NFS allows for sharing a directory between two or more servers. It is needed in case you want to share files between servers.

Setting up NFS server is rather simple, you have to:

1. Create Directory
2. Edit /etc/exports
3. Reload the exports

So let's get started:

root@nfs-server:/backups/mongo# mkdir /backups/mongo/
root@nfs-server:/backups/mongo#

Once you create the directory, you have to edit the export file:

root@nfs-server:/backups/mongo# cat /etc/exports
/backups/oracle             amg-cx-odb1(rw,sync)
/backups/mongo              client-server(rw,sync)
root@nfs-server:/backups/mongo#

You can have many clients accessing the same server directory but you have to specify different fsid as foolows:

root@tain-cx-backup1:/backups# cat /etc/exports
/backups/oracle             server(rw,sync)
/backups/mongo              serverA(rw,sync,fsid=1)
/backups/mongo              serverB(rw,sync,fsid=2)

And reload the export rules:

root@nfs-server:/backups/mongo# exportfs -ra
exportfs: /etc/exports [1]: Neither 'subtree_check' or 'no_subtree_check' specified for export "amg-cx-odb1:/backups/oracle".
  Assuming default behaviour ('no_subtree_check').
  NOTE: this default has changed since nfs-utils version 1.0.x

exportfs: /etc/exports [2]: Neither 'subtree_check' or 'no_subtree_check' specified for export "client-server:/backups/mongo".
  Assuming default behaviour ('no_subtree_check').
  NOTE: this default has changed since nfs-utils version 1.0.x

root@nfs-server:/backups/mongo#

On the client we have to do practically following:

1. Edit /etc/fstab
2. Mount the NFS

[root@client-server log]# cat /etc/fstab

#
# /etc/fstab
# Created by anaconda on Thu Jan 26 16:24:16 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=8333d912-ce24-4819-bd32-940e6cca13e0 /                       xfs     defaults        0 0
UUID=cf0dbc16-627e-4909-bdb3-c02793e93418 /boot                   xfs     defaults        0 0
UUID=e01dacfa-e912-420d-908c-fe5481eb8145 swap                    swap    defaults        0 0

/dev/mapper/mongovg-mongllv1    /var/mongodb/db ext4    defaults        0 0

nfs-server:/backups/mongo  /apphome/backup/mongo   nfs      rw,sync,hard,intr  0 0 <- This line
[root@client-server log]#

And Mount the file system:

[root@client-server log]# mount /apphome/backup/mongo
[root@client-server log]# df
Filesystem                      1K-blocks       Used  Available Use% Mounted on
/dev/sda3                        16006144    2520376   13485768  16% /
devtmpfs                          1922740          0    1922740   0% /dev
tmpfs                             1932524         12    1932512   1% /dev/shm
tmpfs                             1932524       8672    1923852   1% /run
tmpfs                             1932524          0    1932524   0% /sys/fs/cgroup
/dev/sda1                         1038336     172520     865816  17% /boot
/dev/mapper/mongovg-mongllv1    192942346     360476  185379394   1% /var/mongodb/db
tmpfs                              386508          0     386508   0% /run/user/10177
tmpfs                              386508          0     386508   0% /run/user/10163
nfs-server:/backups/mongo 6289912064 2255692288 4034219776  36% /apphome/backup/mongo <- New NFS

Errors which you can have is:

Apr  5 10:57:12 tain-cx-mdb1 snmpd[811]: Cannot statfs /apphome/backup/mongo#012: Stale file handle

This means that the header isn't correct, there are 2 ways to resolve this:

1. umount/mount
2. In case you use XFS anywhere in the configuration, edit the /etc/exports as follows
3. Refresh the export rules and try to mount on the client:

root@tain-cx-backup1:/backups/mongo# cat /etc/exports
/backups/oracle             amg-cx-odb1(rw,sync)
/backups/mongo              tain-cx-mdb1(rw,sync,fsid=1)
root@tain-cx-backup1:/backups/mongo#

You can check if there are problems with the mount using verbose mode:

mount -t nfs IP_ADDR:/backups/mongo /apphome/backup/mongo -vvv -o mountproto=tcp