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


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

  - Create Directory
  - Edit /etc/exports
  - Reload the exports

So let's get started:


<sxh>
root@nfs-server:/backups/mongo# mkdir /backups/mongo/
root@nfs-server:/backups/mongo#
</sxh>


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

<sxh bash>
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#
</sxh>

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

<sxh bash>
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)
</sxh>

And reload the export rules:
<sxh bash>
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#
</sxh>


======Set up NFS Client======
On the client we have to do practically following:


  - Edit /etc/fstab
  - Mount the NFS

<sxh bash>
[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]#
</sxh>


And Mount the file system:


<sxh bash>
[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
</sxh> 

======Errors======
Errors which you can have is:

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


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

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


===Edit the /etc/exports===
<sxh bash>
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#
</sxh>

===Check for possible problems===
You can check if there are problems with the mount using verbose mode:

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


======Unmount NFS======
To unmount NFS, you can use the standard:
<sxh bash>
umount /apphome/backup/mongo
</sxh>

However in a lot of cases, you cannot do that because, the NFS isn't responsible or the device is busy. You have to use the force option:

===Force===
<sxh bash>
umount -f /apphome/backup/mongo
</sxh>

===Lazy===
However, the upper command relies that the NFS is responsible up to some amount and that isn't always teh case. In such cases, we can do so called Lazy unmount which will unmount the FS now and clean the slots when the NFS becoms available again:

<sxh bash>
umount -l /apphome/backup/mongo
</sxh>

===Really force===
In some cases, we really need to shutdown the NFS and kick everyone and burn the server to the ground :)
For that we can use:

<sxh bash>
fuser -ks /apphome/backup/mongo
</sxh>

That will kick all the processes from that file system and most probably kick you from the server as well.