to create a unique name. At step  254 , SFS  124  truncates the file to zero bytes in length. This truncation de-allocates the file data blocks. SFS  124  then removes the inode file  180  from the remove directory  166  in step  256  by issuing a remove command to MFS  132 . Data block de-allocation is further explained in connection with  FIG. 12 . 
         [0101]    Inode File Read 
         [0102]    SFS software  124  periodically needs to read inode files  180  from MFS  132 . The process is illustrated in the flow chart shown in  FIG. 9 . Starting at step  260 , SFS  124  reads the inode file  180  from MFS  132  as a regular file. Since the inode file  180  may contain many extents and occupy thousands of bytes of data, SFS  124  reads only a fixed size buffer of data at one time. MFS  132  transfers a portion of this file to the SFS buffer memory. At step  262 , SFS  124  unpacks the inode header  182  from the inode file  180  into a memory structure. At step  264 , SFS software  124  verifies the consistency of the inode magic number. If this magic number is invalid, the process terminates with an error. If the magic number is valid, SFS  124  assumes the inode header  182  is valid. At step  266 , SFS  124  checks the number of extents field of the header  182 . If this field indicates that there are zero extents in the inode extent list  184 , the process terminates successfully. 
         [0103]    If the inode header  182  indicates the inode contains extents, the process continues to step  268 . At step  268 , SFS  124  unpacks all extents in the current buffer into SFS client  106  memory. At step  270 , SFS  124  verifies the consistency of each extent by checking for valid extent flags. If any extent is invalid, the process terminates with an error. If all extents in the buffer are valid, at step  272 , SFS  124  determines if the inode file  180  contains extents not yet read. When copies of all extents are in SFS client  106  memory, the process completes successfully. If more extents need to be read, SFS  124  reads another buffer from MFS  132  and returns to step  268 . 
         [0104]    Inode File Write 
         [0105]    SFS software  124  periodically needs to write inode files  180  to MFS  132 . The process is illustrated in the flow chart shown in  FIG. 10 . Starting at step  280 , SFS  124  determines if the inode file  180  can be transferred with one write request. If the inode file size  190  is less than or equal to the write buffer size, the process proceeds to step  282 . At step  282 , SFS  124  packs the inode header  182 , extent list  184 , and last-byte  188  into the write buffer. SFS  124  then writes this buffer to MFS  132  as file real-data. The process completes successfully after step  284 . 
         [0106]    If at step  280  the inode file size  190  is greater than the write buffer, the process continues to step  286 . SFS  124  proceeds to request that MFS  132  truncate the inode file  180  to zero bytes in length. At step  288 , SFS  124  writes the last-byte  188  to MFS  132  at the inode file size  190  offset. Then, SFS  124  packs the buffer with the inode header  182 . At step  292 , the buffer is not full, so SFS  124  packs the remaining buffer space with inode extents. Once the buffer is full, SFS  124  writes the buffer to MFS  132  as file real-data. At step  296 , if more extents need to be written, the process returns to step  292 . Once the entire extent list  184  is written, the process completes. 
         [0107]    Block Allocation 
         [0108]    During a file write, SFS software  124  may need to allocate storage from the network attached storage devices  110 . The process is illustrated in the flow chart shown in  FIG. 11 . Starting at step  300 , SFS  124  chooses the segment from which to allocate data blocks. This selection may be random or follow a more advanced heuristic. The goal of the section is to balance system level accesses across all segments as well as attempt to allocate sequential storage blocks for the file. Once a segment is chosen, the process continues to step  302 . At step  302 , SFS  124  requests a write-lock of the segment file  170  from MFS  132 . SFS  124  proceeds to read the segment data from this MFS  132  file. At step  304 , SFS  124  searches through the segment allocation table for free blocks. SFS  124  allocates blocks until the allocation request is satisfied or all segment blocks are allocated. At step  306 , SFS  124  proceeds to write and unlock the modified segment file  170  stored on MFS  132 . If the allocation request is complete, the process ends successfully. If the allocation request is not complete, SFS  124  attempts to select a different segment. If such a segment exists, the process returns to step  300  and selects this segment for allocation. If SFS  124  has exhausted all segments and requires additional blocks, the process terminates with an “out of space” error. 
         [0109]    File Truncation 
