Abstract:
By incorporating a referral mechanism, a failover method and system for remotely mirrored clustered file servers enables a client computer to transparently access a remotely mirrored file system during a failover of a first NAS (network attached storage) storing the file system. The first NAS is clustered with a second NAS that takes over the IP address of the first NAS on failover. A mirroring relationship is established between the first NAS and a third NAS whereby a copy of the file system is replicated to the third NAS. A referral is created on the second NAS, such that an access attempt from the client computer for accessing the first file system that is directed to the second NAS following IP address takeover from the first NAS after failover is referred to the third NAS by the referral to enable the client computer to access the copy of the file system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to file servers, such as Network Attached Storage (NAS) systems and, in particular, to failover methods in a clustered NAS system. 
         [0003]    2. Description of Related Art 
         [0004]    Clustering is the use of multiple computers, multiple storage devices, and redundant interconnections, to form what appears to users as a single highly-available system. Clustering can be used for load balancing as well as to provide high availability. A clustered file system (also referred to as a clustered Network Attached Storage (NAS)) system may include a plurality of file systems, and creates at least one single namespace. A namespace is a set of valid names recognized by a file system that identifies the directory tree structure of the directories and file path names that combine to form a complete file system. 
         [0005]    In a clustered NAS file system, the file system (sometimes referred to as a “global file system” or “GFS”) may be distributed across multiple NAS devices or nodes, while appearing to a user as a complete single file system located on a single device. One method of achieving this under the Network File System (NFS) version 4 (NFSv4) protocol involves providing network file system or server software on the NAS hosts, whereby “referrals” on one host are used to indicate the storage location of directories and files on another host. 
         [0006]    The present invention is directed mainly to NAS systems incorporating NFSv4 technology (see, e.g., Shepler, S. et al., “IETF RFC3530: Network File System (NFS) version 4 Protocol”, www.ietf.org, April 2004 “NFS version 4 Protocol”, www.ietf.org/rfc/rfc3530.txt; and Novack, D. et al., “Implementation Guide for Referrals in NFSv4 draft-ietf-nfsv4-referrals-00.txt”, www.ietf.org, July 2005). NFSv4 is a network file sharing protocol that is the next generation of NFSv3, which is currently one of the most popular network file sharing protocols. However, while the NFSv4 protocol sets forth a “migration” function, it does not disclose or suggest any remote copy function or failover methods in a clustered file system or clustered NAS environment. 
         [0007]    When a failure occurs on one of the NAS nodes in a cluster, the NFSv4 client is not able to access the file system or portion of the file system on the failed NAS node until file system has been recovered from the remotely-mirrored file system back onto the failed NAS node. Alternatively, the IP address of the failed NAS node can be taken over to another NAS node in the cluster using existing clustering software. However, even if the IP address is taken over in this matter, there is normally not the same data in the NAS node which took over the IP address as in the failed NAS. The mirrored file system is typically located on a remote NAS node, which means it is located on the different subnet, separated by a router or the like. Thus, the NFSv4 client is not able to continue accessing the file system on the failed node until backup data on the remote node is recovered to the failed node. The present invention seeks to address the need for providing a backup method and failover system in the NFSv4 environment. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    This invention discloses a failover method for remotely-mirrored clustered file servers. According to the present invention, the file servers or NAS nodes form a cluster. Within each cluster, the internet protocol (IP) address of one NAS node can be taken over by another NAS node. In addition, for purposes of providing backup, each file system in a NAS is mirrored in a file system at a remote site. Under the present invention, a failover method is provided for remotely-mirrored clustered file servers, such as clustered NAS nodes (also referred to herein as NAS servers or NAS systems), using referral information to enable NFSv4 clients to access the remotely-mirrored file system upon failover of a NAS storing the primary file system. This enables the NFSv4 clients to transparently access the backed-up data in one or more remotely-mirrored file systems at a failover of the primary node containing the primary file system. 
         [0009]    These and other features and advantages of the present invention will become apparent to those of ordinary skill in the art in view of the following detailed description of the preferred embodiments. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, in conjunction with the general description given above, and the detailed description of the preferred embodiments given below, serve to illustrate and explain the principles of the preferred embodiments of the best mode of the invention presently contemplated. 
           [0011]      FIG. 1  illustrates an example of a hardware configuration in which the method and apparatus of this invention is applied. 
           [0012]      FIG. 2  illustrates an exemplary software configuration under which the method and apparatus of this invention is applied. 
           [0013]      FIG. 3  illustrates a conceptual diagram of a migration feature provided in NFSv4 protocol. 
           [0014]      FIG. 4  illustrates an exemplary data structure of a referral in a file system for use with the invention. 
           [0015]      FIG. 5  illustrates a conceptual diagram of a problematic situation that can occur in conventional clustered NAS systems. 
           [0016]      FIG. 6  illustrates a conceptual diagram of redirection mechanisms provided by the NFSv4 protocol as modified and applied according to the invention. 
           [0017]      FIG. 7  illustrates a process flow for creating referrals for mirrored file systems according to the present invention. 
