Veritas File System 5.1 SP1 Administrator"s Guide (5900-1499, April 2011)

Block size

The unit of allocation in VxFS is an extent. Unlike some other UNIX file systems,

VxFS does not make use of block fragments for allocation because storage is

allocated in extents that consist of one or more blocks.

You specify the block size when creating a file system by using the mkfs -o bsize

option. The block size cannot be altered after the file system is created. The

smallest available block size for VxFS is 1K. The default block size is 1024 bytes

for file systems smaller than 1 TB, and 8192 bytes for file systems 1 TB or larger.

Choose a block size based on the type of application being run. For example, if

there are many small files, a 1K block size may save space. For large file systems,

with relatively few files, a larger block size is more appropriate. Larger block sizes

use less disk space in file system overhead, but consume more space for files that

are not a multiple of the block size. The easiest way to judge which block sizes

provide the greatest system efficiency is to try representative system loads against

various sizes and pick the fastest.

For 64-bit kernels, the block size and disk layout version determine the maximum

size of the file system you can create.

See “About disk layouts” on page 231.

Intent log size

You specify the intent log size when creating a file system by using the mkfs -o

logsize option. With the Version 6 and later disk layouts, you can dynamically

increase or decrease the intent log size using the log option of the fsadm command.

The mkfs utility uses a default intent log size of 16 megabytes for disk layout

Versions 5 and 6. 64 megabytes is the default for Version 7 and later. The default

size is sufficient for most workloads. If the system is used as an NFS server or for

intensive synchronous write workloads, performance may be improved using a

larger log size.

With larger intent log sizes, recovery time is proportionately longer and the file

system may consume more system resources (such as memory) during normal

operation.

There are several system performance benchmark suites for which VxFS performs

better with larger log sizes. As with block sizes, the best way to pick the log size

is to try representative system loads against various sizes and pick the fastest.

VxFS performance: creating, mounting, and tuning file systems

Creating a VxFS file system