VERITAS File System 4.1 Administrator's Guide

Application Interface
Extent Information
Chapter 482
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.