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

Some of the extent attributes are persistent and become part of the on-disk
information about the file, while other attributes are temporary and are lost after
the file is closed or the system is rebooted. The persistent attributes are similar
to the file's permissions and are written in the inode for the file. When a file is
copied, moved, or archived, only the persistent attributes of the source file are
preserved in the new file.
See “Other controls” on page 65.
In general, the user will only set extent attributes for reservation. Many of the
attributes are designed for applications that are tuned to a particular pattern of
I/O or disk alignment.
See the setext(1M) manual page.
See “About Veritas File System I/O” on page 69.
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.
A persistent reservation is not released when a file is truncated. The reservation
must be cleared or the file must be removed to free the 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. VxFS accomplishes these
goals by allocating 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 the VxFS
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 large extents 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
Extent attributes
About extent attributes
64