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

28 Chapter1
The VxFS File System
Online System Administration
Online System Administration
With the HP OnLineJFS product, a VxFS file system can be
defragmented and resized while it remains online and accessible to
users. The following sections contain detailed information about these
features.
NOTE The online administration features, defragmentation, resizing, and
online backup, are available only with the optional HP OnLineJFS
product.
Defragmentation
Free resources are initially aligned and allocated to files in the most
efficient order possible to provide optimal performance. On an active file
system, the original order of free resources is lost over time as files are
created, removed, and resized. The file system is spread further and
further along the disk, leaving unused gaps or fragments between areas
that are in use. This process is also known as fragmentation and leads
to degraded performance because the file system has fewer options when
assigning a file to an extent (a group of contiguous data blocks).
The HFS file system uses the concept of cylinder groups to limit
fragmentation. Cylinder groups are self-contained sections of a file
system that indicate free inodes and data blocks. Allocation strategies in
HFS attempt to place inodes and data blocks in close proximity. This
reduces fragmentation but does not eliminate it.
HP OnLineJFS provides the online administration utility fsadm to
resolve the problem of fragmentation. The fsadm utility defragments a
mounted file system by:
• removing unused space from directories
• making all small files contiguous
• consolidating free blocks for file system use
This utility can run on demand and should be scheduled regularly as a
cron job.