         [0110]    File truncation occurs during file removal or a truncation request from an application program  120 . During truncation, SFS  124  modifies the inode file size  190  and possibly de-allocates storage of network attached storage device  110  blocks. The process is illustrated in the flow chart shown in  FIG. 12 . Starting at step  320 , SFS  124  locks the inode file  180  by acquiring a write-lock of the file from MFS  132 . Once the lock is granted by MFS  132 , SFS  124  reads the inode file  180 . At step  322 , SFS  124  appropriately modifies the inode extent list  184 . In the process, SFS  124  builds a memory list of extents to free. At step  324 , SFS  124  writes the modified inode file  180  back to MFS  132  and then unlocks the inode file. The process proceeds to step  326 . From the list of extents to free, SFS  124  selects a segment to begin de-allocation. At step  328 , SFS  124  locks and reads the segment file  170  from MFS  132 . At step  330 , SFS  124  frees all blocks in free list corresponding to the current segment. SFS  124  then writes the modified segment file  170  to MFS  132  and unlocks the segment file  170 . If the extent list contains additional extents to free, the process returns to step  326 . Once all extents are freed, the process terminates successfully. 
         [0111]    Failure Recovery 
         [0112]    Failure recovery is a vital element of distributed systems. Recovery must be timely, and damage caused by corrupted meta-data must be limited to single files rather than entire file systems. File system layering provides the present invention with a great deal of protection. By layering SFS  124  on top of MFS  132 , MFS  132  manages name space, file locking, and meta-data file recovery. 
         [0113]    MFS  132 , however, does not facilitate consistency between meta-data update operations. Without atomic meta-data updates, a failure while modifying meta-data may leave an SFS file system in an inconsistent state. To compensate, SFS  124  sequences meta-data update operations in an order that contains such inconsistencies to single files or permits repairable inconsistencies. 
         [0114]    For instance, during file allocation, SFS  124  updates segment file  170  allocation tables before updating inode files  180 . If a failure occurs before the inode  180  is updated, the segment file  170  becomes inconsistent with the remainder of the file system. The allocation table reflects data blocks as allocated, though no inode  180  points to these blocks. 
         [0115]    During file de-allocation, SFS  124  updates inode files  180  before modifying segment files  170 . If a failure occurs before the segment files  170  are updated, the allocation tables indicate blocks as erroneously allocated. These inconsistencies are temporary, since SFS utilities can dynamically repair segment files  170 . 
         [0116]    Segment inconsistencies do not affect normal file system operation, with the temporary exception that less storage space is available. SFS utilities provide a recovery tool to repair this inconsistency. The tool reads through all inode files  180  and creates a list of blocks that are allocated to files. Using this list, the tool reconstructs consistent segment files  170  in order to restore data blocks for file allocation. This utility can be run while the file system is on-line. 
       Alternative Embodiments 
       [0117]    Several known alternative embodiments to the file system of the present invention exist that may improve the performance of the file system in one or more areas. Some improvements break the separation of meta-data and real-data. Benefits of these improvements may vary according to MFS server  108  performance, network performance  102 , and workload. 
         [0118]    Stuffed Inodes 
         [0119]    SFS  124  is optimized for large file performance. Every file access requires SFS  124  to read the file inode  180  from MFS  132  and access real-data on the shared storage devices  110 . For small files, this overhead time may be longer than the time needed to access a similar sized file directly from the MFS server  108 . Furthermore, each SFS  124  file requires a minimum fixed storage overhead. For instance, a one byte file may require a minimum of four kilobytes of data on the MFS server  108  as well as four kilobytes on the shared storage devices  110 . This internal fragmentation is significant for small files. 
         [0120]    To reduce small file access times and internal fragmentation, SFS  124  could stuff inode files  180  with real-data. Stuffed inodes store real-data on the MFS server  108 . Transparent to the users, SFS  124  would forward stuffed file requests to MFS  132  similar to directory and link files. SFS  124  would convert files that grow beyond the stuffing threshold into normal, shared-storage files. SFS  124  also would stuff non-stuffed files during file truncation. SFS  124  could determine whether an inode file  180  is stuffed with real-data by examining a flag in the header  182  or by examining the file size  190  of the inode file  180 . 
