Dedicated article for common problems and solutions.
- 1 Server-side issues
- 2 Client-side issues
- 2.1 mount.nfs4: No such device
- 2.2 mount.nfs4: access denied by server while mounting
- 2.3 Unable to connect from OS X clients
- 2.4 Unreliable connection from OS X clients
- 2.5 Intermittent client freezes when copying large files
- 2.6 mount.nfs: Operation not permitted
- 2.7 mount.nfs: Protocol not supported
- 2.8 Problems with Vagrant and synced_folders
- 3 Performance issues
- 4 Debugging
- 5 Other issues
exportfs: /etc/exports:2: syntax error: bad option list
Delete all space from the option list in
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 than 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. MOUNTD_OPTS="--manage-gids"
"Permission denied" when trying to write files as root
- 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
- You must also add no_root_squash to the first line in
"RPC: Program not registered" when showmount -e command issued
Make sure that
rpcbind.service are running on the server site, see systemd. If they are not, start and enable them.
mount.nfs4: No such device
Check that you have loaded the
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.
or try "exportfs -rav" reload
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
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.
Intermittent client freezes when copying large files
If you copy large files from your client machine to the NFS server, the transfer speed is very fast, but after some seconds the speed drops and your client machine intermittently locks up completely for some time until the transfer is finished.
Try adding sync as a mount option on the client (e.g. in /etc/fstab) to fix this problem.
mount.nfs: Operation not permitted
nfs-utils versions 1.2.1-2 or higher use NFSv4 by default, resulting in NFSv3 shares failing on upgrade. The problem can be solved by using either mount option
'nfsvers=3' on the command line:
# mount.nfs remote target directory -o ...,vers=3,... # mount.nfs remote target directory -o ...,nfsvers=3,...
remote target directory nfs ...,vers=3,... 0 0 remote target directory nfs ...,nfsvers=3,... 0 0
mount.nfs: Protocol not supported
This error occurs when you include the export root in the path of the NFS source. For example:
# mount SERVER:/srv/nfs4/media /mnt mount.nfs4: Protocol not supported
Use the relative path instead:
# mount SERVER:/media /mnt
Problems with Vagrant and synced_folders
If Vagrant scripts are unable to mount folders over NFS, installing the net-tools package may solve the issue.
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)?
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
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 set the value in the
For example, to set the number of threads to 32:
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.
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
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 manual page 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 manual page for more about rsize and wsize.
rpcdebug is the easiest way to manipulate the kernel interfaces in place of echoing bitmasks to /proc.
|-c||Clear the given debug flags|
|-s||Set the given debug flags|
|-m module||Specify which module's flags to set or clear.|
|-v||Increase the verbosity of rpcdebug's output|
|-h||Print a help message and exit. When combined with the -v option, also prints the available debug flags.|
For the -m option, the available modules are:
|nfsd||The NFS server|
|nfs||The NFS client|
|nlm||The Network Lock Manager, in either an NFS client or server|
|rpc||The Remote Procedure Call module, in either an NFS client or server|
rpcdebug -m rpc -s all # sets all debug flags for RPC rpcdebug -m rpc -c all # clears all debug flags for RPC rpcdebug -m nfsd -s all # sets all debug flags for NFS Server rpcdebug -m nfsd -c all # clears all debug flags for NFS Server
Once the flags are set you can tail the journal for the debug output, usually
journalctl -fl or similar.
mountstats tool, which can retrieve a lot of statistics about NFS mounts, including average timings and packet size.
