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Troubleshooting: Unable to access an NFS backup target

· 2 min read
James Lu

Applicable versions​

All Longhorn versions.

Prerequisite​

Longhorn supports only NFS versions 4.0, 4.1, and 4.2. Users should ensure that both their backup server and client support one of the supported versions.

How to determine which NFS version is used​

  • Server side:

Check which versions of NFS are currently enabled.

  # cat /proc/fs/nfsd/versions
  • Client side:

What versions the NFS mount is configured to support:

  # nfsstat -m

Symptom 1​

The Backup page pops up with a "No such file or directory" error, for example,

, error exit status 32: vers=4.2: Failed to execute: mount [-t nfs4 -o nfsvers=4.2 -o actimeo=1 192.168.121.170:/opt/nfs-server /var/lib/longhorn-backupstore-mounts/192_168_121_170/opt/nfs-server], output mount.nfs4: mounting 192.168.121.170:/opt/nfs-server failed, reason given by server: No such file or directory
, error exit status 32: Cannot mount using NFSv4

Possible Reason 1​

The backup target is not correct or exported directory does not exist on NFS server.

Solution​

Correct the backup target or create the directory on NFS server.

Possible Reason 2​

If the configuration file /etc/exports is as the below example:

/opt/nfs-server 192.168.121.0/24(rw,sync,no_subtree_check,crossmnt,fsid=0)

For NFSv4, the fsid=0 or fsid=root option means the exported directory is the root of all exported filesystems. The example here is /opt/nfs-server.

If users try to use the absolute path of the exported directory to list backups, that mounting error will happen.

Solution​

  1. Mount with the absolute path of the exported directory (ex: /opt/nfs-server) on the client side without the option fsid=0 or fsid=root on the server side.

  2. Mount with the path "/" on the client side still with the option fsid=0 or fsid=root on the server side.

Symptom 2​

The Backup page pops up with a permission denied error, for example,

error running create backup command: failed to create backup to nfs://192.168.121.170:/opt/nfs-server for volume test-for-backup: rpc error: code = Unknown desc = mkdir /var/lib/backupstore/192_168_121_170/opt/nfs-server/backup/longhorn/backupstore: permission denied" , error exit status 1

Reason​

The exported directory is not accessible by non-root users due to the root_squash option being used.

Solution​

  1. Use the option no_root_squash instead of root_squash in the exported directory

  2. Execute chmod o+w [exported directory path] or change the owner of the directory to nobody.

  • Related Longhorn issues:

    • https://github.com/longhorn/longhorn/issues/3576
    • https://github.com/longhorn/longhorn/discussions/3805

  • Linux Man page: exports, nfs.