VERITAS File System 4.1 Administrator's Guide

Disk Layout
Disk Layout
Appendix C 253
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.
inode list file Both the primary fileset and the structural fileset have their own set of
inodes stored in an inode list file. Only the inodes in the primary fileset are
visible to users. When the number of inodes is increased, the kernel
increases the size of the inode list file.
inode allocation unit file Holds the free inode map, extended operations map, and a summary
of inode resources.
log file Maps the block used by the file system intent log.
extent allocation unit state file Indicates the allocation state of each AU by defining whether
each AU is free, allocated as a whole (no bitmaps allocated), or expanded, in
which case the bitmaps associated with each AU determine which extents
are allocated.