           [0018]      FIG. 8  illustrates a conceptual diagram of a failover process under the present invention. 
           [0019]      FIG. 9  illustrates a process flow for handling failover according to the present invention. 
           [0020]      FIG. 10  illustrates a conceptual diagram of a failback process under the present invention. 
           [0021]      FIG. 11  illustrates a process flow of failback in a case in which there was an access to the mirrored file system during the failover period. 
           [0022]      FIG. 12  illustrates a process flow of failback in a case in which there was no access to the mirrored file system during the failover period. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In the following detailed description of the invention, reference is made to the accompanying drawings which form a part of the disclosure, and, in which are shown by way of illustration, and not of limitation, specific embodiments by which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. Further, the drawings, the foregoing discussion, and following description are exemplary and explanatory only, and are not intended to limit the scope of the invention or this application in any fashion. 
       First Embodiment—System Architecture 
       [0024]      FIG. 1  illustrates an example of a hardware configuration in which the method and apparatus of the invention are applied. The system is composed of one or more NAS clients  1000 , a management host  1100 , and plural NAS Systems  2001 - 2004  (NAS 1 -NAS 4 ). Each NAS client  1000  includes a CPU  1001  and memory  1002  for storing and executing programs, such as one or more applications and NFSv4 client software (illustrated in  FIG. 2 ). Each NAS client  1000  also includes a network interface card or adapter (NIC)  1003  for connecting the NAS client  1000  to NAS 1 -NAS 4   2001 - 2004  via a network  2500 , a router  2600 , and another network  2700 . The typical media/protocol of the networks  2500  and  2700  may be Ethernet, but other network types and protocols may also be used. 
         [0025]    Management Host  1100  includes a memory  1102  and a CPU  1101  for storing and executing management software (illustrated in  FIG. 2 ) and other programs. Management host  1100  includes a NIC  1103  for connecting the NAS client  1000  to NAS 1 -NAS 4   2001 - 2004 , respectively, via a network  2500 , a router  2600 , and another network  2700 . Networks  2500 ,  2700  may be Ethernet protocol LANs (local area networks) or sub-networks. 
         [0026]    NAS Systems NAS 1   2001 , NAS 2   2002 , NAS 3   2003 , and NAS 4   2004  consist of two main parts: a NAS head  2100  and a storage system  2400 . Because NAS 1 -NAS 4   2001 - 2004  may include essentially the same components, only NAS 1   2001  is described, with it being understood that NAS systems NAS 2 -NAS 4   2002 - 2004  may contain the same or functionally equivalent components. Further, while only four NAS systems  2001 - 2004  are illustrated, it should be understood that any number of NAS systems may be connected for communication and implemented in the invention. 
         [0027]    NAS head  2100  includes a CPU  2101 , a memory  2102 , a cache  2103 , a front-end network interface (NIC)  2104 , and a back-end interface (I/F)  2105 . As described in greater detail with reference to  FIG. 2 , a plurality of programs for processing NFS requests or other operations are stored in memory  2102  and executed by CPU  2002 . Cache memory  2103  temporarily stores NFS write data from NFS clients  1000  before the data is forwarded into the storage system  2400 , and cache  2103  also may store NFS read data that are requested by the NFS clients  1000 . Cache memory  2103  may be a-battery backed-up non-volatile memory. In another implementation, memory  2102  and cache memory  2103  are combined as a common memory. 
         [0028]    Front-end I/F (NIC)  2104  is used to connect NAS head  2100  to network  2500  to enable communication between NAS clients  1000  and NAS head  2100 , and may be an Ethernet protocol device. Back-end I/F  2105  is used to connect between NAS head  2100  and storage system  2400 . Fibre Channel (FC) and Ethernet are typical examples of connection formats that may be used. Alternatively, in the case of an internal connection between NAS head  2100  and controller  2200 , a system bus, such as PCI (peripheral component interface), is a typical example of the connection. 
         [0029]    NAS head  2100  processes requests from the NFS clients  1000  and management host  1100 . The storage system  2400  consists of a storage controller  2200  and disk drives  2300 . NAS head  2100  and storage system  2400  can be connected via interface  2105  and  2214 . NAS head  2100  and storage system  2400  can exist in one storage unit. In such a case, the two elements are connected via a system bus such as a PCI bus. In alternative embodiments, the NAS head and controller may be physically separated. In this case, the two elements can be connected via network connections such as Fibre Channel or Ethernet. 
         [0030]    The storage controller  2200  in storage system  2400  includes a CPU  2211 , a memory  2212 , a cache memory  2213 , a host interface  2214 , and a disk interface (DKA)  2215 . Storage controller  2200  processes I/O requests from the NAS head  2100 . As will be discussed in greater detail below with reference to  FIG. 2 , programs to process I/O requests and perform other operations are stored in the memory  2212  and executed by CPU  2211 . Cache memory  2213  stores the write data from the NAS head  2100  temporally before the data is stored into disk drives  2300 , and cache memory  2213  can also be used to store the read data that is requested by the NAS head  2100 . Cache memory  2213  may be a battery backed-up non-volatile memory. Further, in another implementation, memory  2212  and cache memory  2213  can be combined as a single common memory. 
