Abstract:
A shared storage distributed file system is presented that provides applications with transparent access to a storage area network (SAN) attached storage device. This is accomplished by providing clients read access to the devices over the SAN and by requiring most write activity to be serialized through a network attached storage (NAS) server. Both the clients and the NAS server are connected to the SAN-attached device over the SAN. Direct read access to the SAN attached device is provided through a local file system on the client. Write access is provided through a remote file system on the client that utilizes the NAS server. A supplemental read path is provided through the NAS server for those circumstances where the local file system is unable to provide valid data reads. 
     Consistency is maintained by comparing modification times in the local and remote file systems. Since writes occur over the remote file systems, the consistency mechanism is capable of flushing data caches in the remote file system, and invalidating metadata and real-data caches in the local file system. It is possible to utilize unmodified local and remote file systems in the present invention, by layering over the local and remote file systems a new file system. This new file system need only be installed at each client, allowing the NAS server file systems to operate unmodified. Alternatively, the new file system can be combined with the local file system.

Description:
FIELD OF THE INVENTION 
   The present invention relates general to computer file systems. More specifically, the present invention involves a distributed file system that transfers data using both network attached storage (NAS) and storage area network (SAN) protocols. 
   BACKGROUND OF THE INVENTION 
   File Systems 
   The term “file system” refers to the system designed to provide computer application programs with access to data stored on storage devices in a logical, coherent way. File systems hide the details of how data is stored on storage devices from application programs. For instance, storage devices are generally block addressable, in that data is addressed with the smallest granularity of one block; multiple, contiguous blocks form an extent. The size of the particular block, typically 512 bytes in length, depends upon the actual devices involved. Application programs generally request data from file systems byte by byte. Consequently, file systems are responsible for seamlessly mapping between application program address-space and storage device address-space. 
   File systems store volumes of data on storage devices. The term “volume” refers to the collection of data blocks for one complete file system instance. These storage devices may be partitions of single physical devices or logical collections of several physical devices. Computers may have access to multiple file system volumes stored on one or more storage devices. 
   File systems maintain several different types of files, including regular files and directory files. Application programs store and retrieve data from regular files as contiguous, randomly accessible segments of bytes. With a byte-addressable address-space, applications may read and write data at any byte offset within a file. Applications 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, applications can truncate files by reducing the file size to any particular length. Applications are solely responsible for organizing data stored within regular files, since file systems are not aware of the content of each regular file. 
   Files are presented to application programs through directory files that form a tree-like hierarchy of files and subdirectories containing more files. Filenames are unique to directories but not to file system volumes. Application programs identify files by pathnames comprised of the filename and the names of all encompassing directories. The complete directory structure is called the file system namespace. For each file, file systems maintain attributes such as ownership information, access privileges, access times, and modification times. 
   File systems often utilize the services of operating system memory caches known as buffer caches and page caches. These caches generally consist of system memory buffers stored in volatile, solid-state memory of the computer. Caching is a technique to speed up data requests from application programs by saving frequently accessed data in memory for quick recall by the file system without having to physically retrieve the data from the storage devices. Caching is also useful during file writes; the file system may write data to the memory cache and return control to the application before the data is actually written to non-volatile storage. Eventually, the cached data is written to the storage devices. 
   The state of the cache depends upon the consistency between the cache and the storage devices. A cache is “clean” when its contents are exactly the same as the data stored on the underlying storage devices. A cache is “dirty” when its data is newer than the data stored on storage devices; a cache becomes dirty when the file system has written to the cache, but the data has not yet been written to the storage devices. A cache is “stale” when its contents are older than data stored on the storage devices; a cache becomes stale when it has not been updated to reflect changes to the data stored on the storage devices. 
   In order to maintain consistency between the caches and the storage devices, file systems perform “flush” and “invalidate” operations on cached data. A flush operation writes dirty cached data to the storage devices before returning control to the caller. An invalidation operation removes stale data from the cache without invoking calls to the storage devices. File systems may flush or invalidate caches for specific byte-ranges of the cached files. 
   Many file systems utilize data structures called inodes to store information specific to each file. Copies of these data structures are maintained in memory and within the storage devices. Inodes contain attribute information such as file type, ownership information, access permissions, access times, modification times, and file size. Inodes also contain lists of pointers that 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 stored by the application programs or lists of pointers to other data blocks. With the information specified by these pointers, the contents of a file can be read or written by application programs. When an application programs write to files, data blocks may be allocated by the file system. Such allocation modifies the inodes. 
   Additionally, file systems maintain information, called “allocation tables”, that indicate which data blocks are assigned to files and which are available for allocation to files. File systems modify these allocation tables during file allocation and de-allocation. Most modem file systems store allocation tables within the file system volume as bitmap fields. File systems set bits to signify blocks that are presently allocated to files and clear bits to signify blocks available for future allocation 
   The terms real-data and metadata classify application program data and file system structure data, respectively. In other words, real-data is data that application programs store in regular files. Conversely, file systems create metadata to store volume layout information, such as inodes, pointer blocks, and allocation tables. Metadata is not directly visible to applications. Metadata requires a fraction of the amount of storage space that real-data occupies and has significant locality of reference. As a result, metadata caching drastically influences file system performance. 
   Metadata consistency is vital to file system integrity. Corruption of metadata may result in the complete destruction of the file system volume. Corruption of real-data may have bad consequences to users but will not affect the integrity of the whole volume. 
   I/O Interfaces 
   I/O interfaces transport data among computers and storage devices. 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 typically span short distances and provide low connectivity. Performance requirements often dictate that hardware mechanisms control channel operations. The Small Computer System Interface (SCSI) is a common channel interfaces. Storage devices that are connected directly to computers are known as direct-attached storage (DAS) devices. 
   Computers communicate with other computers through networks. Networks are interfaces with more flexibility than channels. Software mechanisms control substantial network operations, providing networks with flexibility but large latencies and low bandwidth performance. Local area networks (LAN) connect computers medium distances, such as within buildings, whereas wide area networks (WAN) span long distances, like across campuses or even across the world. LANs normally consist of shared media networks, like Ethernet, while WANs are often point-to-point connections, like Asynchronous Transfer Mode (ATM). Transmission Control Protocol/Internet Protocol (TCP/IP) is a popular network protocol for both LANs and WANs. Because LANs and WANs utilize very similar protocols, for the purpose of this application, the term LAN is used to include both LAN and WAN interfaces. 
