File system tuning guidelines

If the file system is being used with VxVM, it is advisable to let the VxFS I/O parameters be set to default values based on the volume geometry.


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:

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 performing sequential I/O to large files, the application should try to issue requests larger than 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.