$ mountstats Stats for example:/tank mounted on /tank: NFS mount options: rw,sync,vers=4.2,rsize=524288,wsize=524288,namlen=255,acregmin=3,acregmax=60,acdirmin=30,acdirmax=60,soft,proto=tcp,port=0,timeo=15,retrans=2,sec=sys,clientaddr=xx.yy.zz.tt,local_lock=none NFS server capabilities: caps=0xfbffdf,wtmult=512,dtsize=32768,bsize=0,namlen=255 NFSv4 capability flags: bm0=0xfdffbfff,bm1=0x40f9be3e,bm2=0x803,acl=0x3,sessions,pnfs=notconfigured NFS security flavor: 1 pseudoflavor: 0 NFS byte counts: applications read 248542089 bytes via read(2) applications wrote 0 bytes via write(2) applications read 0 bytes via O_DIRECT read(2) applications wrote 0 bytes via O_DIRECT write(2) client read 171375125 bytes via NFS READ client wrote 0 bytes via NFS WRITE RPC statistics: 699 RPC requests sent, 699 RPC replies received (0 XIDs not found) average backlog queue length: 0 READ: 338 ops (48%) avg bytes sent per op: 216 avg bytes received per op: 507131 backlog wait: 0.005917 RTT: 548.736686 total execute time: 548.775148 (milliseconds) GETATTR: 115 ops (16%) avg bytes sent per op: 199 avg bytes received per op: 240 backlog wait: 0.008696 RTT: 15.756522 total execute time: 15.843478 (milliseconds) ACCESS: 93 ops (13%) avg bytes sent per op: 203 avg bytes received per op: 168 backlog wait: 0.010753 RTT: 2.967742 total execute time: 3.032258 (milliseconds) LOOKUP: 32 ops (4%) avg bytes sent per op: 220 avg bytes received per op: 274 backlog wait: 0.000000 RTT: 3.906250 total execute time: 3.968750 (milliseconds) OPEN_NOATTR: 25 ops (3%) avg bytes sent per op: 268 avg bytes received per op: 350 backlog wait: 0.000000 RTT: 2.320000 total execute time: 2.360000 (milliseconds) CLOSE: 24 ops (3%) avg bytes sent per op: 224 avg bytes received per op: 176 backlog wait: 0.000000 RTT: 30.250000 total execute time: 30.291667 (milliseconds) DELEGRETURN: 23 ops (3%) avg bytes sent per op: 220 avg bytes received per op: 160 backlog wait: 0.000000 RTT: 6.782609 total execute time: 6.826087 (milliseconds) READDIR: 4 ops (0%) avg bytes sent per op: 224 avg bytes received per op: 14372 backlog wait: 0.000000 RTT: 198.000000 total execute time: 198.250000 (milliseconds) SERVER_CAPS: 2 ops (0%) avg bytes sent per op: 172 avg bytes received per op: 164 backlog wait: 0.000000 RTT: 1.500000 total execute time: 1.500000 (milliseconds) FSINFO: 1 ops (0%) avg bytes sent per op: 172 avg bytes received per op: 164 backlog wait: 0.000000 RTT: 2.000000 total execute time: 2.000000 (milliseconds) PATHCONF: 1 ops (0%) avg bytes sent per op: 164 avg bytes received per op: 116 backlog wait: 0.000000 RTT: 1.000000 total execute time: 1.000000 (milliseconds)
A bitmask of the debug flags can be echoed into the interface to enable output to syslog; 0 is the default:
/proc/sys/sunrpc/nfsd_debug /proc/sys/sunrpc/nfs_debug /proc/sys/sunrpc/nlm_debug /proc/sys/sunrpc/rpc_debug
Sysctl controls are registered for these interfaces, so they can be used instead of echo:
sysctl -w sunrpc.rpc_debug=1023 sysctl -w sunrpc.rpc_debug=0 sysctl -w sunrpc.nfsd_debug=1023 sysctl -w sunrpc.nfsd_debug=0
At runtime the server holds information that can be examined:
grep . /proc/net/rpc/*/content cat /proc/fs/nfs/exports cat /proc/net/rpc/nfsd ls -l /proc/fs/nfsd
A rundown of
/proc/net/rpc/nfsd (the userspace tool
nfsstat pretty-prints this info):
* rc (reply cache): <hits> <misses> <nocache> - hits: client it's retransmitting - misses: a operation that requires caching - nocache: a operation that no requires caching * fh (filehandle): <stale> <total-lookups> <anonlookups> <dir-not-in-cache> <nodir-not-in-cache> - stale: file handle errors - total-lookups, anonlookups, dir-not-in-cache, nodir-not-in-cache . always seem to be zeros * io (input/output): <bytes-read> <bytes-written> - bytes-read: bytes read directly from disk - bytes-written: bytes written to disk * th (threads): <threads> <fullcnt> <10%-20%> <20%-30%> ... <90%-100%> <100%> - threads: number of nfsd threads - fullcnt: number of times that the last 10% of threads are busy - 10%-20%, 20%-30% ... 90%-100%: 10 numbers representing 10-20%, 20-30% to 100% . Counts the number of times a given interval are busy * ra (read-ahead): <cache-size> <10%> <20%> ... <100%> <not-found> - cache-size: always the double of number threads - 10%, 20% ... 