JFS Tuning and Performance

Understanding VxFS

In order to fully understand some of the various file system creation options, mount options, and

tunables, a brief overview of VxFS is provided.

Software versions vs. disk layout versions

VxFS supports several different disk layout versions (DLV). The default disk layout version can be

overridden when the file system is created using mkfs(1M). Also, the disk layout can be upgraded

online using the vxupgrade(1M) command.

Table 1. VxFS software versions and disk layout versions (* denotes default disk layout)

OS Version

SW Version

Disk layout version

11.23

VxFS 3.5

VxFS 4.1

VxFS 5.0

3,4,5*

4,5,6*

4,5,6,7*

11.31

VxFS 4.1

VxFS 5.0

VxFS 5.0.1

4,5,6*

4,5,6,7*

Several improvements have been made in the disk layouts which can help increase performance. For

example, the version 5 disk layout is needed to create file system larger than 2 TB. The version 7 disk

layout available on VxFS 5.0 and above improves the performance of large directories.

Please note that you cannot port a file system to a previous release unless the previous release

supports the same disk layout version for the file system being ported. Before porting a file system

from one operating system to another, be sure the file system is properly unmounted. Differences in

the Intent Log will likely cause a replay of the log to fail and a full fsck will be required before

mounting the file system.

Variable sized extent based file system

VxFS is a variable sized extent based file system. Each file is made up of one or more extents that

vary in size. Each extent is made up of one or more file system blocks. A file system block is the

smallest allocation unit of a file. The block size of the file system is determined when the file system is

created and cannot be changed without rebuilding the file system. The default block size varies

depending on the size of the file system.

The advantages of variable sized extents include:

 Faster allocation of extents

 Larger and fewer extents to manage

 Ability to issue large physical I/O for an extent

However, variable sized extents also have disadvantages:

 Free space fragmentation

 Files with many small extents