NFS

It is important to know them NFS export and mount options especially when you are facing a performance issue or a functional issue with the NFS mount over network.

Basic command

Commands	Description	Command on
# exportfs -r	Re-export your shares	Server
# exportfs -a	Export your shares	Server
# exportfs -v	Verify the NFS Share permissions	Server
$nfsstat -m	Verify Current NFS Mount Options	Client

NFS export on Server

NFS exports options are the permissions that is applied on NFS Server when we create a NFS Share under /etc/exports

Here are most common(important) options that administrator must understand, full list of options are available on the man pages

Export Options	NFS Server	Default
secure/insecure	NFSv4 only uses port 2049 while to check the list of ports used by NFSv3 use in server port With secure the port number from which the client requests a mount must be lower than 1024. To allow client any available free port use insecure in the NFS share	secure
rw/ro	ro means read-only access to the NFS Share rw means read write access to the NFS Share
root_squash/no_root_squash	squash literally means to squash (destroy) the power of the remote root user. root_squash prevents remote root users from having superuser (root) privileges on remote NFS-mounted volumes. no_root_squash allows root user on the NFS client host to access the NFS-mounted directory with the same rights and privileges that the superuser would normally have
all_quash/no_all_squash	all_squash will map all User IDs (UIDs) and group IDs (GIDs) to the anonymous user. all_squash is useful for NFS-exported public FTP directories, news spool directories	no_all_squash
sync/aysnc	sync reply to requests are done only after the changes have been committed to stable storage async allows the NFS server to violate the NFS protocol and reply to requests before any changes made by that request have been committed to stable storage Using aysnc option usually improves performance, but at the cost that an unclean server restart (i.e. a crash) can cause data to be lost or corrupted

Check exports list and options

#with following set
$ cat /etc/exports 
/nas * (rw,sync,no_root_squash,no_subtree_check)

$ rpcinfo -p | grep -i nfs
    100003    3   tcp   2049  nfs
    100003    4   tcp   2049  nfs
    100003    3   udp   2049  nfs
    
# detailed export status with default export options
$ sudo exportfs -v
/nas   <world>(ro,wdelay,root_squash,no_subtree_check,sec=sys,ro,secure,root_squash,no_all_squash)