         [0031]    Host interface  2214  is used to connect between NAS head  2100  and controller  2200 . Fibre Channel and Ethernet are typical examples of connection protocols used. Alternatively, a system bus connection, such as PCI, may be used. Disk adapter (DKA)  2215  is used to connect disk drives  2300  with the storage controller  2200 . Each of disk drives  2300  processes I/O requests in accordance with SCSI (small computer system interface) device commands. Further, it should be noted that the architecture discussed above is exemplary for explanation purposes, and numerous other appropriate hardware architectures may be applied to the invention. 
         [0032]    Software Configuration 
         [0033]      FIG. 2  illustrates an example of a software configuration in which the method and apparatus of the invention are applied. The system includes software on NAS clients  1000 , management host  1100 , and NAS systems or nodes NAS 1 -NAS 4   2001 - 2004 . 
         [0034]    As discussed above, NAS client  1000  is a computer on which an application (AP)  1011  generates file manipulating operations, such as read and write operations or I/O (input/output). An NFSv4 client program  1012  is also active on the NAS client node  1000 . The NFSv4 client program communicates with an NFSv4 server program  2121  on NAS 1   2001  through network protocols such as TCP/IP. The NFSv4 clients  1012  and NFSv4 server  2121  are able to communicate via network  2500  and/or  2700 . Moreover, router  2600  can be applied to connect separate sub LAN networks  2500 ,  2700  to act as a single LAN. 
         [0035]    Management software  1111  resides on the management host  1100 . Storage management operations such as remote copy operations and recovery operations can be issued from management software  1111 . 
         [0036]    As discussed above, each NAS system or node NAS 1   2001 , NAS 2   2002 , NAS 3   2003 , and NAS 4   2004  consists of two main parts, NAS head  2100  and storage system  2400 , each of which performs distinct functions, as described below: 
         [0037]    NAS head  2100 : File related operations are processed in NAS head  2100 . NFSv4 server  2121  resides on NAS head  2100  in order to communicate with NFSv4 client  1012  on the NAS clients  1000 . A number of service program modules also run on the NAS head  2100 , such as service daemons  2123  and NAS manager software  2161 . The local file system  2124  processes file I/O operations to the storage system  2400 , a NAS operating system (OS) kernel  2125  provides functional infrastructure, and drivers of storage system  2126  translate the file I/O operations to the block level operations, and communicate with storage controller  2200  via SCSI commands. The software modules discussed above and below are stored in memory on their respective devices, or on other computer readable medium such as hard disk devices, either locally or remotely. 
         [0038]    Storage system  2400 : Storage controller  2200  processes SCSI commands received from NAS head  2100  for performing read and write operations to logical volumes  2310  which are created from one or more of disk devices  2300 . A volume  2310  may be composed of one or more disk drives  2300 , or multiple volumes  2310  may be created on a single disk drive  2300 . NFSv4 file systems are created in volumes  2310  for storing files according to a file system directory tree data structure having files and directories. As will be described in greater detail below, file system or a portion thereof may be created on a single volume or on multiple volumes in the same NAS server, or on multiple volumes in multiple NAS servers. Also, multiple file systems or portions thereof may be created on a single volume. 
         [0039]    NFSv4 Migration 
         [0040]    The NFSv4 protocol includes a migration feature for migrating all or part of a file system from one storage system to another. By using a special file system location attribute, “fs_location”, the ability to migrate server file systems is enabled within the NFSv4 protocol. The file system locations attribute provides a method for the NAS client to query the server about the location of a file system or a portion of a file system, such as a directory or file. In the event that all or some of a file system has been migrated from its original storage system, the NAS client will receive an error “NFS4ERR_MOVED” response when attempting to, access or otherwise operate on the moved file system, and the NAS client can then query as to the new location of the file system or file system portion. The original NAS file server will then send information to the NAS client to indicate the new location of the migrated file system or portion thereof. From this information, the client can use its own policies to access the appropriate file system location. 
         [0041]    The fs_location attribute provides a list of file system locations. These locations are specified by providing the server name (either DNS (domain name system) domain or IP address) and the path name representing the root of the file system. Depending on the type of service being provided, the list will provide a new location or a set of alternate locations for the file system. The client will use this information to redirect its requests to the new NAS server. 
