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 . fig1 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 fig2 ). 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 . management host 1100 includes a memory 1102 and a cpu 1101 for storing and executing management software ( illustrated in fig2 ) 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 . 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 . 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 fig2 , 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 . 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 . 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 . 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 fig2 , 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 . 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 . fig2 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 . 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 . 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 . 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 : 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 . 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 . 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 . 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 . fig3 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 . fig4 illustrates an example of a data structure of a referral in a file system , such as the file system illustrated in fig3 , 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 fig4 , 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 . fig5 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 fig5 . 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 . 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 . 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 . fig6 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 fig5 . 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 fig5 , is eliminated from the example of fig6 , 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 fig6 , 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 . 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 . 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 fig6 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 . fig7 illustrates a flow of the replication process for creating the referral files on nas 2 2002 that includes the following steps : 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 . 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 fig6 , 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 . step 4002 : nas manager 2171 on nas 2 2002 receives the request . step 4003 : nas manager 2171 creates a new file system for “ export1 ” step 4004 : nas manager 2171 locally mounts the file system as “/ export1 ” or just makes a directory for “ export1 ”. 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 . step 4006 : the same procedure is applied to the file system 2332 . fig8 illustrates a conceptual diagram of failover processing according to the invention . the system configurations are the same as in fig6 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 . fig9 illustrates a process flow for when a failure occurs on nas 1 , which includes the following steps : step 5000 : when a failure occurs on nas 1 , the ip address of nas 1 2001 is taken over by nas 2 . 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 )”. 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 . 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 . 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 ”. the getattr operation obtains attributes for the file system object specified by the current file handle such as file type or file size . 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 ”. 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 . step 5005 : the nfsv4 server 2141 on nas 3 successfully looks up “/ export2 ” and returns a file handle of the directory “/ export2 ”. 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 . 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 ). 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 ). the read operation instructs the server to read the file and return the data to the client . 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 . fig1 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 fig6 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 . fig1 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 : 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . fig1 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 : 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 . 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 . 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 . 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 . step 7004 : nfsv4 server on nas 1 2001 returns the redirection information set at step 7002 , which is similar to step 5003 discussed above . step 7005 : then , the nfs operations are redirected to nas 3 , which is the same as step 6002 discussed above . step 7006 : the nfs operations are processed by nfsv4 server 2141 on nas 3 , which is the same as step 6003 discussed above . 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 . 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 . 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 . 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 .