VERITAS File System 3.5 (HP OnlineJFS/JFS 3.5) Administrator's Guide (August 2003)

Chapter 2
VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
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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.