100%: how deep it found what was looking for - not-found: not found in the read-ahead cache * net: <netcnt> <netudpcnt> <nettcpcnt> <nettcpconn> - netcnt: counts every read - netudpcnt: counts every UDP packet it receives - nettcpcnt: counts every time it receives data from a TCP connection - nettcpconn: count every TCP connection it receives * rpc: <rpccnt> <rpcbadfmt+rpcbadauth+rpcbadclnt> <rpcbadfmt> <rpcbadauth> <rpcbadclnt> - rpccnt: counts all rpc operations - rpcbadfmt: counts if while processing a RPC it encounters the following errors: . err_bad_dir, err_bad_rpc, err_bad_prog, err_bad_vers, err_bad_proc, err_bad - rpcbadauth: bad authentication . does not count if you try to mount from a machine that it's not in your exports file - rpcbadclnt: unused * procN (N = vers): <vs_nproc> <null> <getattr> <setattr> <lookup> <access> <readlink> <read> <write> <create> <mkdir> <symlink> <mknod> <remove> <rmdir> <rename> <link> <readdir> <readdirplus> <fsstat> <fsinfo> <pathconf> <commit> - vs_nproc: number of procedures for NFS version . v2: nfsproc.c, 18 . v3: nfs3proc.c, 22 - v4, nfs4proc.c, 2 - statistics: generated from NFS operations at runtime * proc4ops: <ops> <x..y> - ops: the definition of LAST_NFS4_OP, OP_RELEASE_LOCKOWNER = 39, plus 1 (so 40); defined in nfs4.h - x..y: the array of nfs_opcount up to LAST_NFS4_OP (nfsdstats.nfs4_opcount[i])
NFSD debug flags
/* * knfsd debug flags */ #define NFSDDBG_SOCK 0x0001 #define NFSDDBG_FH 0x0002 #define NFSDDBG_EXPORT 0x0004 #define NFSDDBG_SVC 0x0008 #define NFSDDBG_PROC 0x0010 #define NFSDDBG_FILEOP 0x0020 #define NFSDDBG_AUTH 0x0040 #define NFSDDBG_REPCACHE 0x0080 #define NFSDDBG_XDR 0x0100 #define NFSDDBG_LOCKD 0x0200 #define NFSDDBG_ALL 0x7FFF #define NFSDDBG_NOCHANGE 0xFFFF
NFS debug flags
/* * NFS debug flags */ #define NFSDBG_VFS 0x0001 #define NFSDBG_DIRCACHE 0x0002 #define NFSDBG_LOOKUPCACHE 0x0004 #define NFSDBG_PAGECACHE 0x0008 #define NFSDBG_PROC 0x0010 #define NFSDBG_XDR 0x0020 #define NFSDBG_FILE 0x0040 #define NFSDBG_ROOT 0x0080 #define NFSDBG_CALLBACK 0x0100 #define NFSDBG_CLIENT 0x0200 #define NFSDBG_MOUNT 0x0400 #define NFSDBG_FSCACHE 0x0800 #define NFSDBG_PNFS 0x1000 #define NFSDBG_PNFS_LD 0x2000 #define NFSDBG_STATE 0x4000 #define NFSDBG_ALL 0xFFFF
NLM debug flags
/* * Debug flags */ #define NLMDBG_SVC 0x0001 #define NLMDBG_CLIENT 0x0002 #define NLMDBG_CLNTLOCK 0x0004 #define NLMDBG_SVCLOCK 0x0008 #define NLMDBG_MONITOR 0x0010 #define NLMDBG_CLNTSUBS 0x0020 #define NLMDBG_SVCSUBS 0x0040 #define NLMDBG_HOSTCACHE 0x0080 #define NLMDBG_XDR 0x0100 #define NLMDBG_ALL 0x7fff
RPC debug flags
/* * RPC debug facilities */ #define RPCDBG_XPRT 0x0001 #define RPCDBG_CALL 0x0002 #define RPCDBG_DEBUG 0x0004 #define RPCDBG_NFS 0x0008 #define RPCDBG_AUTH 0x0010 #define RPCDBG_BIND 0x0020 #define RPCDBG_SCHED 0x0040 #define RPCDBG_TRANS 0x0080 #define RPCDBG_SVCXPRT 0x0100 #define RPCDBG_SVCDSP 0x0200 #define RPCDBG_MISC 0x0400 #define RPCDBG_CACHE 0x0800 #define RPCDBG_ALL 0x7fff
- While the number of threads can be increased at runtime via an echo to
/proc/fs/nfsd/threads, the cache size (double the threads, see the ra line of /proc/net/rpc/nfsd) is not dynamic. The NFS daemon must be restarted with the new thread size during initialization in order for the thread cache to properly adjust.
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 systemd, ensure that the
nfs-idmapdservice has been started.
- For initscripts, ensure that
NEED_IDMAPDis set to
On some systems detecting the domain from FQDN minus hostname does not seem to work reliably. If files are still showing as
nobody after the above changes, edit /etc/idmapd.conf, ensure that
Domain is set to
FQDN minus hostname. For example:
[General] Verbosity = 7 Pipefs-Directory = /var/lib/nfs/rpc_pipefs Domain = yourdomain.local [Mapping] Nobody-User = nobody Nobody-Group = nobody [Translation] Method = nsswitch
If nfs-idmapd.service refuses to start because it cannot open the Pipefs-directory (defined in /etc/idmapd.conf and appended with '/nfs'), issue a mkdir-command and restart the daemon.