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, 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 utility uses a default intent log size of 16 megabytes for disk layout Versions 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.