Veritas File System 5.0 Administrator's Guide (September 2006)

on the same system. VxFS allocates disk space to files in extents. An extent is a
set of contiguous blocks.
VxFS Version 4 disk layout
The Version 4 disk layout allows the file system to scale easily to accommodate
large files and large file systems.
The original 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 4 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.
The Version 4 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 previous layouts.
All Version 4 structural files reside in the structural fileset.
The structural files in the Version 4 disk layout are:
DescriptionFile
Contains the object location table (OLT). The OLT, which is referenced
from the super-block, is used to locate the other structural files.
object location
table file
Encapsulates the super-block and super-block replicas. Although the
location of the primary super-block is known, the label file can be used
to locate super-block copies if there is structural damage to the file
system.
label file
Records device information such as volume length and volume label,
and contains pointers to other structural files.
device file
275Disk layout
VxFS Version 4 disk layout