VERITAS File System 4.1 Administrator's Guide

Disk Layout
Disk Layout
Appendix C252
Some of the disk layout versions were not supported on all UNIX operating systems. Version 2
and 3 file systems can still be mounted, but this will be disallowed in future releases.
Currently, the Version 4, Version 5, and Version 6 disk layouts can be created and mounted.
Version 6 is the default disk layout version.
The vxupgrade command is provided to upgrade an existing VxFS file system to the Version
4, Version 5, or Version 6 disk layout while the file system remains online. You must do an
upgrade in steps from older to newer layouts. See the vxupgrade(1M) manual page for details
on upgrading VxFS file systems.
The vxfsconvert command is provided to upgrade Version 2 and 3 disk layouts to the Version
4 disk layout while the file system is not mounted. Using vxfsconvert, the file system can be
converted to the Version 4 layout while offline, then using vxupgrade, you can convert it to
Version 5 while online. See the vxfsconvert(1M) manual page for details on upgrading VxFS
disk layouts.
The following additional topics are covered in this appendix:
• Disk Space Allocation
• The VxFS Version 4 Disk Layout
• The VxFS Version 5 Disk Layout
• The VxFS Version 6 Disk Layout
Disk Space Allocation
Disk space is allocated by the system in 1024-byte sectors. An integral number of sectors are
grouped together to form a logical block. VxFS supports logical block sizes of 1024, 2048, 4096,
and 8192 bytes. The default block size is 1024 bytes. The block size may be specified as an
argument to the mkfs utility and may vary between VxFS file systems mounted on the same
system. VxFS allocates disk space to files in extents. An extent is a set of contiguous blocks.
The 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.