Installation guide

Using the Cache With NFS

tune2fs -o user_xattr /dev/device

Alternatively, extended attributes for a file system can be enabled at mount time, as in:

mount /dev/device /path/to/cache -o user_xattr

The cache back-end works by maintaining a certain amount of free space on the partition hosting

the cache. It grows and shrinks the cache in response to other elements of the system using up free

space, making it safe to use on the root file system (for example, on a laptop). FS-Cache sets defaults

on this behavior, which can be configured via cache cull limits. For more information about configuring

cache cull limits, refer to Section 11.4, “Setting Cache Cull Limits”.

Once the configuration file is in place, start up the cachefilesd daemon:

service cachefilesd start

To configure cachefilesd to start at boot time, execute the following command as root:

chkconfig cachefilesd on

11.3. Using the Cache With NFS

NFS will not use the cache unless explicitly instructed. To configure an NFS mount to use FS-Cache,

include the -o fsc option to the mount command, as in:

mount nfs-share:/ /mount/point -o fsc

All access to files under /mount/point will go through the cache, unless the file is opened for

direct I/O or writing (refer to Section 11.3.2, “Cache Limitations With NFS” for more information). NFS

indexes cache contents using NFS file handle, not the file name; this means that hard-linked files

share the cache correctly.

Caching is supported in version 2, 3, and 4 of NFS. However, each version uses different branches for

caching.

11.3.1. Cache Sharing

There are several potential issues to do with NFS cache sharing. Because the cache is persistent,

blocks of data in the cache are indexed on a sequence of four keys:

• Level 1: Server details

• Level 2: Some mount options; security type; FSID; uniquifier

• Level 3: File Handle

• Level 4: Page number in file

To avoid coherency management problems between superblocks, all NFS superblocks that wish

to cache data have unique Level 2 keys. Normally, two NFS mounts with same source volume and

options will share a superblock, and thus share the caching, even if they mount different directories

within that volume. Take the following two mount commands:

mount home0:/disk0/fred /home/fred -o fsc

mount home0:/disk0/jim /home/jim -o fsc