         [0042]      FIG. 3  illustrates a conceptual diagram of the migration feature carried out under the NFSv4 protocol. A file system “export1”  2311  is created on a volume in NAS 1   2001  and a second file system “export2”  2313  is created on a volume in NAS 2   2002  for exporting to the NFSv4 client. An NFSv4 client  1012  mounts file systems exported by NAS 1  “export1”  2311  as “/mnt/nas1” and NAS 2  “export2”  2313  as “/mnt/nas2”. Then, application software  1011  is able to see a file system as illustrated by directory tree  1500 , which virtually appears to application software  1011  to have a unitary structure of a complete file system, even though the directory tree is actually split among two different NAS systems  2001 ,  2002 . At some point in time, the file system /export 1   2311  on NAS 1   2001  is migrated to a file system /export 1   2312  on NAS 2 . After the migration process, when the NFSv4 client  1000  attempts to access the directory tree “/export1”, such as by using an NFSv4 read operation to the file system  2311 , NFSv4 server  2121  returns NFS4ERR_MOVED and a referral file which describes a location of the migrated file system export  1   2311  as now being on NAS 2 . Then, the NFSv4 client  1012  can redirect the operation to the current location of file system exportl  2312  by making the access request to an NFSv4 server  2131  on NAS 2   2002 . 
         [0043]      FIG. 4  illustrates an example of a data structure of a referral in a file system, such as the file system illustrated in  FIG. 3 , wherein an inode is able to refer a client to a migrated file system. An inode can be used to indicate a file, a directory, or a referral. Each inode includes an inode number  3001 , a file type  3002 , and a block pointer  3003  that indicates a disk block address where file or directory information is stored. If the inode indicates a file (if its file type field  3002  is “file”), the data block pointed to from the pointer  3003  in the inode contains actual data of the file. If a file is stored in a plurality of blocks (such as ten blocks), the addresses of the ten disk blocks are recorded in block pointer  3003 . On the other hand, if the inode is for a directory, such as inode  3000  then the file type field  3002  is “directory”, and the data blocks pointed to from block pointer  3003  store a list  3004  of inode numbers and names of all files and directories (subdirectories) in the directory (i.e., directory entry). Thus, in  FIG. 4 , root inode  3000  is a directory-type inode that includes an inode number “10” to a directory “export1”. However, since the directory tree for “export1” has been migrated from NAS 1   2001  to NAS 2   2002 , a referral file inode  3005  has replaced the original “export1” directory inode. The referral inode  3005  can point to a block address for a file  3006  in the local file system  2124  that describes the location of the file system “NAS2:/export1”. Additionally, it should be noted that there can be other implementations of referrals, and the implementation method should not be interpreted as restricting the invention. 
         [0044]      FIG. 5  illustrates a conceptual diagram of a problematic situation that can occur with the current system described above. A complete file system  1600  is visible to application  1011  on NAS client  1000 , and is distributed on NAS 1   2001  and NAS 2   2002 , as illustrated. The NFSv4 client  1012  NFS mounts file systems “export1”  2321  as “/mnt/1” and “export2”  2322  as “/mnt/2”, and “export3”  2323  as “/mnt/3”. Once the file systems are mounted, application software  1011  on client  1000  is able to perceive the directory tree  1600  as a complete file system, as illustrated in  FIG. 5 . 
         [0045]    In order to provide remote copy backup of NAS 1   2001  and NAS 2   2002 , the exported file systems are replicated to remote NAS 3   2003  and remote NAS 4   2004 , respectively. Thus, in this example, the file system  2321  is replicated to a file system  2324  on NAS 3   2003 ; the file system  2322  is replicated to a file system  2325  on NAS 3   2003 , and the file system  2323  is replicated to a file system  2326  on NAS 4   2004 . NAS 1   2001  and NAS 2   2002  are located on the same sub-network or subnet  2500 . NAS 3   2003  and NAS 4   2004  are located on the same subnet  2700 . The subnets  2500  and  2700  are different subnets but are in communication via router  2600 . Now, when a failure occurs on NAS 1   2001 , the NFSv4 client cannot keep accessing the file systems  2321  and  2322  until the backup data on NAS 3   2003  has been recovered to NAS 1   2001 . 
         [0046]    The IP address of NAS 1   2001  can be taken over to the NAS 2   2002  using clustering software. However, even if the IP address is taken over, there is no data in NAS 2   2002  corresponding to the file systems  2321  and  2322  of NAS  1   2001 . The backup data is located on NAS 3   2003 , and because NAS 3   2003  is located on the different subnet  2700  and not part of the same clustered NAS system as NAS 1   2001  and NAS 2   2002 , NAS 3   2003  cannot take over the IP address of NAS 1   2001 . Accordingly, the NFSv4 client  1000  is unable to access to the file systems  2321  and  2322  until the backup data on NAS 3   2003  is recovered to NAS 1   2001 . 
         [0047]    Under the example described above, a two-node clustering (NAS 1  and NAS 2  make a cluster) arrangement and a two-node replication scenario (NAS 1  and NAS 3  make a pair, and NAS 2  and NAS 4  make a pair) is employed. However, it should be understood that the invention can be applied to more than two-node clustering and replication. In that case, the IP address can be taken over to one of the other nodes within a cluster. Further, it should be noted that the failover process changes a mapping between IP address and MAC (Media Access Control) address of a NAS front end port. To do that, a NAS sends a Gratuitous-ARP (Address Resolution Protocol) request for the IP address in order to replace the ARP cache for NAS clients, LAN switches, and the like. The gratuitous ARP message is in layer  2  (data link), which is not possible to distribute across routers, which handle layer  3  (network) in the conventional open system interconnection model. Accordingly, typical failover from a NAS on one subnet to a NAS on another subnet cannot be carried out. 