NFS mount on Client

# mount -t nfs -o [options] remote:/nfs /mount

Mount Options	NFS Client	Default	Notes
nfsvers=n or vers=n	The version of the NFS protocol to use. By default, the local NFS client will attempt to mount the file system using NFS version 3. If the NFS server does not support version 3, the file system will be mounted using version 2. If you know that the NFS server does not support version 3, specify vers=2, and you will save time during the mount, because the client will not attempt to use version 3 before using version 2	3
rw (read/write) / ro (read-only)	Use rw for data that users need to modify. In order for you to mount a directory read/write, the NFS server must export it read/write. Use ro for data you do not want users to change. A directory that is automounted from several servers should be read-only, to keep versions identical on all servers.	rw.
suid / nosuid	Specify suid if you want to allow mounted programs that have setuid permission to run with the permissions of their owners, regardless of who starts them. If a program with setuid permission is owned by root, it will run with root permissions, regardless of who starts it. Specify nosuid to protect your system against setuid programs that may run as root and damage your system.	suid
hard / soft	Specify hard if users will be writing to the mounted directory or running programs located in it. When NFS tries to access a hard-mounted directory, it keeps trying until it succeeds or someone interrupts its attempts. If the server goes down, any processes using the mounted directory hang until the server comes back up and then continue processing without errors. Interruptible hard mounts may be interrupted with CTRL-C or kill (see the intr option, later). Specify soft if the server is unreliable and you want to prevent systems from hanging when the server is down. When NFS tries to access a soft-mounted directory, it gives up and returns an error message after trying retrans times (see the retrans option, later). Any processes using the mounted directory will return errors if the server goes down.	hard
nconnect=<value>	NFS-over-TCP mount for one or more NFS shares from an individual NFS server, the traditional behavior is that all those mounts share one TCP connection if they are using the same NFS protocol version.  In cases of high NFS work load at the client, this connection sharing may result in lower performance or unnecessary bottlenecks.	16	In Linux kernel 5.3 (and higher), the nconnect option allows multiple TCP connections for a single NFS mount.
intr / nointr	Specify intr if users are not likely to damage critical data by manually interrupting an NFS request. If a hard mount is interruptible, a user may press [CTRL]-C or issue the kill command to interrupt an NFS mount that is hanging indefinitely because a server is down. Specify nointr if users might damage critical data by manually interrupting an NFS request, and you would rather have the system hang while the server is down than risk losing data between the client and the server.	intr	In Linux kernel 2.6.25 (and higher), the intr and nointr mount options are deprecated. If you use the hard option on modern Linux kernels, you must use the kill -9 (SIGKILL) command to interrupt a stuck NFS mount.
fg (foreground) / bg (background)	Specify fg for directories that are necessary for the client machine to boot or operate correctly. If a foreground mount fails, it is retried again in the foreground until it succeeds or is interrupted. All automounted directories are mounted in the foreground; you cannot specify the bg option with automounted directories. Specify bg for mounting directories that are not necessary for the client to boot or operate correctly. Background mounts that fail are re-tried in the background, allowing the mount process to consider the mount complete and go on to the next one. If you have two machines configured to mount directories from each other, configure the mounts on one of the machines as background mounts. That way, if both systems try to boot at once, they will not become deadlocked, each waiting to mount directories from the other. The bg option cannot be used with automounted directories.	fg
devs / nodevs	Specify devs if you are mounting device files from a server whose device files will work correctly on the client. The devs option allows you to use NFS-mounted device files to read and write to devices from the NFS client. It is useful for maintaining a standard, centralized set of device files, if all your systems are configured similarly. Specify nodevs if device files mounted from a server will not work correctly for reading and writing to devices on the NFS client. The nodevs option generates an error if a process on the NFS client tries to read or write to an NFS-mounted device file.	devs
timeo=n	The timeout, in tenths of a second, for NFS requests (read and write requests to mounted directories). If an NFS request times out, this timeout value is doubled, and the request is retransmitted. After the NFS request has been retransmitted the number of times specified by the retrans option, a soft mount returns an error, and a hard mount retries the request. The maximum timeo value is 30 (3 seconds). – Try doubling the timeo value if you see several servers not responding messages within a few minutes. This can happen because you are mounting directories across a gateway, because your server is slow, or because your network is busy with heavy traffic.	7
retrans=n	The number of times an NFS request (a read or write request to a mounted directory) is retransmitted after it times out. If the request does not succeed after n retransmissions, a soft mount returns an error, and a hard mount retries the request. Increase the retrans value for a directory that is soft-mounted from a server that has frequent, short periods of downtime. This gives the server sufficient time to recover, so the soft mount does not return an error	4
retry=n	The number of times the NFS client attempts to mount a directory after the first attempt fails. If you specify intr, you can interrupt the mount before n retries. However, if you specify nointr, you must wait until n retries have been made, until the mount succeeds, or until you reboot the system. If mounts are failing because your server is very busy, increasing the retry value may fix the problem	1
rsize=n	The number of bytes the NFS client requests from the NFS server in a single read request. If packets are being dropped between the client and the server, decrease rsize to 4096 or 2048. To find out whether packets are being dropped, issue the “nfsstat -rc” command at the HP-UX prompt. If the timeout and retrans values returned by this command are high, but the badxid number is close to zero, then packets are being dropped somewhere in the network.	8192