   Recent interface trends combine channel and network technologies into single interfaces capable of supporting multiple protocols. For instance, Fibre Channel (FC) is a serial interface that supports network protocols like TCP/IP as well as channel protocols such as SCSI-3. Other technologies, such as iSCSI, map the SCSI storage protocol onto TCP/IP network protocols, thus utilizing LAN infrastructures for storage transfers. 
   The term “storage area network (SAN)” is used to describe network interfaces that support storage protocols. Storage devices connected to SANs are referred to as SAN-attached storage devices. These storage devices are block and object-addressable and may be dedicated devices or general purpose computers serving block and object-level data. 
   Block and object-addressable devices connect to SANs and share storage among multiple computers. Block-address devices are common storage devices that are addressable by fixed length data blocks or sectors. In contrast, object-addressable devices are impending devices that are addressable by an object identifier and an offset into the object. Each object-addressable device may support numerous objects. Two proposed object-addressable devices are the Seagate Object Oriented Device (OOD) and the Carnegie Mellon University Network Attached Secure Disks (NASD). 
   SANs are often dedicated networks specifically designed to transport block data; however, SANs may also operate as subsets of general purpose LANs and share the same physical network connections. Therefore, the type of data moving on the network dictates whether a network is a SAN or a LAN. 
   Local Files Systems 
   Local file systems service file-level requests for application programs only running on the same computer that maintains the non-shared file system volume. To achieve the highest levels of performance, local file systems extensively cache metadata and real-data within operating system buffer caches and page caches. Because local file systems do not share data among multiple computer systems, performance is generally very good. 
   Local file systems traditionally store volumes on DAS devices connected directly to the computer. The weakness of using DAS is that should the computer fail, volumes located on the DAS devices become inaccessible. To reclaim access to these volumes, the DAS devices must be physically detached from the original computer and connected to a backup computer. 
   SAN technologies enable local file system volumes to be stored on SAN-attached devices. These volumes are accessible to several computers; however, at any point in time, each volume is only assigned to one computer. Storing local file system volumes on SAN-attached devices rather than DAS devices has the benefit that the volumes may be easily reassigned to other computers in the event of failures or maintenance. 
   Distributed Files Systems 
   Distributed file systems provide users and application programs with transparent access to files from multiple computers networked together. Distributed file systems lack the high-performance found in local file systems due to resource sharing and lack of data locality. However, the sharing capabilities of distributed file systems often compensate for poor performance. 
   Architectures for distributed file systems fall into two main categories: network attached storage (NAS)-based and storage area network (SAN)-based. NAS-based file sharing, also known as “shared nothing”, places server computers between storage devices and client computers connected via LANs. In contrast, SAN-based file sharing, traditionally known as “shared disk” or “share storage”, uses SANs to directly transfer data between storage devices and networked computers. 
   NAS-based Distributed File Systems 
   NAS-based distributed file systems transfer data between server computers and client computers across LAN connections. The server computers store volumes in units of blocks on DAS devices and present this data to client computers in a file-level format. These NAS servers communicate with NAS clients via NAS protocols. Both read and write data-paths traverse from the clients, across the LAN, to the NAS servers. In turn, the servers read from and write to the DAS devices. NAS servers may be dedicated appliances or general-purpose computers. 
   The Sun Microsystems Network File System (NFS) is a popular NAS protocol that uses central servers and DAS devices to store real-data and metadata for the file system volume. These central servers locally maintain metadata and transport only real-data to clients. The central server design is simple yet efficient, since all metadata remains local to the server. Like local file systems, central servers only need to manage metadata consistency between main memory and DAS devices. In fact, central server distributed file systems often use local file systems to manage and store data for the file system. In this regard, the only job of the central server file system is to transport real-data between clients and servers. 
   Central server designs were the first NAS-based distributed file systems. As the need for greater parallelism and enhanced availability grew, 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 DAS devices connected to server computers. Since multiple servers cooperatively manage the file system, servers may share metadata between computers. The complexity of these designs increases an order of magnitude, since distributed system integrity requires strong metadata consistency between servers. Such systems often cannot use local file systems to store data. As a result, server software must manage, store, and transport metadata and real-data between servers. Two examples of distributed server file systems are the Andrew File System (AFS) from Carnegie Mellon University and the Sprite File System from the University of California at Berkeley. 
   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 metadata and real-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. 
   One aspect of NAS-based file system designs that has remained unchanged among central server, distributed server, and merged client-server designs is the direct attachment of storage devices to computers. With devices directly attached to computers, however, a single computer failure renders data stored on the storage devices inaccessible. Although redundant devices on separate computers can be added to improve availability, such techniques add complexity and cost to the system. 
   Furthermore, the NAS architecture limits performance when clients access data stored on remote devices, because the data-path between client and storage device includes server computers. These servers add overheads caused by server workloads as well as overheads relating to the translations from channel interface protocols to network interface protocols. Server computers designed to support large workloads are very expensive. 
     FIG. 1  illustrates the data-paths and components of a typical, prior art NAS-based file sharing environment  100 . NAS clients  102  are connected to the NAS server  106  via network-based I/O interface links  110  connected to the LAN  104 . The LAN  104  consists of network components such as routers, switches, and hubs. The NAS server  106  connects to DAS devices  108  via channel-based I/O interface links  112 . The DAS devices  108  are block addressable, non-volatile storage devices. These interface links  110  and  112  include one or more physical connections. 
   The NAS read data-path  114  begins at the DAS devices  108  and leads to the NAS server  106 . The read data-path  114  continues through the NAS server  106 , across the LAN  104 , to the NAS clients  102 . Conversely, the NAS write data-path  116  begins at the NAS clients  102  and traverses through the LAN  104  to the NAS server  106 . The NAS server  106 , in turn, writes across the channel interface link  112  to the DAS devices  108 . 
   SAN-based Distributed Files Systems 
   Distributed file system designs that use SAN technologies have followed a different evolutionary path. Instead of storing data on storage devices connected directly to computers, SAN-based designs store data on SAN-attached devices shared among several client computers. SAN-based designs have high-bandwidth, low-latency data-paths between clients and devices. 