         [0048]      FIG. 6  illustrates a conceptual diagram of a first embodiment of the invention. The invention employs the migration and redirection mechanisms provided by the NFSv4 protocol to solve the problem described with reference to  FIG. 5 . In order to make the explanation of the invention simpler, an exported file system created on the NAS 2   2002 , such as “export3”  2323  in  FIG. 5 , is eliminated from the example of  FIG. 6 , but it should be understood that the invention could be equally applied to any number of NAS systems in a clustered NAS arrangement having any number of file systems. As illustrated in  FIG. 6 , file systems  2321  and  2322  are created for exporting to the NFSv4 client  1012 . An NFSv4 client  1012  on NAS client  1000  mounts the file systems exported by NAS 1  “export1”  2321  as “/mnt/1” and “export2”  2322  as “/mnt/2”. Following this, application software  1011  is able see a directory tree that appears as a virtually complete directory tree  1700 . For creating a remote data backup, the exported file systems  2321  and  2322  are replicated to NAS 3   2003 , so that the file system  2321  is replicated to a file system  2324  on NAS 3   2003  and the file system  2322  is replicated to a file system  2325 . NAS 1   2001  and NAS 2   2002  are located on the same subnet  2500  and make up a first clustered NAS system  201 . NAS 3   2003  is located on the subnet  2700 , and may form part of a second clustered NAS system  202  with NAS 4   2004 , which may be at a remote location from the first clustered NAS system  201 . The subnets  2500  and  2700  are different subnets able to communicate via network equipment such as via router  2600 . NAS 1   2001  and NAS 2   2002  make a cluster  201  which enables IP address takeover should one of NAS 1   2001  or NAS 2   2002  fail. Further, while the clustered NAS systems  201 ,  202  in the example are shown as having only two NAS nodes in each cluster, it should be understood that a larger number of NAS nodes may be used to form each cluster and not adversely affect the functionality of the invention. 
         [0049]    To initiate remote copy, management software  1111  on a management host  1100 , or other authorized source, issues a replication request command to NAS manager  2161  on NAS 1   2001  and provides parameters such as the source file system a destination node and a destination file system. In the present example, the parameters are as follows: replication( 2321 , NAS 3 ,  2324 ) and replication( 2322 , NAS 3 ,  2325 ). NAS manager  2161 , other replication software on NAS 1   2001 , or other known means can be used to manage the replication operations. Thus, the file system  2321  on NAS 1   2001  serves as a source (primary) file system for the portion of the file system having the directory tree “export1” under it and the file system  2324  on NAS  3  serves as the destination (secondary or mirror) file system for this portion of the file system, such that file systems  2321  and  2324  make up a replication pair. Similarly, the file system  2322  on NAS 1   2001  serves as a source (primary) file system for the portion of the file system having the directory tree “export2” under it, and the file system  2325  on NAS 3   2003  serves as the destination (secondary) file system for this portion of the file system, such that file systems  2322  and  2325  make up a second replication pair. Further, while the file system of the present example is illustrated as being split among two volumes on the same NAS system, it should be understood that multiple portions of the file system may exist on a single volume, or the file system may be split among any number of volumes on any number of NAS systems within a NAS cluster. It should be further understood that multiple file systems may coexist on a NAS cluster or NAS system and that some or all may implement the present invention. 
         [0050]    Under the invention, when the replication initialization process is completed, the NAS manager  2161  on NAS 1   2001  sends a request for the creation of a referral file to NAS manager  2171  on NAS 2   2002 . NAS manager  2171  on NAS 2   2002  requests the NFSv4 server  2131  on NAS 2   2002  to create a referral file “NAS3:/export1” for “/export1” a file system  2331  and “NAS3:/export2” for “/export2” a file system  2332 . While the file systems  2331  and  2332  are illustrated as separate file systems in  FIG. 6  for ease of discussion, physically the referral files can be stored in a single file system on NAS 2 , i.e., with the directories being in the same file system. Further it is assumed that the same file or directory name does not exist on NAS 2 , or that some naming rule may be applied to the entire clustered NAS systems  201 ,  202  to avoid creating the same names for export directories, such as using NAS node names with the directory name. By creating these referral files on NAS 2   2002 , the file systems  2321  and  2322  will appear on NAS 2   2002  as having been migrated to NAS 3   2003  should a failover from NAS 1   2001  to NAS 2   2002  occur. This will utilize the NFSv4 redirection mechanism for NFSv4 clients  1012  to cause them to access the remote replicated data without their realizing that NAS 1   2001  has failed and that IP address takeover has occurred in response to the failover. 
         [0051]      FIG. 7  illustrates a flow of the replication process for creating the referral files on NAS 2   2002  that includes the following steps: 
         [0052]    Step  4000 : The file system is initially replicated to the designated mirror secondary file system in the second cluster  202  that will form replication pairs with the primary file system in the first cluster  201 . The replication of file systems takes place until all designated data have been replicated from the primary file systems to their secondary file systems. 
