HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

Chapter 5 93
Performance and Tuning
Choosing a Block Size
Choosing a Block Size
You specify the block size when a file system is created; it cannot be
changed later. The standard HFS file system defaults to a block size of
8K with a 1K fragment size. This means that space is allocated to small
files (up to 8K) in 1K increments. Allocations for larger files are done in
8K increments except for the last block, which may be a fragment.
Because many files are small, the fragment facility saves a large amount
of space compared to allocating space 8K at a time.
The unit of allocation in VxFS is a block. There are no fragments because
storage is allocated in extents that consist of one or more blocks. The
smallest block size available is 1K, which is also the default block size for
VxFS file systems created on file systems of less than 8 gigabytes.
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. The trade-off of specifying larger block sizes is a decrease in
the amount of space used to hold the free extent bitmaps for each
allocation unit, an increase in the maximum extent size, and a decrease
in the number of extents used per file versus an increase in the amount
of space wasted at the end of files that are not a multiple of the block
size. 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.