wsize=n	The number of bytes the NFS client sends to the NFS server in a single write request. If packets are being dropped between the client and the server, decrease wsize to 4096 or 2048. To find out whether packets are being dropped, issue the “nfsstat -rc” command at the HP-UX prompt. If the timeout and retrans values returned by this command are high, but the badxid number is close to zero, then packets are being dropped somewhere in the network.	8192
O (Overlay mount)	Allows the file system to be mounted over an existing mount point, making the underlying file system inaccessible. If you attempt to mount a file system over an existing mount point without the -O option, the mount will fail with the error device busy. Caution: Using the -O mount option can put your system in a confusing state. The -O option allows you to hide local data under an NFS mount point without receiving any warning. Local data hidden beneath an NFS mount point will not be backed up during regular system backups. On HP-UX, the -O option is valid only for NFS-mounted file systems. For this reason, if you specify the -O option, you must also specify the -F nfs option to the mount command or the nfs file system type in the /etc/fstab file.	The default value is not specified
remount	If the file system is mounted read-only, this option remounts it read/write. This allows you to change the access permissions from read-only to read/write without forcing everyone to leave the mounted directory or killing all processes using it	The Default value is not specified
noac	If specified, this option prevents the NFS client from caching attributes for the mounted directory. Specify noac for a directory that will be used frequently by many NFS clients. The noac option ensures that the file and directory attributes on the server are up to date, because no changes are cached on the clients. However, if many NFS clients using the same NFS server all disable attribute caching, the server may become overloaded with attribute requests and updates. You can also use the actimeo option to set all the caching timeouts to a small number of seconds, like 1 or 3. If you specify noac, do not specify the other caching options.	The Default value is not specified
nocto	If specified, this option suppresses fresh attributes when opening a file. Specify nocto for a file or directory that never changes, to decrease the load on your network	The Default value is not specified
acdirmax=n	The maximum number of seconds a directory’s attributes are cached on the NFS client. When this timeout period expires, the client flushes its attribute cache, and if the attributes have changed, the client sends them to the NFS server. For a directory that rarely changes or that is owned and modified by only one user, like a user’s home directory, you can decrease the load on your network by setting acdirmax=120 or higher	60
acdirmin=n	The minimum number of seconds a directory’s attributes are cached on the NFS client. If the directory is modified before this timeout expires, the timeout period is extended by acdirmin seconds. For a directory that rarely changes or that is owned and modified by only one user, like a user’s home directory, you can decrease the load on your network by setting acdirmin=60 or higher	30
acregmax=n	The maximum number of seconds a file’s attributes are cached on the NFS client. When this timeout period expires, the client flushes its attribute cache, and if the attributes have changed, the client sends them to the NFS server. For a file that rarely changes or that is owned and modified by only one user, like a file in a user’s home directory, you can decrease the load on your network by setting acregmax=120 or higher	60
actimeo=n	Setting actimeo to n seconds is equivalent to setting acdirmax, acdirmin, acregmax, and acregmin to n seconds. Set actimeo=1 or actimeo=3 for a directory that is used and modified frequently by many NFS clients. This ensures that the file and directory attributes are kept reasonably up to date, even if they are changed frequently from various client locations. Set actimeo=120 or higher for a directory that rarely or never changes. If you set the actimeo value, do not set the acdirmax, acdirmin, acregmax, or acregmin values	The Default value is not specified
grpid	Forces a newly created file in the mounted file system to inherit the group ID of the parent directory. By default, a newly created file inherits the effective group ID of the calling process, unless the GID bit is set on the parent directory. If the GID bit is set, the new file inherits the group ID of the parent directory	The Default value is not specified
lock / nolock	Selects whether to use the NLM sideband protocol to lock files on the server. If neither option is specified (or if lock is specified), NLM locking is used for this mount point. When using the nolock option, applications can lock files, but such locks provide exclusion only against other applications running on the same client. Remote applications are not affected by these locks. NLM locking must be disabled with the nolock option when using NFS to mount /var because /var contains files used by the NLM implementation on Linux. Using the nolock option is also required when mounting exports on NFS servers that do not support the NLM protocol.
local_lock=mechanism	Specifies whether to use local locking for any or both of the flock and the POSIX locking mechanisms. mechanism can be one of all, flock, posix, or none. The Linux NFS client provides a way to make locks local. This means, the applications can lock files, but such locks provide exclusion only against other applications running on the same client. Remote applications are not affected by these locks. If all is specified, the client assumes that both flock and POSIX locks are local. If flock is specified, the client assumes that only flock locks are local and uses NLM sideband protocol to lock files when POSIX locks are used. If posix is specified, the client assumes that POSIX locks are local and uses NLM sideband protocol to lock files when flock locks are used. To support legacy flock behavior similar to that of NFS clients < 2.6.12, use Samba as Samba maps Windows share mode locks as flock. Since NFS clients > 2.6.12 implement flock by emulating POSIX locks, this will result in conflicting locks.	none	This option is supported in kernels 2.6.37 and later. NOTE: When used together, the 'local_lock' mount option will be overridden by 'nolock'/'lock' mount option.

Recommended Options

• In Linux kernel 5.3 (and higher), the nconnect option allows multiple TCP connections for a single NFS mount. Note: Currently, the maximum number of concurrent TCP connections is 16.
• In Linux kernel 2.6.25 (and higher), the intr and nointr mount options are deprecated. If you use the hard option on modern Linux kernels, you must use the kill -9 (SIGKILL) command to interrupt a stuck NFS mount.

Client with Server-Side Network Lock Manager (NLM) Enabled	mount -t nfs -o rsize=65536,wsize=65536,intr,hard,tcp,rdirplus,readahead=128 server:/path mountpath
Client with Local Locking Enforced	mount -t nfs -o rsize=65536,wsize=65536,intr,hard,tcp,locallocks,rdirplus,readahead=128 \ server:/path mountpath