         [0053]    Step  4001 : After the file system replication process is completed, the NAS Manager  2161  requests the creation of referral files for each replicated file system. In the system illustrated in  FIG. 6 , a referral creation request for “/export1” file system  2321  is sent to NAS Manager  2171  on NAS 2   2002  with parameters of replication destination node and directory. Thus, here NAS 2   2002  is asked to create the referral. However, this does not mean that the invention is restricted by the number of nodes, and it is possible to configure more than two nodes to make up cluster  201 . In this case, one of the clustered nodes will take over the IP address of NAS 1   2001  following a failure, and the referral should be made on the node that will take over the IP address, or the referral should be made on all of the nodes in the cluster  201  in the same way. 
         [0054]    Step  4002 : NAS manager  2171  on NAS 2   2002  receives the request. 
         [0055]    Step  4003 : NAS manager  2171  creates a new file system for “export1” 
         [0056]    Step  4004 : NAS manager  2171  locally mounts the file system as “/export1” or just makes a directory for “export1”. 
         [0057]    Step  4005 : NAS manager  2171  asks the NFSv4 server to create a referral file for the file system such as “NAS3:/export1” that refers to the remote NAS in which the original file system is mirrored. Physically the referral file can be stored in a disk drive on NAS 2 . 
         [0058]    Step  4006 : The same procedure is applied to the file system  2332 . 
         [0059]    Failover Processing Under the Invention 
         [0060]      FIG. 8  illustrates a conceptual diagram of failover processing according to the invention. The system configurations are the same as in  FIG. 6  described above. When a failure occurs on NAS 1   2001 , the IP address of NAS 1   2001  is taken over by NAS 2   2002 . Then, when the NFSv4 client  1012  attempts to access the data on NAS 1 , such as reading a file “f” in the file system  2322  using a command such as “READ(/mnt/2/b/f)”, the NFS operation is sent to the NFSv4 server  2131  on NAS 2   2002  rather than to NAS 1   2001 . The NFSv4 server  2131  on NAS 2   2002  receives the request and processes it. The NFSv4 server  2131  looks up the directory “/export2” and finds the referral file “NAS3:/export2”. Then, NFSv4 server  2131  on NAS 2   2002  returns the location information (“fs_location”) to the NFSv4 client  1012  on NAS client  1000 . The NFSv4 client  1012  redirects the operation to the NFSv4 server  2141  on NAS 3  according to the location information received from NAS 2 . NFSv4 server  2141  on NAS 3  looks up the directory and opens the file “/export2/b/f” on the mirror file system  2325 . 
         [0061]      FIG. 9  illustrates a process flow for when a failure occurs on NAS 1 , which includes the following steps: 
         [0062]    Step  5000 : When a failure occurs on NAS 1 , the IP address of NAS 1   2001  is taken over by NAS 2 . 
         [0063]    Step  5001 : The NFSv4 client attempts to access data in the file system  2322  on NAS 1 , such as by issuing a command “READ(/mnt/2/b/f)”. 
         [0064]    Step  5002 : Because of the IP address takeover, the client operation is sent to the NFSv4 server  2131  on NAS 2 . The NFSv4 client looks up the mount point “/export2” first, by issuing a compound procedure containing PUTROOTFH, LOOKUP, GETATTR, and GETFH operations. 
         [0065]    The PUTROOTFH operation instructs the server (NAS 2 ) to set the “current” file handle to the root of the server&#39;s file tree. The root file handle is one of the special file handles in the NFS version 4 protocol. The root file handle is the “conceptual” root of the file system name space at the NFS server. The client uses or starts with the root file handle by employing the PUTROOTFH operation. Once this PUTROOTFH operation is used, the client can then traverse the entirety of the server&#39;s file tree with the LOOKUP operation. 
         [0066]    The LOOKUP operation instructs the server to look up a specified file name or directory name on the server, and return the file handle for the file or the directory. In this case, the NAS 2  server is instructed to look up the directory “export2”. 
         [0067]    The GETATTR operation obtains attributes for the file system object specified by the current file handle such as file type or file size. 
         [0068]    The GETFH operation obtains the current file handle. 
         [0069]    Step  5003 : The NFSv4 server  2131  on NAS 2   2002  receives the compound procedure from the NFSv4 client, and processes the procedure by looking up the directory “/export2”. The NFSv4 server  2131  looks up the directory “/export2” and finds the referral file “NAS3:/export2”. Then, NFSv4 server  2131  returns an error message “NFS4ERR_MOVED” and the location of the file system in an attribute of “fs_location” to the NFSv4 client. The “fs_location” is a structure containing the referral information. In this case, the “fs_location” contains the referral information “NAS3:/export2”. In reference to this information, the NFSv4 client  1000  can determine to which location it has to redirect the operations. In this case, the NFSv4 client  1000  can determine from the referral information that the desired file is located in “NAS3:/export2”. 