   SAN-based file systems 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 hardware and software. 
   There are two distinct SAN-based file system designs. The first design uses private file managers, in which client computers independently access metadata and real-data directly from the storage devices. Private file manager schemes do not require dedicated servers, since all necessary data is taken directly from the SAN-attached devices. With private file manager designs, clients only service local file requests. Examples of such systems include the Cray Research Shared File System, the Digital Equipment Corporation VAXcluster,™ and the Global File System from the University of Minnesota. 
   As a result of their designs, clients utilizing private file managers 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 have several disadvantages. First, the designs can only support a primitive form of caching. Clients may only access data cached locally in memory or data stored on the SAN-attached devices; data cached in the memory of other clients is not accessible. The second disadvantage deals with complications encountered during failure recovery. Since clients are not aware of other clients, clients must indirectly determine data corruption caused by other client failures. 
   The second type of SAN-based distributed file system design utilizes file manager server computers. These file servers manage file system namespace and metadata. SAN clients make requests to the SAN servers, and the servers determine the location of real-data on SAN devices by examining and modifying file metadata. Once the location is determined, the servers either initiate transfers between clients and storage devices or inform the clients how to invoke the transfers. Servers must maintain and store metadata, manage real-data, and control transfers between clients and storage devices. These SAN-based file server designs suffer from many of the same difficulties as NAS architectures. 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. The SANergy file system from Tivoli Systems, the CentraVision File System (CVFS) from Advanced Digital Information Corporation (ADIC), and the Celerra HighRoad multiplex file system (MPFS) from EMC Corporation are examples of SAN-based file systems that utilize SAN server file managers to facilitate file transfers between SAN devices and SAN clients. 
     FIG. 2  illustrates the data-paths and components of a typical, prior art SAN-based file sharing environment  120 . SAN clients  122  are connected to the SAN server  124  via network-based I/O interface links  110  connected to the LAN  104 . The LAN  104  consists of network components such as routers, switches, and hubs. Typically only control and consistency information passes across the LAN  104 . In some SAN-based file system designs, the SAN server  124  and the LAN  104  are unnecessary. In other designs, the SAN-based file system may actually utilize the services of a NAS-based file system to pass control information between the servers  124  and clients  122 . Regardless of the control data-path, SAN clients  122  access all real-data via SAN protocols. 
   The SAN clients  122  and the SAN server  124  connect to the SAN-attached devices  126  via channel-based I/O interface links  130  capable of transferring storage protocols over network connections. As with the LAN links  110 , the channel links  130  include one or more physical connections. The I/O interface  130  links connect to the SAN  128 , which consists of network components such as routers, switches, and hubs. The SAN  128  may also include components that perform storage virtualization, caching, and advanced storage management functions. The SAN-attached devices  126  are typically block addressable, non-volatile storage devices. SAN-attached devices  126  may also support object-addressable interfaces. SAN-attached devices  126  often have multiple ports that connect via channel links  130  to the SAN  128 . 
   The SAN read data-path  132  begins at the SAN devices  126 , passes across the SAN  128 , and leads to the SAN clients  122  and the SAN server  124 . The SAN write data-path  134  begins at the SAN clients  122  and the SAN server  124  and passes through the SAN  128  to the SAN-attached devices  126 . 
   SAN-based File Sharing using Local File Systems 
   Local file systems may be used in SAN file sharing environments  120  under various restrictions. For instance, most local file system volumes may be mounted by multiple SAN clients  122  as long as all clients  122  mount the volume in read-only mode. Since the volume does not change, caching performed by the clients  122  does not affect the state of the SAN environment  120 . When files of the volume need to be modified, however, all clients  122  must unmount the volume and then one client  122  re-mounts the volume in read-write mode. This client  122  makes the appropriate modifications and then unmounts the volume. Finally, all clients  122  re-mount the volume in read-only mode. This scheme promotes high-speed file sharing yet is tremendously restrictive and inefficient with respect to modifying volumes. 
   Some local file systems are specifically designed to support SAN file sharing environments  120  where one SAN client  122  mounts the volume in read-write mode and all other SAN clients  122  mount the volume read-only. These SAN-based local file system must frequently flush dirty caches on the read-write client  122  and regularly invalidate stale caches on the read-only clients  122 . Given that only one computer is capable of modifying the volumes, this solution lacks transparency required by most applications and thus possess limited usefulness. 
   SAN Clients that Serve NAS Clients 
   A SAN-based file sharing environment  120  may be configured to serve a large number of NAS client computers  102  using NAS file sharing protocols. SAN clients  122  act as NAS servers  106  that serve volumes stored on the SAN-attached devices  126  to a large number of NAS clients  102  connected to the NAS servers  106  though LANs  104 . Such systems, also known as clusters, combine SAN and NAS technologies into a two tiered scheme. In effect, a NAS cluster can be viewed as a single, large NAS server  106 . 
   SAN Appliances 
   SAN appliances are prior art systems that consist of a variety of components including storage devices, file servers, and network connections. SAN appliances provide block-level, and possibly file-level, access to data stored and managed by the appliance. Despite the ability to serve both block-level and file-level data, SAN appliances do not possess the needed management mechanisms to actually share data between the SAN and NAS connections. The storage devices are usually partitioned so that a portion of the available storage is available to the SAN  128  and a different portion is available for NAS file sharing. Therefore, for the purpose of this application, SAN appliances are treated as the subsystems they represent. 
     FIG. 3  illustrates an example of a SAN appliance  136  that possess an internal SAN  138  that shares data between SAN-attached devices  126 , the NAS server  124 , and the SAN  128  external to the appliance  136 . The appliance  136  serves block-level data, through channel-based interface links  130 , to the SAN  128 . From the perspective of the SAN, the appliance  136  appears as a prior art SAN-attached device  126 . The appliance  136  also serves file-level data, through network-based interface links  110 , to the LAN  104 . From the perspective of the LAN, the appliance  136  appears as a prior art NAS server  124 . 
   Another adaptation of a SAN appliance is simply a general purpose computer with DAS devices. This computer converts the DAS protocols into SAN protocols in order to serve block-level data to the SAN  128 . The computer may also act as a NAS server  124  and serve file-level data to the LAN  104 . 
