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

54 Chapter2
Disk Layout
The VxFS Version 2 Disk Layout
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)).
Object Location Table
The object location table (OLT) can be considered an extension of the
super-block. The OLT contains information used at mount time to locate
file system structures that are not in fixed locations. The OLT is typically
located immediately after the super-block and is 8K long. However, if a
Version 1 file system is upgraded to Version 2, the placement of the OLT
depends on the availability of space.
The OLT is replicated and its replica is located immediately after the
intent log. The OLT and its replica are separated in order to minimize
the potential for losing both copies of the vital OLT information in the
event of localized disk damage.
The contents and use of the OLT are described in “Locating Dynamic
Structures”.
Intent Log
The VxFS file system uses intent logging to guarantee file system
integrity in the event of system failure
The intent log is a circular activity log with a default size of 512
blocks. If the file system is less than 4 MB, the log size will be reduced 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 separatesub-functions for each
data structure that needs to be updated. A composite log record of the
transaction is created that contains the subventions that constitute the
transaction.
For example, the creation of a file that would expand the directory in
which the file is contained will produce a transaction consisting of the
following subventions:
• a free extent map update for the allocation of the new directory block