VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
Choosing mkfs Command Options
Chapter 2 39
Choosing mkfs Command Options
There are several characteristics that you can select when you create a file system. The most
important options pertaining to system performance are the block size and intent log size.
Block Size
The unit of allocation in VxFS is a block. 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-obsize option. The
block size cannot be altered after the file system is created. The smallest available block size
for VxFS is 1K, which is also the default block size.
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.
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 disk layout, you can dynamically increase or decrease the intent log
size using the log option of the fsadm command. The mkfs utilty uses a default intent log size
of 16 megabytes for disk layout Version 4, 5, and 6. 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.