   File System Layering 
   File system designers can construct complete file systems by layering, or stacking, partial designs on top of existing file systems. The new designs reuse existing services by inheriting functionality of the lower level file system software. For instance, NFS is a central-server architecture that utilizes existing local file systems to store and retrieve data from storage device attached directly to servers. By layering NFS on top of local file systems, NFS software is free from the complexities of namespace, 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. 
   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 adding compression or encryption to file systems without such support. 
   Installable File System Interfaces 
   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, installable interface. 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. 
   VFS occupies the level between the 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 to perform tasks such as mounting, unmounting, and reading file system statistics. Vnode operations manipulate individual files. Vnode operations include opening, closing, looking up, creating, removing, reading, writing, and renaming files. 
   Vnode structures are the objects upon which vnode functions operate. The VFS interface creates and passes vnodes to file system vnode functions. A vnode is the VFS virtual equivalent of an inode. Each vnode maintains a pointer called “v_data” to attached file system specific, in-core memory structures such as inodes. 
   Many file system interfaces support layering. With layering, file systems are capable of making calls to other file systems though the virtual file system interface. For instance, NFS server software may be implemented to access local file systems through VFS. In this manner, the server software does not need to be specifically coded for any particular local file system type; new local file systems may be added to an operating system without reconfiguring NFS. 
   SUMMARY OF THE INVENTION 
   The present invention is a distributed file system that utilizes aspects of a NAS server system along with a storage area network having at least one SAN-attached storage device. By combining these two architectures, it is possible to achieve the benefits of fast data reads over a SAN as well as some of the consistency benefits of using a NAS server. The present invention combines these two architectures by creating separate data paths for write and read requests. 
   The write data-path of the present invention is similar to the write data-path of prior art NAS, with the DAS storage device being replace with a SAN-attached storage device accessed over a SAN. This is accomplished so that all write activities to the SAN attached storage device are serialized through one server, while still allowing each client write access to the volume stored on the SAN-attached device. 
   The primary read data-path of the present invention is similar to the read data-path of prior art SAN environments, whereas the secondary read data-path is similar to the read data-path of prior art NAS environments. Since most reads pass directly from the SAN-attached storage device to the clients, the present invention takes full advantage of high-speed SAN protocols. In those rare instances where the primary read data path is not available, the present invention can utilize the secondary data path of typical NAS environments. 
   The present invention is able to maintain consistency between the local and remote file system layers by comparing modification times for files and related directories, such as is accomplished during file lookup. To perform a lookup operation, the present invention requests that the remote file system lookup the file. If this is successful, the present invention then compares the modification times for the lookup directory in both the local and remote file systems. If these times are not the same, the local file system cache for the directory is invalidated, and the directory inode for the local file system is read again from the SAN-attached device. A lookup is then performed through the local file system. If this is unsuccessful, the system will note that reads for this file should occur through the remote file system. 
   Similarly, consistency is maintained during read operations by examining the modification times for the file in both the remote and local file systems. If the times are the same, then the local file system is used to read the file. If the times differ, the local file system cache is invalidated and the modification time is then read from the storage device and again compared. If the modification times remain different, then the remote file system is used to read the file. If the modification times are the same, the local file system is used to read the file. In some cases, it may be necessary to flush the cache in the remote file system before performing the read operation with the local file system. 
   In a first embodiment of the present invention, a new file system is loaded into each client. This file system is layered on top of separate local and remote file systems, which handle actual data transfers over the SAN and actual data transfers with the NAS server. No modification of the file systems of the NAS server is necessary in this embodiment. 
   In a second embodiment, the file system of the present invention is merged with a local file system. In this embodiment, this combined file system is used on the client in conjunction with a remote file system that handles communication with the NAS server. The new, combined file system is also used on the NAS server as the local file system. By using this combined data system, this second embodiment ensures that all clients and servers accessing the SAN-attached devices will be able to coexist. 
   In a third embodiment, the file system of the present invention is merged with a local file system and given support to write directly to the SAN-attached devices. In this embodiment, the client coordinates with the server to maintain consistency while updating on-disk inodes and block allocate tables. The client has multiple options concerning which data-path to transfer data; however, in a common scenario, the client transfers small files and small file requests across the LAN using NAS protocols and transfers large files and large file requests across the SAN using SAN protocols. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representational drawing of a prior art NAS-based file sharing environment. 
       FIG. 2  is a representational drawing of a prior art SAN-based file sharing environment. 
       FIG. 3  is a representational drawing of a prior art SAN appliance. 
       FIG. 4  is a representational drawing of a file sharing environment utilizing a file system of the present invention. 
       FIG. 5  is a representational drawing of the network environment of  FIG. 4 , showing additional details of the client and server elements. 
       FIG. 6  is a representational drawing of an in-core inode structure of the present invention. 
       FIG. 7  is a flowchart showing the basic procedure of the present invention for locating a file within a directory. 
       FIG. 8  is a flowchart showing the basic procedure of the present invention for reading from a file. 
       FIG. 9  is a flowchart showing the basic procedure of the present invention for writing to a file. 
       FIG. 10  is a representational drawing of the network environment of  FIG. 4  with local file system layers merged into the file system of the present invention. 
       FIG. 11  is a representational drawing of the network environment of  FIG. 10  showing the data-path that enables the present invention to directly write data to the SAN-attached devices. 
       FIG. 12  is a flowchart showing the basic procedure of the present invention for writing data to a file using a data-path directly connecting the client to the SAN-attached devices. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a distributed file system that provides users and application programs with transparent access to shared data found on storage devices attached directly to a network. For purposes of this application, the present invention will be referred to as the Nasan file system. This file system incorporates two technologies into a single file system: network attached storage (NAS) and storage area networks (SANs). 
   Referring to  FIG. 4 , a file sharing environment  140  is shown that utilizes a file system of the present invention. A Nasan environment  140  consists of Nasan client computers  142 , SAN-attached devices  126 , and at least one NAS server  106 . Nasan clients  142  are connected to the NAS server  106  via network-based I/O interface links  110  connected to the LAN  104 . The LAN  104  consists of network components such as routers, switches, and hubs. 
