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.
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