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

68 Chapter2
Disk Layout
The VxFS Version 3 Disk Layout
The VxFS Version 3 Disk Layout
The Version 3 disk layout allows the file system to scale easily to
accommodate large files and large file systems.
The Version 1 and 2 disk layouts divided up the file system space into
allocation units. The first AU started part way into the file system which
caused potential alignment problems depending on where the first AU
started. Each allocation unit also had its own summary, bitmaps, and
data blocks. Because this AU structural information was stored at the
start of each AU, this also limited the maximum size of an extent that
could be allocated. By replacing the allocation unit model of previous
versions, the need for alignment of allocation units and the restriction on
extent sizes was removed.
The VxFS Version 3 disk layout divides the entire file system space into
fixed size allocation units. The first allocation unit starts at block zero
and all allocation units are a fixed length of 32K blocks. (An exception
may be the last AU, which occupies whatever space remains at the end of
the file system). Because the first AU starts at block zero instead of part
way through the file system as in previous versions, there is no longer a
need for explicit AU alignment or padding to be added when creating a
file system (see mkfs_vxfs(1M)).
The Version 3 file system also moves away from the model of storing AU
structural data at the start of an AU and puts all structural information
in files. So expanding the file system structures simply requires
extending the appropriate structural files. This removes the extent size
restriction imposed by the Version 1 and Version 2 layouts.
All Version 3 structural files reside in the structural fileset , which is
similar to the Version 2 attribute fileset. The structural files in the
Version 3 disk layout are:
object location table file Contains the object location table
(OLT). As with the Version 2 disk
layout, the OLT, which is referenced
from the super-block, is used to locate
the other structural files.
label file Encapsulates the super-block and
super-block replicas. Although the
location of the primary super-block is