VERITAS File System 3.5 (HP OnlineJFS/JFS 3.5) Administrator's Guide (August 2003)

Appendix C
Disk Layout
Disk Space Allocation
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Disk Space Allocation
Disk space is allocated by the system in 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.
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:
object location table file Contains the object location table (OLT). 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 known, the label file can be used to
locate super-block copies if there is structural damage to the file system.
device file Records device information such as volume length and volume label, and
contains pointers to other structural files.
fileset header file Holds information on a per-fileset basis. This may include the inode of the
fileset's inode list file, the maximum number of inodes allowed, an
indication of whether the file system supports large files, and the inode
number of the quotas file if the fileset supports quotas. When a file
system is created, there are two filesets—the structural fileset defines the
file system structure, the primary fileset contains user data.