Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 11-05-2007 Installation Manual page 492

25.2.5 XFS
Originally intended as the file system for their IRIX OS, SGI started XFS development
in the early 1990s. The idea behind XFS was to create a high-performance 64-bit jour-
naling file system to meet the extreme computing challenges of today. XFS is very
good at manipulating large files and performs well on high-end hardware. However,
even XFS has a drawback. Like ReiserFS, XFS takes great care of metadata integrity,
but less of data integrity.
A quick review of XFS's key features explains why it may prove a strong competitor
for other journaling file systems in high-end computing.
High Scalability through the Use of Allocation Groups
At the creation time of an XFS file system, the block device underlying the file
system is divided into eight or more linear regions of equal size. Those are referred
to as allocation groups. Each allocation group manages its own inodes and free
disk space. Practically, allocation groups can be seen as file systems in a file system.
Because allocation groups are rather independent of each other, more than one of
them can be addressed by the kernel simultaneously. This feature is the key to
XFS's great scalability. Naturally, the concept of independent allocation groups
suits the needs of multiprocessor systems.
High Performance through Efficient Management of Disk Space
Free space and inodes are handled by B
use of B
delayed allocation. It handles allocation by breaking the process into two pieces.
A pending transaction is stored in RAM and the appropriate amount of space is
reserved. XFS still does not decide where exactly (speaking of file system blocks)
the data should be stored. This decision is delayed until the last possible moment.
Some short-lived temporary data may never make its way to disk, because it may
be obsolete by the time XFS decides where actually to save it. Thus XFS increases
write performance and reduces file system fragmentation. Because delayed allocation
results in less frequent write events than in other file systems, it is likely that data
loss after a crash during a write is more severe.
Preallocation to Avoid File System Fragmentation
Before writing the data to the file system, XFS reserves (preallocates) the free space
needed for a file. Thus, file system fragmentation is greatly reduced. Performance
is increased because the contents of a file are not distributed all over the file system.
474 Installation and Administration
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trees greatly contributes to XFS's performance and scalability. XFS uses
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trees inside the allocation groups. The