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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 mkfs_vxfs(1M) manual page.

See VxFS I/O interface.

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. For users who do not have the appropriate privileges, there is a variant request that prevents such users from viewing uniinitialized data.