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

46 Chapter2
Disk Layout
The VxFS Version 1 Disk Layout
Super-Block
The super-block contains important information about the file system,
such as:
• the file system type
• creation and modification dates
• label information
• information about the size and layout of the file system
• the count of available resources
• the file system disk layout version number
The super-block is always in a fixed location, offset from the start of the
file system by 8192 bytes. This fixed location enables utilities to easily
locate the super-block when necessary. The super-block is 1024 bytes
long.
Copies of the super-block are kept in allocation unit headers: thesecopies
can be used for recovery purposes if the super-block is corrupted or
destroyed (see fsck_vxfs(1M) for more details).
Intent Log
In the event of system failure, the VxFS file system uses intent logging to
guarantee file system integrity.
The intent log is a circular activity log with a default size of 1024
blocks. If the file system is smaller than 4 MB, the default log size is
reduced (by mkfs_vxfs(1M)) to avoid wasting space. The intent log
contains records of the intention of the system to update a file system
structure. An update to the file system structure (a transaction ) is
divided into separate sub-functions for each data structure that needs to
be updated. A composite log record of the transaction is created,
containing the sub-functions constituting the transaction.
For example, the creation of a file that would expand the directory in
which the file is contained would produce a transaction consisting of the
following sub-functions:
• a free extent map update for the allocation of the new directory block
• a directory block update