   The Nasan clients  142  and NAS server  106  connect to the SAN-attached devices  126  via an I/O interface capable of transferring storage protocols over network connections. The NAS server  106  may actually be comprised of a cluster of computers serving file-level data via NAS protocols. The NAS server  106  may also be part of the file server component of a SAN appliance  136 . 
   The I/O interface links  130  connect to the SAN  128 , which consists of network components such as routers, switches, and hubs. The SAN  128  may also include components that perform storage virtualization, caching, and advanced storage management functions. The SAN devices  126  are block and object-addressable, non-volatile storage devices. The SAN devices  126  may be part of a SAN appliance  136  or dedicated storage devices attached to the SAN  128 . 
   The primary read data-path  144  of Nasan is similar to the read data-path  132  of prior art SAN environments  120 , whereas the secondary read data-path  146  is similar to the read data-path  114  of prior art NAS environments  100 . The majority of read transfers take place over the primary data-path  144 , which passes from the SAN devices  126 , through the SAN  128 , directly to the Nasan clients  142 . The primary data-path  144  takes full advantage of high-speed SAN protocols. However, some read transfers follow the secondary data-path  146  and pass from the SAN-attached devices  126 , through the NAS server  106 , across the LAN  104 , en-route to the Nasan clients  142 . The state of the Nasan environment  140  dictates whether the primary data-path  144  or the secondary data-path  146  is used for read transfers. 
   The write data-path  148  of Nasan is similar to the write data-path of prior art NAS  116  with the difference being the Nasan write data-path  148  also includes the SAN  128 . The write data-path  148  begins at the Nasan clients  142  and passes through the LAN  104  to the NAS server  106 . The server  106 , in turn, writes across the SAN  128  to the SAN-attached devices  126 . 
   Due to high-speed SAN reads  144 , the Nasan file system significantly exceeds the file sharing performance and scalability of prior art NAS solutions. Although Nasan write performance is similar to prior art NAS write performance, Nasan reads are often ten times faster. Because read operations generally outnumber writes five to one, the performance improvement made to reads dramatically increases overall system throughput. Furthermore, by offloading reads from the NAS servers  106 , the Nasan file system substantially reduces server  106  workloads. With reduced workloads, servers  106  exhibit shorter response times, sustain more simultaneous file transfers, and support considerably larger throughputs than servers  106  supporting traditional NAS  100 . 
   The Nasan file system transfers read requests across the high-speed SAN  128  while serializing writes through a central NAS server  106 . This serialization leads to write transfer rates that are slower than reads; however, writeserialization facilitates extremely low-latency consistency management. Low-latency consistency enables Nasan clients  142  to efficiently transfer files of all sizes. Therefore, the Nasan file system is a general-purpose solution for readintensive workloads. 
   Nasan Layering 
   One embodiment of the Nasan file system utilizes a two-tiered layering scheme. Nasan software occupies the upper level, while non-modified local and remote file systems comprise the lower. The Nasan layer provides a management framework that facilitates data consistency and routes file requests to the appropriate lower level file system. All remaining file management functionality is derived from these lower layer file systems. 
   Referring to  FIG. 5 , application programs  150  running on the Nasan client  142  make file requests to the Nasan file system software layer  152 . Nasan software  152  redirects read requests to the either the local file system level  154  or the client-side remote file system layer  156  and redirects write requests to the remote file system layer  156 . These lower layer file systems conduct the actual data management, transport, and storage. 
   The local file system  154  of the client provides the primary read data-path  144  for Nasan transfers. Because the clients  142  do not directly modify the volume stored on the SAN devices  126 , Nasan software  152  maintains low-latency consistency by simply invalidating stale caches of the local file system layer  154 . 
   The remote file system facilitates the secondary read data-path  146  as well as write access to files managed by the NAS server  106 . The Nasan client  142  passes file requests to the client-side remote file system layer  156 . In turn, the remote file system  156  on the client  142  transmits these requests via NAS protocols to the server-side remote file system layer  158  on the server  106 . The NAS server  106  completes the requests by reading data from or writing data through the local file system  155  of the server  106  to volumes stored on SAN-attached devices  126 . Write-serialization, through the NAS server  106 , enables low-latency consistency. 
   Components of the Preferred Embodiment 
   The components and protocols that form the environment  140  of the present invention range in price, performance, and compatibility. In the preferred embodiment, the interface links  110 , 130  that connect to the LAN  104  and to the SAN  128  may include Ethernet, InfiniBand, and Fibre Channel. Over these links  110 , 130  run a number of different network and channel protocols, including Internet Protocol (IP), SCSI-3, Virtual Interface (VI), iSCSI, FCIP, and iFCP. The NAS protocols used by the remote file system  156 , 158  include Network File System (NFS), Server Message Block (SMB), and Common Internet File System (CIFS). The present invention is not limited to these specific components and protocols. 
   Local File System Consistency 
   In general, local file systems perform extensive metadata and real-data caching. The only consistency management typically required of local file systems is periodic updates to on-disk data structures. Cached data is never invalidated because on-disk data is always assumed to be older than cached data. 
   Within a Nasan environment  140 , the NAS server  106  has read-write access to the local file system volume stored on SAN-attached disks  126 , while Nasan clients  142  have read-only access to this volume. Because the client local file systems  154  and the server local file systems  155  may not be designed to support SAN environments with multiple computers, Nasan software  152  must explicitly maintain data consistency between storage devices  126  and caches of the client local file system  154 . 
   Consistency Between Local and Remote File System Layers 
   Local  154  and remote  156  file systems utilize separate caches within client  142  main memories. After file writes, the remote file system  156  cache contains newer data than the local file system  154  cache. Nasan software  152  makes the local file system  154  cache consistent with the Nasan environment  140  by explicitly invalidating stale data within the cache. 
   Nasan software  152  has the option to read from the local file system  154  or the remote file system  156 . When reading from the primary data-path  144 , Nasan software  152  first determines if data is being cached by the client-side remote file system  156 . If data is cached, Nasan software  152  flushes the remote file system  156  cache and invalidates the local file system  154  cache. The read operation continues by reading file data from the local file system layer  154 . 
