Veritas File System 5.1 SP1 Administrator"s Guide (5900-1499, April 2011)

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 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.
Other controls
The auxiliary controls on extent attributes determine the following conditions:
■ Whether allocations are aligned
■ Whether allocations are contiguous
■ Whether the file can be written beyond its reservation
■ Whether an unused reservation is released when the file is closed
■ Whether the reservation is a persistent attribute of the file
■ When the space reserved for a file will actually become part of the file
Alignment
Specific alignment restrictions coordinate a file's allocations with a particular
I/O pattern or disk alignment. Alignment can only be specified if a fixed extent
size has also been set. Setting alignment restrictions on allocations is best left to
well-designed applications.
See the setext(1M) manual page.
See “About Veritas File System I/O” on page 69.
Contiguity
A reservation request can specify that its allocation remain contiguous (all one
extent). Maximum contiguity of a file optimizes its I/O characteristics.
Note: Fixed extent sizes or alignment cause a file system to return an error message
reporting insufficient space if no suitably sized (or aligned) extent is available.
This can happen even if the file system has sufficient free space and the fixed
extent size is large.
65Extent attributes
About extent attributes