Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Extent Attributes
Attribute Specifics
Chapter 3 55
Reservation: Preallocating Space to a File
VxFS makes it possible to preallocate space to a file at the time of the request rather than when data is
written into the file. This space cannot be allocated to other files in the file system. VxFS prevents any
unexpected out-of-space condition on the file system by ensuring that a file’s required space will be
associated with the file before it is required.
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 methods 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.
• 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.