Veritas™ File System 5.0.1 Programmer's Reference Guide

■ 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
Allocation flags with reservation
The VX_TRIM, VX_NOEXTEND, VX_CHGSIZE, VX_NORESERVE, VX_CONTIGUOUS, and
VX_GROWFILE flags can be used to modify reservation requests. VX_NOEXTEND is
the only flag that is persistent; the other flags may have persistent effects, but
the VX_GETEXT ioctl does not return the flags. The non-persistent flags remain
active for a file in the file system cache until the file is no longer accessed and is
removed from the cache.
Reservation trimming
The VX_TRIM flag specifies that the reservation amount must be be trimmed to
match the file size when the last close occurs on the file. At the last close, the
VX_TRIM flag is cleared and any unused reservation space beyond the size of the
file is freed. This flag can be useful if an application needs enough space for a file,
but it is not known how large the file will become. Enough space can be reserved
to hold the largest expected file, and when the file has been written and closed,
any extra space is released.
Non-persistent reservation
If you do not want reservation to be a persistent attribute, you can specify the
VX_NORESERVE flag to request allocation of space without making reservation a
persistent attribute of the file. Applications that are interested in temporary
reservation but want to free any space past the end of the file when the file is
closed case use this flag. For example, if an application is copying a file that is 1
MB long, it can request a 1 MB reservation with the VX_NORESERVE flag set. The
space is allocated, but the reservation in the file is left at 0. If the program aborts
for any reason or the system crashes, the unused space past the end of the file is
released. When the program finishes, there is no clean up because the reservation
was never recorded on disk.
81Veritas File System I/O
Extents