   Nasan software  152  reads from the secondary data-path  146  when the local file system  154  inode is temporarily inconsistent with inode of the remote file system  156 . Nasan software  152  may also read from the secondary data-path  146  when performance benefits are possible. For example, if the client  142  has recently written to the file, the remote file system  156  cache likely contains a cached copy of the most recently written data. In this particular case, reading from the secondary data-path  146  will benefit from the enhanced performance of the cache. However, if a different client  142  writes to the file before the read is requested, the cached data is no longer valid, and therefore the read will propagate across the LAN  104  to the NAS server  106 . 
   Another example of when the Nasan file system  152  may read from the secondary data-path  146  rather than the primary data-path  144  relates to the size of the read request and the size of the file. For small files or small read requests, read transfer times may actually be smaller when reading from the remote file system  156 , because reading from the primary data-path  144  entails reading metadata and real-data from the local file system  154 . 
   Inode Structure 
   The VFS interface of a Nasan client  142  maintains a vnode structure in memory for each active Nasan file. These vnodes are passed to Nasan software  152  functions such as lookup, read, write, create and remove. The Nasan functions, in turn, use the v_data field of the vnode to store and locate the in-core Nasan inode of the specific file. 
     FIG. 6  illustrates the primary fields found within an in-core Nasan inode  160  structure as well as pointers to closely related data structures within the main memory of the client  142 . The i_handle  164 , i_rhandle  174  and i_lhandle  176  fields point to the Nasan vnode  162 , the remote file system vnode  166 , and the local file system vnode  170 , respectively. These vnodes,  162 ,  166 , and  170 , point to their corresponding file system specific inodes  160 ,  168 , and  172  through the v_data pointer field of each vnode. In situations where the remote file system layer  156  and local file system layer  154  are inconsistent, Nasan software  152  may set the i_lhandle  176  field to point to the remote file system vnode  166  rather than the local file system vnode  170 . This situation is temporary but signifies to Nasan software  152  that the remote file system layer  156  should be accessed for all operations of the corresponding file. The i_rmtime  178  and i_lmtime  180  fields maintain the last know file modification times returned by the remote file system layer  156  and the local file system layer  154 , respectively. 
   Basic File System Operations 
   The basic file system operations are searching for a file within a directory, reading from a file, and writing to a file. Other operations include reading a directory, creating a file, removing a file, retrieving file attributes, modifying file attributes, and locking a file. 
   File Lookup 
   A lookup is a common file system operation that searches a directory for a given file name. If the lookup function locates the file within the directory, the function returns a vnode pointer for the corresponding file. 
   The lookup operation of the present invention is illustrated in the flowchart shown in  FIG. 7 . A lookup operation receives as arguments the directory vnode  162  and the name of the file for which to search, and returns a newly created vnode  162  and inode  160  combination for the found file. The process starts at step  200  by the operating system of a Nasan client  142  invoking the Nasan lookup function with the appropriate arguments. At step  202 , the Nasan lookup function calls the lookup routine of the remote file system layer  156  using i_rhandle  174  as the directory vnode argument. If the file is not found by the remote file system  156 , at step  206  the Nasan lookup exits with a “file not found” error. 
   If the file is found, it is necessary to determine if the found file is available through the local file system  154 . The first step for accomplishing this task is step  208 , which compares the lower-level vnode pointers, i_rhandle  174  and i_lhandle  176 , of the directory. If these pointers are identical, the function proceeds to step  224 , because the directory inode does not contain a pointer to the local file system vnode  170 . This indicates that the directory in which the file is being searched is not presently available through the local file system  154 . 
   If the lower-level vnode pointers of the directory, i_rhandle  174  and i_lhandle  176 , are not the same, Nasan lookup continues to step  210  which calls the remote file system  156  function that reads file attributes of the directory and then saves the file modification time in i_rmtime  178 . 
   At step  212 , directory inode modification times, i_rmtime  178  and i_lmtime  180  are compared. If these times are the same, the local file system  154  cache for the directory is clean, so the function proceeds to step  218 . Otherwise, the cache of the local file system  154  for the directory must be invalidated in step  214  before a lookup can be performed in the directory. The actual technique for invalidating the cache associated with the directory is dependent upon the operating system of the Nasan client  142 . In the preferred embodiment, directory metadata is cached in the operating system buffer cache, whereas the directory real-data is cached in the page cache. The buffer cache of the directory metadata is invalidated by explicitly marking each cached block as stale. The page cache of the directory real-data is invalidated by calling page cache invalidate routines. 
   At step  216 , Nasan reads the local file system directory inode  172  from the SAN-attached devices  126  and saves the modification time in i_lmtime  180  of the Nasan directory inode  160 . In the preferred embodiment, reading the local file system directory inode  172  merely involves calling the local file system  154  function that reads the directory attributes; the local file system layer  154  invokes the actual calls to the SAN-attached devices  126 . 
   At step  218 , the Nasan lookup function calls the lookup routine of the local file system layer  154  using i_lhandle  176  of the directory inode  160  as the directory argument. If the file is not found by the local file system  154 , the Nasan lookup proceeds to step  224  which saves the vnode  166  returned by the remote file system  156  lookup routine at step  202  in both i_rhandle  174  and i_lhandle  176  fields of a newly created Nasan inode  160 . The Nasan lookup routine finishes and returns control to the operating system. 
   If the local file system  154  lookup function finds the file at step  218 , control passes to step  222  where Nasan  152  creates a new Nasan inode  160 , saves the vnode  166  returned by the remote file system  156  lookup routine at step  202  in the i_rhandle  174  field of a newly created Nasan inode  160 , and saves the vnode  170  returned by the local file system  154  lookup routine at step  218  in the i_lhandle  176  field. The Nasan lookup routine finishes and returns control to the operating system. 
   File Read 
   The file read operation of the present invention is illustrated in the flowchart of  FIG. 8 . The process starts by an application program  150  running on a Nasan client  142  desiring to read data from a file. The application program  150  submits the read request to the operating system, which in turn invokes a call to the Nasan, read function, as shown in step  230 . At step  232 , the Nasan read function tests whether the lower level vnode pointers, i_rhandle  174  and i_lhandle  176 , are identical. If these pointers are the same or if Nasan otherwise selects to read from the secondary data-path  146 , the Nasan read function at step  252  invokes a call to the client-side remote file system layer  156 , which in turn reads data from the NAS server  106  across the LAN  104 . 
