VERITAS File SystemÖ 3.5 (HP OnlineJFS/JFS3.5) AdministratorÆs Guide (December 2002)

Chapter 2
VxFS Performance: Creating, Mounting, and Tuning File Systems
Monitoring Free Space
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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
• Less than 5 percent of free space in extents of less than 64 blocks in length
• More than 5 percent of the total file system size available as free extents in lengths of 64 or more blocks
A badly fragmented file system will have one or more of the following characteristics:
• Greater than 5 percent of free space in extents of less than 8 blocks in length
• More than 50 percent of free space in extents of less than 64 blocks in length
• Less than 5 percent of the total file system size available as free extents in lengths of 64 or more blocks
The optimal period for scheduling of extent reorganization runs can be determined by choosing a reasonable
interval, scheduling fsadm runs at the initial interval, and running the extent fragmentation report feature of
fsadm before and after the reorganization.