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Server-side Issues

exportfs: /etc/exports:2: syntax error: bad option list

Delete all space from the option list in /etc/exports

Group/gid permissions issues

If NFS shares mount fine, and are fully accessible to the owner, but not to group members; check the number of groups that user belongs to. NFS has a limit of 16 on the number of groups a user can belong to. If you have users with more then this, you need to enable the --manage-gids start-up flag for rpc.mountd on the NFS server.

# Options for rpc.mountd.
# If you have a port-based firewall, you might want to set up
# a fixed port here using the --port option.
# See rpc.mountd(8) for more details.


"Permission denied" when trying to write files

  • If you need to mount shares as root, and have full r/w access from the client, add the no_root_squash option to the export in /etc/exports:

Client-side Issues

mount.nfs4: No such device

Check that you have loaded the nfs module

lsmod | grep nfs

and if previous returns empty or only nfsd-stuff, do

modprobe nfs

mount.nfs4: access denied by server while mounting

Check that the permissions on your client's folder are correct. Try using 755.

Unable to connect from OS X clients

When trying to connect from a OS X client, you will see that everything is ok at logs, but MacOS X refuses to mount your NFS share. You have to add insecure option to your share and re-run exportfs -r.

Unreliable connection from OS X clients

OS X's NFS client is optimized for OS X Servers and might present some issues with Linux servers. If you are experiencing slow performance, frequent disconnects and problems with international characters edit the default mount options by adding the line nfs.client.mount.options = intr,locallocks,nfc to /etc/nfs.conf on your Mac client. More information about the mount options can be found here.

Lock problems

If you got error such as this:

mount.nfs: rpc.statd is not running but is required for remote locking.
mount.nfs: Either use '-o nolock' to keep locks local, or start statd.
mount.nfs: an incorrect mount option was specified

To fix this, you need to change the "NEED_STATD" value in /etc/conf.d/nfs-common.conf to YES.

Remember to start all the required services (see NFS or NFSv3), not just the nfs service.

Performance Issues

This NFS Howto page has some useful information regarding performance. Here are some further tips:

Diagnose the problem

  • Htop should be your first port of call. The most obvious symptom will be a maxed-out CPU.
  • Press F2, and under "Display options", enable "Detailed CPU time". Press F1 for an explanation of the colours used in the CPU bars. In particular, is the CPU spending most of its time responding to IRQs, or in Wait-IO (wio)?

Server Threads

Symptoms: Nothing seems to be very heavily loaded, but some operations on the client take a long time to complete for no apparent reason.

If your workload involves lots of small reads and writes (or if there are a lot of clients), there may not be enough threads running on the server to handle the quantity of queries. To check if this is the case, run the following command on one or more of the clients:

# nfsstat -rc
Client rpc stats:
calls      retrans    authrefrsh
113482     0          113484

If the retrans column contains a number larger than 0, the server is failing to respond to some NFS requests, and the number of threads should be increased.

To increase the number of threads on the server, edit the file /etc/conf.d/nfs-server.conf and change the value of the NFSD_COUNT variable. The default number of threads is 8. Try doubling this number until retrans remains consistently at zero. Don't be afraid of increasing the number quite substantially. 256 threads may be quite reasonable, depending on the workload. You will need to restart the NFS server daemon each time you modify the configuration file. Bear in mind that the client statistics will only be reset to zero when the client is rebooted.

Use htop (disable the hiding of kernel threads) to keep an eye on how much work each nfsd thread is doing. If you reach a point where the retrans values are non-zero, but you can see nfsd threads on the server doing no work, something different is now causing your bottleneck, and you'll need to re-diagnose this new problem.

Close-to-Open / Flush-on-Close

Symptoms: Your clients are writing many small files. The server CPU is not maxed out, but there is very high wait-IO, and the server disk seems to be churning more than you might expect.

In order to ensure data consistency across clients, the NFS protocol requires that the client's cache is flushed (all data is pushed to the server) whenever a file is closed after writing. Because the server is not allowed to buffer disk writes (if it crashes, the client won't realise the data wasn't written properly), the data is written to disk immediately before the client's request is completed. When you're writing lots of small files from the client, this means that the server spends most of its time waiting for small files to be written to its disk, which can cause a significant reduction in throughput.

See this excellent article or the nfs manpage for more details on the close-to-open policy. There are several approaches to solving this problem:

The nocto mount option

Note: The linux kernel doesn't seem to honour this option properly. Files are still flushed when they're closed.

Does your situation match these conditions?

  • The export you have mounted on the client is only going to be used by the one client.
  • It doesn't matter too much if a file written on one client doesn't immediately appear on other clients.
  • It doesn't matter if after a client has written a file, and the client thinks the file has been saved, and then the client crashes, the file may be lost.

If you're happy with the above conditions, you can use the nocto mount option, which will disable the close-to-open behaviour. See the nfs manpage for details.

The async export option

Does your situation match these conditions?

  • It's important that when a file is closed after writing on one client, it is:
    • Immediately visible on all the other clients.
    • Safely stored on the server, even if the client crashes immediately after closing the file.
  • It's not important to you that if the server crashes:
    • You may loose the files that were most recently written by clients.
    • When the server is restarted, the clients will believe their recent files exist, even though they were actually lost.

In this situation, you can use async instead of sync in the server's /etc/exports file for those specific exports. See the exports manpage for details. In this case, it does not make sense to use the nocto mount option on the client.

Buffer Cache Size and MTU

Symptoms: High kernel or IRQ CPU usage, a very high packet count through the network card.

This is a trickier optimisation. Make sure this is definitely the problem before spending too much time on this. The default values are usually fine for most situations.

See this excellent article for information about I/O buffering in NFS. Essentially, data is accumulated into buffers before being sent. The size of the buffer will affect the way data is transmitted over the network. The Maximum Transmission Unit (MTU) of the network equipment will also affect throughput, as the buffers need to be split into MTU-sized chunks before they're sent over the network. If your buffer size is too big, the kernel or hardware may spend too much time splitting it into MTU-sized chunks. If the buffer size is too small, there will be overhead involved in sending a very large number of small packets. You can use the rsize and wsize mount options on the client to alter the buffer cache size. To achieve the best throughput, you need to experiment and discover the best values for your setup.

It is possible to change the MTU of many network cards. If your clients are on a separate subnet (e.g. for a Beowulf cluster), it may be safe to configure all of the network cards to use a high MTU. This should be done in very-high-bandwidth environments.

See also the nfs manpage for more about rsize and wsize.

Other Issues

Permissions Issues

If you find that you cannot set the permissions on files properly, make sure the user/group you are chowning are on both the client and server.

If all your files are owned by nobody, and you are using NFSv4, on both the client and server, you should:

  • For initscripts, ensure that NEED_IDMAPD is set to YES in /etc/conf.d/nfs-common.conf.
  • For systemd, ensure that the rpc-idmapd service has been started.