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

Chapter 3 75
Extent Attributes
Attribute Specifics
Attribute Specifics
The two basic extent attributes associated with a file are its
reservation and its fixed extent size . The user can preallocate
space to the file by manipulating a file’s reservation; the user can also
override the default allocation policy of the file system by setting a fixed
extent size.
Other policies determine the way these attributes are expressed during
the allocation process. The user can specify that:
• the space reserved for a file must be contiguous
• no allocations should be made for a file beyond the current
reservation
• unused reservation should be released when the file is closed
• space should be allocated but no reservation should be assigned
• the file size should be changed to immediately incorporate the
allocated space
Some of the extent attributes are persistent and become part of the
on-disk information about the file, while other attributes are temporary
and lost after the file is closed or the system is rebooted. The persistent
attributes are similar to the file’s permissions and are actually written in
the inode for the file. When a file is copied, moved, or archived, only the
persistent attributes of the source file can be preserved in the new file
(see “Other Controls” for more information).
In general, the user will only set extent attributes for reservation. Many
of the attributes are designed for applications that are tuned to a
particular pattern of I/O or disk alignment (see mkfs_vxfs(1M) and
Chapter 6 , “Application Interface” for more information).
Reservation: Preallocating Space to a File
VxFS makes it possible to preallocate space to a file at the time of the
request rather than when data is written into the file. This space cannot
be allocated to other files in the file system. VxFS prevents any
unexpected out-of-space condition on the file system by ensuring that a
file’s required space will be associated with the file before it is required.