How do I configure the fsid option in an NFS server's /etc/exports file?

Environment

• Red Hat Enterprise Linux 6
• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 9
• NFSv3 and NFSv4 exports

Issue

• How do I configure the fsid parameter in an NFS server's /etc/exports file?
• What does the fsid option do in NFS?

Resolution

NFSv3 and NFSv4

• The fsid option is used to uniquely identify a given export to the NFS server.

• Usually the UUID or device number is used and this is done transparently, however there may be situations where a filesystem does not have a UUID or does not reside on a device, so the NFS server must be given a unique number to identify the export.

• An example of usage would be:

/example1 *(rw,fsid=10001)
/example2 *(rw,fsid=10002)

• It is not correct to have two exports defined with the same fsid.

NFSv4

• Under NFSv4, the special fsid=0 is used to identify the root of the NFS share. A good example of this usage is given within What happens if two NFS shares are exported with the fsid=0 option?.

• It is not correct to have two exports defined with fsid=0.

Related

• How to identify the fsid of an NFS share on NFS client?

Root Cause

• From man exports:

fsid=num|root|uuid
  NFS needs to be able to identify each filesystem that it exports.  Normally it
  will use a UUID for the filesystem (if the filesystem has such a thing) or the
  device number of the device holding the filesystem (if the filesystem is
  stored on the device).

  As not all filesystems are stored on devices, and not all filesystems have
  UUIDs, it is sometimes necessary to explicitly tell NFS how to identify a
  filesystem. This is done with the fsid= option.

  For NFSv4, there is a distinguished filesystem which is the root of all
  exported filesystem. This is specified with fsid=root or fsid=0 both of which
  mean exactly the same thing.

  Other filesystems can be identified with a small integer, or a UUID which
  should contain 32 hex digits and arbitrary punctuation.

  Linux kernels version 2.6.20 and earlier do not understand the UUID setting so
  a small integer must be used if an fsid option needs to be set for such
  kernels. Setting both a small number and a UUID is supported so the same
  configuration can be made to work on old and new kernels alike.

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