VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
Monitoring Free Space
Chapter 2 55
VxVM Maximum I/O Size
When using VxFS with the VERITAS Volume Manager (VxVM), VxVM by default breaks up
I/O requests larger than 256K. When using striping, to optimize performance, the file system
issues I/O requests that are up to a full stripe in size. If the stripe size is larger than 256K,
those requests are broken up.
See the VERITAS Volume Manager Administrator’s Guide for more information on avoiding
I/O breakup by setting the maximum I/O tunable parameter.
Monitoring Free Space
In general, VxFS works best if the percentage of free space in the file system does not get
below 10 percent. This is because file systems with 10 percent or more free space have less
fragmentation and better extent allocation. Regular use of the df command (see the df_vxfs
(1M) manual page) to monitor free space is desirable. Full file systems may have an adverse
effect on file system performance. Full file systems should therefore have some files removed,
or should be expanded (see the fsadm_vxfs (1M) manual page for a description of online file
system expansion).
Monitoring Fragmentation
Fragmentation reduces performance and availability. Regular use of fsadm’s fragmentation
reporting and reorganization facilities is therefore advisable.
The easiest way to ensure that fragmentation does not become a problem is to schedule
regular defragmentation runs using the cron command.
Defragmentation scheduling should range from weekly (for frequently used file systems) to
monthly (for infrequently used file systems). Extent fragmentation should be monitored with
fsadm or the df -o s commands. There are three factors which can be used to determine the
degree of fragmentation:
• Percentage of free space in extents of less than eight blocks in length
• Percentage of free space in extents of less than 64 blocks in length
• Percentage of free space in extents of length 64 blocks or greater
An unfragmented file system will have the following characteristics:
• Less than 1 percent of free space in extents of less than eight blocks in length