NFS Server Performance and Tuning Guide for Sun Hardware

Adding the Cache File System

The cache file system is client-centered. You use the cache file system on the client to reduce server load. With the cache file system, files are obtained from the server, block by block. The files are sent to the memory of the client and manipulated directly. Data is written back to the disk of the server.

Adding the cache file system to client mounts provides a local replica for each client. The /etc/vfstab entry for the cache file system looks like this:

# device    device    mount    FS    fsck    mount    mount
# to mount  to fsck   point    type  pass    at boot  options
server:/usr/dist      cache    /usr/dist     cachefs  3  yes
ro,backfstype=nfs,cachedir=/cache

Use the cache file system in situations with file systems that are read mainly, such as application file systems. Also, you should use the cache file system for sharing data across slow networks. Unlike a replicated server, the cache file system can be used with writable file systems, but performance will degrade as the percent of writes climb. If the percent of writes is too high, the cache file system may decrease NFS performance.

You should also consider using the cache file system if your networks are high speed networks interconnected by routers.

If the NFS server is frequently updated, do not use the cache file system because doing so would result in more traffic than operating over NFS.

To monitor the effectiveness of the cached file systems use the cachefsstat command (available with the Solaris 2.5 and later operating environment).

The syntax of the cachefsstat command is as follows:
```
system# /usr/bin/cachefsstat [-z] path
```
where:

-z initializes statistics. You should execute cachefs -z (superuser only) before executing cachfsstat again to gather statistics on the cache performance. The statistics printed reflect those just before the statistics are reinitialized.

path is the path the cache file system is mounted on. If you do not specify a path, all mounted cache file systems are used.

Without the -z option, you can execute this command as a regular UNIX user. The statistical information supplied by the cachefsstat command includes cache hits and misses, consistency checking, and modification operation:

Table 4-1 Statistical Information Supplied by the cachefsstat Command


Output	Description
`cache hit rate`	Percentage of cache hits over the total number of attempts (followed by the actual numbers of hits and misses)
`consistency checks`	Number of consistency checks performed. It is followed by the number that passed and the number that failed.
`modifies`	Number of modify operations, including writes and creates.

An example of the cachefsstat command is:

system% /usr/bin/cachefsstat /home/sam
cache hit rate: 73% (1234 hits, 450 misses)
consistency checks:  700 (650 pass, 50 fail)
modifies: 321

In the previous example, the cache hit rate for the file system should be higher than thirty percent. If the cache hit rate is lower than thirty percent, this means that the access pattern on the file system is widely randomized or that the cache is too small.

The output for a consistency check means that the cache file system checks with the server to see if data is still valid. A high failure rate (15 to 20 percent) means that the data of interest is rapidly changing. The cache may be updated more quickly than what is appropriate for a cached file system. When you use the output from consistency checks with the number of modifies, you can learn if this client or other clients are making the changes.

The output for modifies is the number of times the client has written changes to the file system. This output is another method to understand why the hit rate may be low. A high rate of modify operations likely goes along with a high number of consistency checks and a lower hit rate.

Also available, beginning with the Solaris 2.5 software environment, are the commands cachefswssize, which determine the working set size for the cache file system and cachefsstat, which displays where the cache file system statistics are being logged. Use these commands to determine if the cache file system is appropriate and valuable for your installation.