   If i_rhandle  174  and i_lhandle  176  are different, at step  234 , the Nasan read function calls the remote file system  156  function that reads file attributes and saves the modification time of the file within i_rmtime  178  of the inode  160 . At step  236 , the Nasan read function compares the newly acquired modification time with the saved modification time within i_lmtime  180 . If the modification times of i_rmtime  178  and i_lmtime  180  are the same, step  238  is performed; otherwise, control proceeds to step  244 . 
   At step  238 , the Nasan read function checks whether the client-side remote file system layer  156  is caching real-data. If data is being cached, Nasan flushes dirty data from the cache for the remote file system  156  and invalidates any real-data cached by the local file system layer  154 . The flush operation ensures that data last written by the client  142  will be written to the SAN-attached devices  126  prior to reading from the local file system  154 . If data is not cached, flow proceeds to step  250 . 
   At step  244 , because the modification times of i_rmtime  178  and i_lmtime  180  differ, it is necessary to invalidate the cache of the local file system  154  associated with the file. This is accomplished in the preferred embodiment by explicitly marking the appropriate metadata blocks within the buffer cache as stale and invalidating real-data within the page cache by calling page cache invalidation routines. 
   At step  246 , Nasan  152  reads the local file system inode  172  from the SAN-attached devices  126  and saves the modification time within i_lmtime  180  of the inode  160 . In the preferred embodiment, reading the local file system inode  172  merely involves calling the local file system  154  function that reads the file attributes; the local file system layer  154  invokes the actual calls to the SAN-attached devices  126 . 
   At step  248 , if the modification times of i_rmtime  178  and i_lmtime  180  are the same, control proceeds to step  238 . Otherwise, the Nasan read function at step  252  invokes a call to the client-side remote file system layer  156 , which in turn reads data from the NAS server  106  across the LAN  104 . 
   At step  250 , the Nasan read function invokes a call to the local file system layer  154 , which in turns reads data from the SAN-attached devices  126  across the SAN  128 . 
   File Write 
   The file write operation of the present invention is illustrated in the flowchart of  FIG. 9 . The process starts by an application program  150  running on a Nasan client  142  desiring to write data to a file. The application  150  submits the write request to the operating system, which in turn invokes a call to the Nasan write function, as shown in step  260 . At step  262 , the Nasan write function passes the request to the write function of the client-side remote file system layer  156 . The Nasan write completes after the remote file system  156  function completes. 
   File Close 
   The operating system of the Nasan client  142  calls the Nasan close operation when an application program  150  is finished using a file. The Nasan close function simply calls the close function of the local file system  154  and the close function of the client-side remote file system  156 . The client-side remote file system  156  performs a synchronous flush operation of its cache. This flush operation writes the dirty cached data to the NAS server  106  and completes after the NAS server  106  has written the data to the SAN-attached devices  126 . 
   File Locks 
   The Nasan file system derives file locking services from the remote file system layer  156 . On the clients  142 , application programs  150  make lock requests to Nasan file system software  152 . Nasan software  152  redirects these lock requests to the remote file system layer  156 . The client-side remote file system software  156  and the server-side remote file system  158  provide all lock management, process-blocking, and recovery functionality. 
   Other File Operations 
   Other file system operations include reading a directory, creating a file, removing a file, retrieving file attributes, and modifying file attributes. Operations that modify the Nasan volume are similar to file writes in that control is passed from the Nasan  152  function to the client-side remote file system  156  function. These remote file system  156  functions perform the entire operation and then return control to the Nasan  152  function. The Nasan  152  function simply passes error information back to the operating system. 
   Operations that do not modify the Nasan volume may use the primary read data-path  144  or the secondary data-path  146 . Operations that use the primary data-path  144  are similar to file read operations and operations that use the secondary data-path  146  are similar to file write operations. The primary data-path  144  is usually chosen by Nasan software  152  when the i_rmtime  178  and i_lmtime  180  fields of the Nasan inode  160  are identical. Otherwise, the secondary data-path  146  is used to service the file operation. 
   ALTERNATIVE EMBODIMENTS 
   Numerous alternative embodiments to the file system of the present invention are possible, while remaining within the scope of the present invention. Some embodiments may improve the performance of the file system in one or more areas. Other embodiments may improve heterogeneity, availability, and recovery. The following alternative embodiments are examples of the type of files systems that are possible utilizing the present invention. 
   NAS Server Layering 
   It is possible to run Nasan software  152  on the NAS server  106  to enable various consistency and performance optimizations. These optimizations may or may not be beneficial to various environments. On the Nasan client  142 , Nasan software  152  remains layered above the local file system  154  and client-side remote file system  156 . On the NAS server  106 , Nasan software  152  is layered below the server-side remote file system  158  and below the application programs  150  but above the local file system  155 . 
   Merged Layers 
   The Nasan clients  142  and the NAS server  106  must interpret the file system volume stored on the SAN devices  126  in exactly the same manner. However, not many local file systems  154 ,  155  support multiple operating systems, thus Nasan is often limited to a few heterogeneous environments.  FIG. 10  illustrates a system  300  in which the file system of the present invention is merged with local file system software. The system  300  is useful in that it ensures that the file systems which access the volumes stored by the SAN-attached devices interpret the volumes in the same manner, whether from the Nasan clients  142  or from the NAS Server  106 , regardless of the operating systems within the system  300 . 
   The Nasan file system layers  302  and  304  in system  300  incorporate the functionality of the client local file system layer  154  and the server local file system  155 . On the client  142 , the Nasan file system  302  provides read access to volume stored on the SAN-attached storage devices  126 . On the NAS server  106 , the Nasan file system  304  provides both read and write access to the volume. Like the local file system of the NAS server  155 , the Nasan file system  304  on the NAS server  106  is able to create files, remove files, read files, write files, retrieve file attributions, modify file attributes, and lock files ranges. 
   Application programs  150  on the client  142  make file requests to the Nasan file system software layer  302 . Nasan software  302  services most read requests and redirects write requests and other requests to the client-side remote file system layer  156 . 
