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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Tunable VxFS I/O Parameters
read_pref_io The preferred read request size. The file system uses this in
conjunction with the read_nstream value to determine how
much data to read ahead. The default value is 64K.
write_pref_io The preferred write request size. The file system uses this
in conjunction with the write_nstream value to determine
how to do flush behind on writes. The default value is 64K.
read_nstream The number of parallel read requests of size read_pref_io
to have outstanding at one time. The file system uses the
product of read_nstream multiplied by read_pref_io to
determine its read ahead size. The default value for
read_nstream is 1.
write_nstream The number of parallel write requests of size
write_pref_io to have outstanding at one time. The file
system uses the product of write_nstream multiplied by
write_pref_io to determine when to do flush behind on
writes. The default value for write_nstream is 1.
default_indir_
size
On VxFS, files can have up to ten direct extents of variable
size stored in the inode. Once these extents are used up, the
file must use indirect extents which are a fixed size that is
set when the file first uses indirect extents. These indirect
extents are 8K by default. The file system does not use
larger indirect extents because it must fail a write and
return ENOSPC if there are no extents available that are the
indirect extent size. For file systems with many large files,
the 8K indirect extent size is too small. The files that get
into indirect extents use many smaller extents instead of a
few larger ones. By using this parameter, the default
indirect extent size can be increased so large that files in
indirects use fewer larger extents. The tunable
default_indir_size should be used carefully. If it is set
too large, then writes will fail when they are unable to
allocate extents of the indirect extent size to a file. In
general, the fewer and the larger the files on a file system,
the larger the default_indir_size can be set. This
parameter should generally be set to some multiple of the
read_pref_io parameter. default_indir_size is not
applicable on Version 4 and Version 5 disk layouts.