VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
I/O Tuning
Chapter 2 57
I/O Tuning
NOTE The tunables and the techniques described in this section work on a per file
system basis. Use them judiciously based on the underlying device properties
and characteristics of the applications that use the file system.
Performance of a file system can be enhanced by a suitable choice of I/O sizes and proper
alignment of the I/O requests based on the requirements of the underlying special device.
VxFS provides tools to tune the file systems.
Tuning VxFS I/O Parameters
VxFS provides a set of tunable I/O parameters that control some of its behavior. These I/O
parameters are useful to help the file system adjust to striped or RAID-5 volumes that could
yield performance superior to a single disk. Typically, data streaming applications that access
large files see the largest benefit from tuning the file system.
If VxFS is being used with the VERITAS Volume Manager, the file system queries VxVM to
determine the geometry of the underlying volume and automatically sets the I/O parameters.
The mount command also queries VxVM when the file system is mounted and downloads the
I/O parameters.
If the default parameters are not acceptable or the file system is being used without VxVM,
then the /etc/vx/tunefstab file can be used to set values for I/O parameters. The mount
command reads the /etc/vx/tunefstab file and downloads any parameters specified for a
file system. The tunefstab file overrides any values obtained from VxVM. While the file
system is mounted, any I/O parameters can be changed using the vxtunefs command which
can have tunables specified on the command line or can read them from the
/etc/vx/tunefstab file. For more details, see the vxtunefs (1M) and tunefstab (4) manual
pages. The vxtunefs command can be used to print the current values of the I/O parameters:
# vxtunefs -p mount_point
If the default alignment from mkfs is not acceptable, the -o align=n option can be used to
override alignment information obtained from VxVM. The following is an example tunefstab
file:
/dev/vx/dsk/userdg/netbackup
read_pref_io=128k,write_pref_io=128k,read_nstream=4,write_nstream=4
/dev/vx/dsk/userdg/opt
read_pref_io=128k,write_pref_io=128k,read_nstream=4,write_nstream=4