VERITAS File System 3.5 (HP OnlineJFS/JFS 3.5) Administrator's Guide (August 2003)

Chapter 3
Extent Attributes
Attribute Specifics
52
Other Controls
The auxiliary controls on extent attributes determine:
• 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
(see the mkfs_vxfs (1M) manual page and “Application Interface” on page 55 for details). 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.
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.
Write Operations Beyond Reservation
A reservation request can specify that no allocations can take place after a write operation fills up the last
available block in the reservation. This specification can be used in a similar way to ulimit to prevent a file’s
uncontrolled growth.
Reservation Trimming
A reservation request can specify that any unused reservation be released when the file is closed. The file is
not completely closed until all processes open against the file have closed it.
Reservation Persistence
A reservation request can ensure that the reservation does not become a persistent attribute of the file. The
unused reservation is discarded when the file is closed.
Including Reservation in the File
A reservation request can make sure the size of the file is adjusted to include the reservation. Normally, the
space of the reservation is not included in the file until an extending write operation requires it. A reservation
that immediately changes the file size can generate large temporary files. Unlike a ftruncate operation that
increases the size of a file, this type of reservation does not perform zeroing of the blocks included in the file
and limits this facility to users with appropriate privileges. The data that appears in the file may have been
previously contained in another file.