CacheFS performance impacts

CacheFS will not increase the write performance to NFS file systems. However, you have some write options to choose as parameters to the -o option of the mount command, when mounting a CacheFS. They will influence the subsequent read performance to the data.

The write options are as follows:

write around

The write around mode is the default mode and it handles writes the same way that NFS does. The writes are made to the back file system, and the affected file is purged from the cache. This means that write around voids the cache and new data must be obtained back from the server after the write.

non-shared

You can use the non-shared mode when you are certain that no one else will be writing to the cached file system. In this mode, all writes are made to both the front and the back file system, and the file remains in the cache. This means that future read accesses can be done to the cache, rather than going to the server.

Small reads might be kept in memory anyway (depending on your memory usage) so there is no benefit in also caching the data on the disk. Caching of random reads to different data blocks does not help, unless you will access the same data over and over again.

The initial read request still has to go to the server because only by the time a user attempts to access files that are part of the back file system will those files be placed in the cache. For the initial read request, you will see typical NFS speed. Only for subsequent accesses to the same data, you will see local JFS access performance.

The consistency of the cached data is only checked at intervals. Therefore, it is dangerous to cache data that is frequently changed. CacheFS should only be used for read-only or read-mostly data.

Write performance over a cached NFS file system differs from NFS Version 2 to NFS Version 3. Performance tests have shown the following:

• Sequential writes to a new file over NFS Version 2 to a CacheFS mount point can be 25 percent slower than writes directly to the NFS Version 2 mount point.
• Sequential writes to a new file over NFS Version 3 to a CacheFS mount point can be 6 times slower than writes directly to the NFS Version 3 mount point.