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

76 Chapter3
Extent Attributes
Attribute Specifics
Persistent reservation is not released when a file is truncated. The
reservation must be cleared or the file must be removed to free reserved
space.
Fixed Extent Size
The VxFS default allocation policy uses a variety of heuristics to
determine how to make an allocation to a file when a write requires
additional space. The policy attempts to balance the two goals of
optimum I/O performance through large allocations and minimal file
system fragmentation through allocation from space available in the file
system that best fits the data.
Setting a fixed extent size overrides the default allocation policies for a
file and always serves as a persistent attribute. Be careful to choose an
extent size appropriate to the application when using fixed extents. An
advantage of VxFS’s extent based allocation policies is that they rarely
use indirect blocks compared to block based file systems; VxFS
eliminates many instances of disk access that stem from indirect
references. However, a small extent size can eliminate this advantage.
Files with aggressive allocation sizes tend to be more contiguous and
have better I/O characteristics. However, the overall performance of the
file system degrades because the unused space fragments free space by
breaking large extents into smaller pieces. By erring on the side of
minimizing fragmentation for the file system, files may become so
non-contiguous that their I/O characteristics would degrade.
Fixed extent sizes are particularly appropriate in the following
situations:
• If a file is large and sparse and its write size is fixed, a fixed extent
size that is a multiple of the write size can minimize space wasted by
blocks that do not contain user data as a result of misalignment of
write and extent sizes. (The default extent size for a sparse file is 8K.)
• If a file is large and contiguous, a large fixed extent size can minimize
the number of extents in the file.
Custom applications may also use fixed extent sizes for specific reasons,
such as the need to align extents to cylinder or striping boundaries on
disk.