Linux

FS-CACHE for NFS clients

FS-CACHE is a system which caches files from remote network mounts on the local disk. It is a very easy to set up facility to improve performance on NFS clients.

I strongly recommend a recent kernel if you want to use FS-CACHE though. I tried this with the 4.9 based Debian Stretch kernel a year ago, and this resulted in a kernel oops from time to time, so I had to disable it again. I’m currently using it again with a 4.19 based kernel, and I did not encounter any stability issues up to now.

First of all, you will need a dedicated file system where you will store the cache on. I prefer to use XFS, because it has nice performance and stability. Mount the file system on /var/cache/fscache.

Then install the cachefilesd package and edit the file /etc/default/cachefilesd so that it contains:

RUN=yes

Then edit the file /etc/cachefilesd.conf. It should look like this:

dir /var/cache/fscache
tag mycache
brun 10%
bcull 7%
bstop 3%
frun 10%
fcull 7%
fstop 3%

These numbers define when cache culling (making space in the cache by discarding less recently used files) happens: when the amount of available disk space or the amount of available files drops below 7%, culling will start. Culling will stop when 10% is available again. If the available disk space or available amount of files drops below 3%, no further cache allocation is done until more than 3% is available again. See also man cachefilesd.conf.

Start cachefilesd by running

# systemctl start cachefilesd

If it fails to start with these messages in the logs:

cachefilesd[1724]: About to bind cache
kernel: CacheFiles: Security denies permission to nominate security context: error -2
cachefilesd[1724]: CacheFiles bind failed: errno 2 (No such file or directory)
cachefilesd[1724]: Starting FilesCache daemon : cachefilesd failed!
systemd[1]: cachefilesd.service: Control process exited, code=exited status=1
systemd[1]: cachefilesd.service: Failed with result 'exit-code'.
systemd[1]: Failed to start LSB: CacheFiles daemon.

then you are hitting this bug. This happens when you are using a kernel with AppArmor enabled (like Debian’s kernel from testing). You can work around it by commenting out the line defining the security context in /etc/cachefilesd.conf:

#secctx system_u:system_r:cachefiles_kernel_t:s0

and starting cachefilesd again.

Now in /etc/fstab add the fsc option to all NFS file systems where you want to enable caching for. For example for NFS4 your fstab line might look like this:

nfs.server:/home	/home	nfs4	_netdev,fsc,noatime,vers=4.2,nodev,nosuid	0	0

Now remount the file system. Just using the remount option is probably not enough: you have to completely umount and mount the NFS file system. Check with the mount command whether the fsc option is present. You can also run

# cat /proc/fs/nfsfs/volumes 

and check whether the FSC column is set to YES.

Try copying some large files from your NFS mount to the local disk. Then you can check the statistics by running

# cat /proc/fs/fscache/stats

You should sea that not all values are 0 any more. You will also see with

$ df -h /var/cache/fscache

your file system is being filled with cached data.