VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager

Volume Resynchronization

Chapter 144

Volume Resynchronization

When storing data redundantly and using mirrored or RAID-5 volumes,

VxVM 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.

VxVM 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.

Dirty Flags

VxVM 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, and all writes have been completed, VxVM removes the

dirty ﬂag for the volume. Only volumes that are marked dirty when the

system reboots require resynchronization.