VERITAS File System 3.5 (HP OnlineJFS/JFS 3.5) Administrator's Guide (August 2003)

Chapter 2
VxFS Performance: Creating, Mounting, and Tuning File Systems
Choosing mkfs Command Options
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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 –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
minimum block size must be 2K for file systems from 4 TB to 8 TB, 4K for file systems from 8 TB to 16 TB, and 8K
for file systems larger than 16 TB (see “The VxFS Version 5 Disk Layout” on page 124“ for more information).
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. The intent
log size cannot be altered after the file system is created. The mkfs utility uses a default intent log size of
256K for file systems less than 8 megabytes, 1024K for file systems from 8 to 512 megabytes, and 16
megabytes for file systems greater than 512 megabytes. 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.