Veritas File System 5.0 Administrator's Guide (September 2006)

Note: VxFS does not query VxVM with multiple volume sets. To improve I/O
performance when using multiple volume sets, use the vxtunefs command.
If the file system is being used with a hardware disk array or volume manager
other than VxVM, try to align the parameters to match the geometry of the logical
disk. With striping or RAID-5, it is common to set read_pref_io to the stripe unit
size and read_nstream to the number of columns in the stripe. For striped arrays,
use the same values for write_pref_io and write_nstream, but for RAID-5 arrays,
set write_pref_io to the full stripe size and write_nstream to 1.
For an application to do efficient disk I/O, it should use the following formula to
issue read requests:
■ read requests = read_nstream x by read_pref_io
Generally, any multiple or factor of read_nstream multiplied by read_pref_io
should be a good size for performance. For writing, the same rule of thumb applies
to the write_pref_io and write_nstream parameters. When tuning a file system,
the best thing to do is try out the tuning parameters under a real life workload.
If an application is doing sequential I/O to large files, it should try to issue requests
larger than the discovered_direct_iosz. This causes the I/O requests to be
performed as discovered direct I/O requests, which are unbuffered like direct I/O
but do not require synchronous inode updates when extending the file. If the file
is larger than can fit in the cache, using unbuffered I/O avoids removing useful
data out of the cache and lessens CPU overhead.
55VxFS performance: creating, mounting, and tuning file systems
Tuning I/O