Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

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 ensures 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), resynchronization 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 require resynchronization.
Resynchronization process
The process of resynchronization depends on the type of volume. 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. 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.
59Understanding Veritas Volume Manager
Volume resynchronization