   The client-side remote file system layer  156  facilitates write access to files managed by the NAS server  106 . The Nasan layer  302  passes write requests to the client-side remote file system  156 . In turn, the client-side remote file system  156  transmits these requests via NAS protocols to the server-side remote file system layer  158  of the server  106 . The server-side remote file system  158  passes the request to the Nasan layer  304 , which completes the request by writing data to the SAN-attached devices  126 . 
   SAN Write Optimization 
   The SAN write optimization enables Nasan clients  142  to write real-data across the SAN  128  without corrupting the volume stored on the SAN-attached devices  126 . Writing data across the SAN requires a consistency scheme to coordinate simultaneous accesses by multiple computers to metadata and real-data. Although such a consistency scheme adds appreciable overheads to the file transfer times, SAN writes reduce overall transfer times when transferring large amounts of data. Large file transfers allow the high efficiencies of the SAN protocols to overcome the overheads imposed by the consistency scheme. In contrast, small file transfers through the NAS server  106  benefit from the low-latency consistency management of the NAS architecture despite the inefficiencies of NAS protocols. 
     FIG. 11  illustrates a system  320  in which the file system of the present invention is merged with local file system software and the SAN write optimization is enabled. On the Nasan client  142 , write requests from the application programs  150  are passed to the Nasan file system layer  322 . The Nasan layer  322  either forwards the write request to the client-side remote file system  156  or services the request using the SAN write data-path  326 . Numerous factors are considered when determining which write data-path is used, including file sizes, request sizes, explicit user directions, and privileges of the clients  142 . 
   Before real-data may be written across the SAN  128 , the file must be fully allocated for the given range of the write request. Either the server  106  or the clients  142  must allocate this file range. The determination of which computer performs the allocation is typically based upon static policies setup by system administrations for each individual client  142 . 
   Much like prior art SAN-based file systems with file server computers, the server-side Nasan file system  324  allocates blocks to the file range, without writing real-data, after receiving instruction by the remote file system  156 ,  158 . Clients  142  may also allocate the file range; however, special mechanisms must be in place that allow the clients  142  to access and modify block allocation tables of the volumes. In the preferred embodiment, the Nasan file system  322 ,  324  gives access to the allocation tables through a file interface. Each volume has one or more special files that contain allocation tables. These files may be locked, read, and written by only the Nasan file system software  322 ,  324 . 
   On the server  106 , the Nasan file system  324  allocates blocks by locking the allocation table files, modifying their contents, and then releasing the locks. The client-side Nasan file system  322  performs these same tasks by locking the allocation table files using the lock provisions of the remote file system  156 ,  158 . For performance reasons, the client-side Nasan software  322  reads and writes the allocation table files across the SAN data-paths  144 ,  326 . 
   The client-side file write operation of the present invention is illustrated in the flowchart of  FIG. 12 . The process starts by an application program  150  running on a Nasan client  142  desiring to write data to a file. The application  150  submits the write request to the operating system, which in turn invokes a call to the Nasan write function, as shown in step  400 . 
   At step  402 , the Nasan write function determines whether the request will utilize the SAN data-path  326  or the NAS data-path  148 . If the NAS data-path  148  is to be used, control passes to step  404  where the Nasan write function forwards the request to the write function of client-side remote file system layer  156 . The Nasan write completes after the remote file system  156  function completes. 
   If the Nasan write function determines that the SAN data-path  326  is to be used, at step  406 , the Nasan client  142  acquires a lock on the file inode by submitting a file lock request to the client-side remote file system  156 . This lock request is passed, through the LAN  104 , to the server-side remote file system layer  158  of the NAS server  106 . The remote file system  158  forwards the lock request to the Nasan file system layer  324 . Before the server-side Nasan software  324  grants the lock, it flushes the caches of all metadata and real-data associated with the file. 
   Upon receiving acknowledgement from the NAS server  106  that the lock has been acquired, at step  408 , the Nasan write function determines whether the client  142  or the server  106  is to perform allocation. If the client  142  is to perform allocation, the Nasan write function proceeds to step  412 . 
   If the server  106  is to perform the allocation, at step  410 , the client  142  initiates the allocation by invoking a set attribute call to the client-side remote file system layer  156 . The client-size remote file system  156  then forwards this request to the server-side remote file system  158 , which passes the request to the server-side Nasan file system software  326 . In the preferred embodiment, the remote file system software  156 , 158  is able to specify an allocation range within file; in other embodiments, the remote file system software  156 , 158  must specify the allocation for the entire file. Upon receiving an allocation request by the remote file system  158 , the Nasan file system  324  allocates blocks to the specified file range and then flushes the on-disk inode to the SAN-attached devices  126 . 
   At step  412 , the client-side Nasan software  322  reads the on-disk inode structure for the file. Using this inode information, the Nasan software  322  determines if block allocation is necessary to perform the write request. If no allocation is necessary or if the allocation was performed at step  410 , the write function continues to step  418 . 
   If allocation is needed, the Nasan software  322  proceeds with block allocation at step  416  by acquiring the file lock of the allocation tables. Once the allocation tables are locked, the Nasan client  142  reads the allocation tables from the SAN devices  126 , modifies the allocation tables, writes the tables to the SAN devices  126 , and then releases the file lock. 
   At step  418 , the file is fully allocated for the request range. The Nasan write function writes the real-data to the SAN-attached devices  126  via the SAN write data-path  326 . Once this real-data write completes, at step  420 , the modified on-disk inode is written by the client  142  to the SAN-attached devices  126  and the file lock is released by issuing an unlock request to the client-side remote file system  156 . The remote file system  156  passes the unlock request to the server  106  which forwards the unlock request to the server-side Nasan file system  324 . After the file lock is released, the Nasan write operation completes. 
   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. For instance, the present invention was described and shown with the SAN and LAN networks appearing as separate, physical networks. However, as is well known in the prior art, it is possible to send SAN protocols and LAN protocols over the same physical network. The two networks are distinguishable by the protocols that are used to communicate between nodes on the network. In addition, although it is not shown in the drawings, it would be possible to use a client computer in the present invention as a file server that serves file requests from other computers. These other computers would likely have no access to the storage area network, but would have the ability to send file requests to the client computer of the present invention over a local area network. Because many such modifications and variations are present, the scope of the present invention is not to be limited to the above description, but rather is to be limited only by the following claims