Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

55Understanding Veritas Volume Manager

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 the disk group that is automatically imported at boot time (usually

aliased as the reserved system-wide disk group, bootdg), 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.

Resynchronization process

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 by placing the volume in recovery mode (also called read-

writeback recovery mode). Resynchronization of data in the volume is done in the

background. This allows the volume to be available for use while recovery is taking place.

The process of resynchronization can impact system performance. The recovery process

reduces some of this impact by spreading the recoveries to avoid stressing a specific disk

or controller.