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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max_buf_data_size The maximum buffer size allocated for file data; either 8K
bytes or 64K bytes. Use the larger value for workloads
where large reads/writes are performed sequentially. Use
the smaller value on workloads where the I/O is random or
is done in small chunks. 8K bytes is the default value.
max_direct_iosz The maximum size of a direct I/O request that will be issued
by the file system. If a larger I/O request comes in, then it is
broken up into max_direct_iosz chunks. This parameter
defines how much memory an I/O request can lock at once,
so it should not be set to more than 20 percent of memory.
max_diskq Limits the maximum disk queue generated by a single file.
When the file system is flushing data for a file and the
number of buffers being flushed exceeds max_diskq,
processes will block until the amount of data being flushed
decreases.Although this doesn't limit the actual disk queue,
it prevents flushing processes from making the system
unresponsive. The default value is 1 MB.
max_seqio_extent_
size
Increases or decreases the maximum size of an extent.
When the file system is following its default allocation
policy for sequential writes to a file, it allocates an initial
extent which is large enough for the first write to the file.
When additional extents are allocated, they are
progressively larger (the algorithm tries to double the size
of the file with each new extent) so each extent can hold
several writes worth of data. This is done to reduce the total
number of extents in anticipation of continued sequential
writes. When the file stops being written, any unused space
is freed for other files to use. Normally this allocation stops
increasing the size of extents at 2048 blocks which prevents
one file from holding too much unused space.
max_seqio_extent_size is measured in file system blocks.