         [0070]    Step  5004 : The NFSv4 client  1000  redirects the operation to the NFSv4 server  2141  on NAS 3   2003  based on the information in the fs_location attribute received at step  5003 . Thus, the NFSv4 client  1000  looks up the directory “/export2” on NAS 3  using the same compound procedure described above in step  5002 . 
         [0071]    Step  5005 : The NFSv4 server  2141  on NAS 3  successfully looks up “/export2” and returns a file handle of the directory “/export2”. 
         [0072]    Step  5006 : The NFSv4 client requests to open and read the file “b/f” on NAS 3  according to the file handle received at step  5005  by issuing a second compound operation that contains PUTFH, OPEN, GETFH, and READ operations. 
         [0073]    The PUTFH operation replaces the current file handle with the file handle provided as an argument of the operation (in this case, the file handle for “/export2” is provided by the client). 
         [0074]    The OPEN operation instructs the NAS 3  server to open the file with the file handle provided as an argument of the operation (in this case, the file handle for “/export2/b/f” is provided). 
         [0075]    The READ operation instructs the server to read the file and return the data to the client. 
         [0076]    Step  5007 : The NFSv4 server  2141  on NAS 3  successfully opens and reads the file “b/f”. Thus, the procedure described above enables the NFSv4 client to transparently continue to access the file system during the failover period by accessing a backup copy of the file system on a different clustered NAS system. 
         [0077]    Failback Process Under the Invention 
         [0078]      FIG. 10  illustrates a conceptual diagram of a failback process according to the present invention. The system configurations in this example are the same as in the example discussed above in  FIGS. 6 and 8 . After finishing the hardware recovery of NAS 1 , such as by the replacement of hard disk drives, other parts, or the whole of the NAS 1   2001 , the software recovery starts. During the software recovery, the file systems are recovered to file systems  2321  and  2322 . In this case, the mirrored file systems  2324  and  2325  on NAS 3  are copied back to file systems  2321  and  2322 , respectively, on NAS 1 . After completion of copying back the mirrored file system copies, the file system recovery is completed when the local NAS manager  2161  on NAS 1   2001  mounts the recovered file systems. Then, the NFSv4 client  1012  should begin again to access NAS 1   2001  even though it has been redirecting NFS operations to NAS 3   2003  during the failover period. There are two possible situations that are described below: (1) the case wherein there was an access to the mirrored file system during the failover/failback period, and (2) the case in which there were no accesses to the file system during the failover/failback period. 
         [0079]      FIG. 11  illustrates a process flow of the case in which there was an access to the mirrored file system during the failover period, such as an access to the directory “/export2”  2325 , which includes the following steps: 
         [0080]    Step  6000 : Following IP address takeover, the failed NAS system, NAS 1   2001 , is repaired or replaced. After finishing the hardware recovery of NAS 1 , such as by the replacement of hard disk drives, other parts, or the entire NAS system, and the recovery of the operating system, NAS 1   2001  reboots and takes back the IP address from NAS 2   2002 . 
         [0081]    Step  6001 : At some point in time after the reboot, the management software  1111  on management host  1100  issues a recovery operation to NAS manager  2161  on NAS 1   2001  for recovering the file systems “/export1”  2324  and “/export2”  2325  from NAS 3   2003 . NAS managers  2162 ,  2181  or other software on NAS 1   2001  and NAS 3  can take care of the file system recovery process. 
         [0082]    Step  6002 : During the file system recovery, when NFSv4 client attempts to accesses the directory “/export2” the operation is still sent to the “/export2” file system copy  2325  on NAS 3 , so that accesses to the file systems originally on NAS 1   2001  continue to be redirected to NAS 3  during the failback recovery period. 
         [0083]    Step  6003 : If, for example, there is a read access to the file system “/export2” during the recovery period, NFSv4 server  2141  on NAS 3   2003  reads the data from the file system  2325  using normal NFSv4 read operations. If there is a write access to the file system “/export2” during the recovery, NFSv4 server  2141  on NAS 3  writes the data to the file system  2325  on NAS 3  using normal NFSv4 write operations. In addition, the NAS Manager  2181  or some other replication module on NAS 3   2003  mirrors the write data to NAS 1   2001 . Another option is for NAS manager  2181  or other software on NAS 3  to store the write operations and send the write operations to the NFSv4 server on NAS 1   2001  after finishing the recovery process. Still another option is to block all write NFS operations to the failed file system during the failover and failback recovery period. 
         [0084]    Step  6004 : After the completion of copying back the mirrored file systems to NAS 1 , and NAS manager  2161  on NAS 1   2001  locally mounts the file systems  2321  and  2322 , NFSv4 client should now be ready to begin accessing NAS 1   2001  instead of NAS 3   2003 . 
