Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 1, Understanding 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.