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

56 Chapter2
Disk Layout
The VxFS Version 2 Disk Layout
Figure 2-4 Allocation Unit Structure
One or more allocation units exist per file system. Allocation units are
located after the OLT replica. The number and size of allocation units
can be specified when the file system is made. All of the allocation units,
except possibly the last one, are of equal size. If space is limited, the last
allocation unit can have a partial set of data blocks to allow use of all
remaining blocks.
Allocation Unit Header The allocation unit header contains a copy of
the file system’s super-block that is used to verify that the allocation unit
matches the super-block of the file system. The super-block copies
contained in allocation unit headers can also be used for recovery
purposes if the super-block is corrupted or destroyed. The allocation unit
header occupies the first block of each allocation unit.
Allocation Unit Summary The allocation unit summary summarizes
the resources (data blocks) used in the allocation unit. This includes
information such as the number of free extents of each size in the
allocation unit.
Free Extent Map The free extent map is a series of independent
512-byte bitmaps that are each referred to as a free extent map section.
Each section is broken down into multiple regions. The first region of
2048 bits represents a section of 2048 one-block extents. The second
region of 1024 bits represent a section of 1024 two-block extents. This
regioning continues for all powers of 2 up to the single bit that represents
Allocation Unit Header
Allocation Unit Summary
Free Extent Map
Padding
Data Blocks