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:

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:

Here, /home/fred and /home/jim will likely share the superblock as they have the same options, especially if they come from the same volume/partition on the NFS server (home0). Now, consider the next two subsequent mount commands:

In this case, /home/fred and /home/jim will not share the superblock as they have different network access parameters, which are part of the Level 2 key. The same goes for the following mount sequence:

Here, the contents of the two subtrees (/home/fred1 and /home/fred2) will be cached twice.

Another way to avoid superblock sharing is to suppress it explicitly with the nosharecache parameter. Using the same example:

However, in this case only one of the superblocks will be permitted to use cache since there is nothing to distinguish the Level 2 keys of home0:/disk0/fred and home0:/disk0/jim. To address this, add a unique identifier on at least one of the mounts, i.e. fsc=unique-identifier. For example:

Here, the unique identifier jim will be added to the Level 2 key used in the cache for /home/jim.