         [0085]    Step  6005 : There can be two options for accomplishing the switch back to accessing NAS 1 . First, NAS manager  2181  or some other replication module on NAS 3  can request that the NFSv4 server  2141  on NAS 3  create a referral for the recovered file system, such as for file system “/export2”  2325 , creating a referral as “NAS1:/export2”. Physically the referral file can be stored in a disk drive on NAS 3 . Then, when remote copy is restarted, a different file system name can be used for the mirrored file system to enable client access. Alternatively, NAS manager  2181  can request that the NFSv4 server  2141  on NAS 3  return an NFS4ERR_STALE error when the NFSv4 client uses the current file handle. This will cause the NFSv4 client to revert to the original file handle on NAS 1  by obtaining a new file handle for the file system. These operations are invoked by the request from NAS manager  2161  on NAS 1   2001  or NAS manager  2163  on NAS 3  at the completion of the mirrored file system recovery stage. 
         [0086]    Step  6006 : Then, the next time that the NFSv4 client accesses the file system, for example by a “READ(/mnt/2/b/f)” command, since there is still a file handle on the client, NFSv4 client  1012  sends the operation to NAS 3 . 
         [0087]    Step  6007 : When NFSv4 server on NAS 3  receives the operation to the recovered file system, NAS 3  returns the referral file or the NFS4ERR_STALE error. 
         [0088]    Step  6008 : Then, NFSv4 client again accesses the file systems on NAS 1   2001  rather than on NAS 3 , and the operations carried out thereafter are the same as the normal file access operations under NFSv4. 
         [0089]      FIG. 12  illustrates a process flow in the case that there was no access to the mirrored file system during the failover/failback period, such as to the file system “/export1”  2324 . The process flow includes the following steps: 
         [0090]    Step  7000 : After finishing the hardware recovery of NAS 1 , such as by the replacement of disks, other parts, or the entire NAS 1  system, and the recovery of the operating system, NAS 1   2001  reboots and takes back the IP address from NAS 2 . 
         [0091]    Step  7001 : At some time after the reboot, the management software  1111  on management host  1100  issues a recovery operation to NAS manager  2161  on NAS 1   2001  for the file systems  2324  and  2325  on NAS 3 . NAS managers  2161 ,  2181  or other software on NAS 1   2001  and NAS 3  can perform the file system recovery process. 
         [0092]    Step  7002 : In order to redirect NFS operations to the mirrored site, NAS 3  during the recovery, NAS manager  2161  requests that the NFSv4 server  2121  create a referral file for the exported file systems such as  2321  “/export1” as “NAS3:/export1” and  2322  “/export2” as “NAS3:/export1” by using the file system mirror destination information, including destination node and location, stored on NAS 1   2001  by NAS manager  2161  or some replication software on NAS 1 . This is similar to Steps  4001 - 4005  except that it is carried out on NAS 1   2001 , and physically the referral files can be stored in a disk drive on NAS 1 . 
         [0093]    Step  7003 : Once this is accomplished, then, during the file system recovery, when NFSv4 client  1012  attempts to access the directory “/export1” for the first time, the operation is sent to NAS 1 , because there was no access to the file system “/export1” during the failover period and there is no redirection information on the NFSv4 client  1012 . 
         [0094]    Step  7004 : NFSv4 server on NAS 1   2001  returns the redirection information set at Step  7002 , which is similar to Step  5003  discussed above. 
         [0095]    Step  7005 : Then, the NFS operations are redirected to NAS 3 , which is the same as Step  6002  discussed above. 
         [0096]    Step  7006 : The NFS operations are processed by NFSv4 server  2141  on NAS 3 , which is the same as Step  6003  discussed above. 
         [0097]    Step  7007 : After the completion of copying back the mirrored file systems to NAS 1 , the NAS manager  2161  on NAS 1   2001  requests the NFSv4 server  2121  to delete the referral files created at Step  7002 , and then, NAS Manager  2161  locally mounts the recovered file systems. 
         [0098]    Step  7008 : Like  6005 , on NAS 3 , in order to redirect the NFS operations back to NAS 1 , NAS Manager  2181  or some other replication module on NAS 3  asks the NFSv4 server on NAS 3  to create a referral for the recovered file system “/export1”  2324  as “NAS1:/export1”. Physically the referral file can be stored in a disk drive on NAS 3 . In another method, NAS Manager  2181  requests the NFSv4 server on NAS 3  to return an NFS4ERR_STALE error when the NFSv4 client will use the current file handle. 
         [0099]    Steps  7009 - 7011 : NFSv4 client returns to being able to access the file system such as “READ(/mnt/1/a/c)” in the same way as described for steps  6006 - 6008  above. 
         [0100]    Thus, it may be seen that the present invention sets forth a system and method by which NFSv4 clients are provided with a failover method for remotely mirrored clustered NAS file servers. The invention provides a mechanism of redirecting the NFS operations to the remotely mirrored site upon failure of a NAS. Accordingly, an NFSv4 client can transparently access the remotely mirrored file system during the failover/failback period. Further, while specific embodiments have been illustrated and described in this specification, those of ordinary skill in the art appreciate that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments disclosed. This disclosure is intended to cover any and all adaptations or variations of the present invention, and it is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Accordingly, the scope of the invention should properly be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled.