VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Volume Resynchronization
Chapter 3110
Volume Resynchronization
When storing data redundantly, using mirrored or RAID-5 volumes, the
Volume Manager ensures that all copies of the data match exactly.
However, under certain conditions (usually due to complete system
failures), some redundant data on a volume can become inconsistent or
unsynchronized . The mirrored data is not exactly the same as the
original data. Except for normal configuration changes (such as
detaching and reattaching a plex), this can only occur when a system
crashes while data is being written to a volume.
Data is written to the mirrors of a volume in parallel, as is the data and
parity in a RAID-5 volume. If a system crash occurs before all the
individual writes complete, it is possible for some writes to complete
while others do not. This can result in the data becoming
unsynchronized. For mirrored volumes, it can cause two reads from the
same region of the volume to return different results, if different mirrors
are used to satisfy the read request. In the case of RAID-5 volumes, it
can lead to parity corruption and incorrect data reconstruction.
The Volume Manager needs to ensure that all mirrors contain exactly
the same data and that the data and parity in RAID-5 volumes agree.
This process is called volume resynchronization. For volumes that
are part of disk groups that are automatically imported at boot time
(such as rootdg), the resynchronization process takes place when the
system reboots.
Not all volumes require resynchronization after a system failure.
Volumes that were never written or that were quiescent (that is, had no
active I/O) when the system failure occurred could not have had
outstanding writes and do not require resynchronization.
The Volume Manager records when a volume is first written to and
marks it as dirty. When a volume is closed by all processes or stopped
cleanly by the administrator, all writes have been completed and the
Volume Manager removes the dirty flag for the volume. Only volumes
that are marked dirty when the system reboots require
resynchronization.
The process of resynchronization depends on the type of volume. RAID-5
volumes that contain RAID-5 logs can “replay” those logs. If no logs are
available, the volume is placed in reconstruct-recovery mode and all
parity is regenerated. For mirrored volumes, resynchronization is done