         [0121]    Segment Bitmaps on Network Attached Storage Devices 
         [0122]    Segment files  170  contain segment header information as well allocation tables. During file allocation and de-allocation, SFS  124  modify the segment headers and allocation tables. For large file systems, segment files  170  may be hundreds of kilobytes. MFS  132  manages and transports segment files  170  to SFS  124 . With high file system activity, the MFS server  108  may be overloaded. 
         [0123]    To reduce server  108  workloads, SFS  124  could store segment headers information on MFS  132  and segment allocation tables on the network attached storage devices  110 . With this optimization, the NAS devices  110  host meta-data as well as real-data. Segment meta-data could be distributed evenly across several devices  110 . 
         [0124]    Inode File Extents on Network Attached Storage Devices 
         [0125]    Large, highly fragmented files may have relatively large inode files  180 . To reduce server  108  load and distribute extent list accesses, SFS  124  could store inode file extents  184  on the NAS devices  110 . SFS  124  would read inode file headers  182  from MFS  132 . SFS  124  would then use extents in this header  182  to address shared storage device  110  data blocks that contain direct extents. These direct extents address real-data. 
         [0126]    This optimization could have benefits for large files with many extents, since MFS  132  would maintain and transport less data; however, the additional indirection requires extra data transfers. Instead, a combination of both approaches could satisfy extreme cases. A flag within the inode file header  182  could indicate whether direct extents are stored on the MFS server  108  or on the NAS devices  110 . 
         [0127]    Meta-Data Consistency Manager 
         [0128]    Meta-data consistency is extremely important. SFS  124  uses file locks managed by the MFS server  108  to preserve consistency. Although centralized servers can easily manage file locks and recovery, this centralized mechanism eventually becomes a bottleneck. SFS  124  may benefit from a distributed lock manager running on clients  106  or storage devices  110 . Such a distributed lock manager could utilize techniques known in the prior art. 
         [0129]    Store File Size in Inode Header 
         [0130]    The SFS  124  implementation described above fills inode files  180  with holes to extend the inode size  190  to be larger than the file the inode  180  represents. This approach is based upon the assumption that SFS  124  can access MFS  132  file attributes quicker than reading file data. If this assumption ceases, storing the file size in the inode header  182  may improve file performance. 
         [0131]    The invention is not to be taken as limited to all of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Meta-Data File 
                 Contents 
               
               
                   
               
             
             
               
                 Superfile 
                 File system block size 
               
               
                   
                 Device name 
               
               
                   
                 Device capacity 
               
               
                   
                 Number of segments 
               
               
                 Segment File 
                 Segment header 
               
               
                   
                 Number of blocks in segment 
               
               
                   
                 Number of unallocated blocks in segment 
               
               
                   
                 Bitmaps table of with one bit assigned to each block 
               
               
                   
                 in segment 
               
               
                 Inode File 
                 Inode header 
               
               
                   
                 Magic number to verify header integrity 
               
               
                   
                 Number of extents in inode file 
               
               
                   
                 Extent list where each extent contains 
               
               
                   
                 Flag: 0-invalid extent, 1-valid data, 2-hole 
               
               
                   
                 Segment number of extent 
               
               
                   
                 Block offset into segment 
               
               
                   
                 Length in blocks of extent 
               
               
                   
                 Inode hole-size of hole based on file size 
               
               
                   
                 Last-byte