NFS common considerations for replicas and referrals

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If the client encounters two different paths leading to the same data (file system), the client treats the second path as a symbolic link to the file.

For example, server A exports:

/tmp/a               -vers=4,replicas=/tmp/a@B:/tmp/a@A    
/tmp/b               -vers=4,refer=/tmp/a/b@B

And

server
B

exports:

/tmp/a               -vers=4    
/tmp/a/b             -vers=4

In this example, the client mounts / on

server
A

onto /mnt by using the command

mount
-o vers=4 A:/ /mnt

. The client user accesses /tmp/a/b on

server
B

through cd /mnt/tmp/a/b or cd /mnt/tmp/b. If the user changes the directory to cd /mnt/tmp/a/b first, then the path /mnt/tmp/b acts as a symbolic link to /mnt/tmp/a/b. In this scenario, if the user is in /mnt/tmp/b and uses the command /bin/pwd, /bin/pwd > will return /mnt/tmp/a/b.

Note: The above practice is not recommended. The administrator should set up export specifications that result in only one possible namespace path to exported data.

You can list multiple locations in referrals if the target data of the referral is actually replicated. Clients will only use the referral locations to find the referral target at an available server. Once the client establishes access to the referral target, it will obtain new location information for the found data.

Since clients may not immediately detect changes on referral location information, removing or changing referral location frequently is not recommended. When relocating the target of a reference location, it is suggested that the new location be populated along with changing the location information in the export's referral specification. The data at the old location should be kept for several hours or even days to give clients time to see and use the new location.

Both replication and referrals can only run on servers running the 64-bit kernel. Clients can run on both the 32- and 64-bit kernels.

If you are going to specify replicas as read-write, you must keep the data on the replicas coherent with the primary fileset.