HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

116 Chapter6
Application Interface
Extent Information
reservation amount. This flag can be used to create files with
uninitialized data. Because this allows uninitialized data in files, it is
restricted to users with appropriate privileges.
It is possible to use these flags in combination. For example, using
VX_CHGSIZE and VX_NORESERVE changes the file size but does not set any
reservation. When the file is truncated, the space is freed. If the
VX_NORESERVE flag had not been used, the reservation would have been
set on disk along with the file size.
Space reservation is used to make sure applications do not fail because
the file system is out of space. An application can preallocate space for all
the files it needs before starting to do any work. By allocating space in
advance, the file is optimally allocated for performance, and file accesses
are not slowed down by the need to allocate storage. This allocation of
resources can be important in applications that require a guaranteed
response time.
With very large files, use of space reservation can avoid the need to use
indirect extents. It can also improve performance and reduce
fragmentation by guaranteeing that the file consists of large contiguous
extents. Sometimes when critical file systems run out of space, cron jobs,
mail, or printer requests fail. These failures are harder to track if the
logs kept by the application cannot be written due to a lack of space on
the file system.
By reserving space for key log files, the logs will not fail when the system
runs out of space. Process accounting files can also have space reserved
so accounting records will not be lost if the file system runs out of space.
In addition, by using the VX_NOEXTEND flag for log files, the maximum
size of these files can be limited. This can prevent a runaway failure in
one component of the system from filling the file system with error
messages and causing other failures. If the VX_NOEXTEND flag is used for
log files, the logs should be cleaned up before they reach the size limit in
order to avoid losing information.
Fixed Extent Sizes
The VxFS file system uses the I/O size of write requests, and a default
policy, when allocating space to a file. For some applications, this may not
work out well. These applications can set a fixed extent size, so that all
new extents allocated to the file are of the fixed extent size.
By using a fixed extent size, an application can reduce allocations and
guarantee good extent sizes for a file. An application can reserve most of