Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

You can determine the transformation direction by using the vxrelayout status
volume command.
These transformations are protected against I/O failures if there is sufficient
redundancy and space to move the data.
Transformations and volume length
Some layout transformations can cause the volume length to increase or decrease.
If either of these conditions occurs, online relayout uses the vxresize command
to shrink or grow a file system.
See “Resizing a volume” on page 351.
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.
Understanding Veritas Volume Manager
Volume resynchronization
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