VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Volume Resynchronization
38 VERITAS Volume Manager Administrator’s Guide
Transformation Characteristics
Transformation of data from one layout to another involves rearrangement of data in the
existing layout to the new layout. During the transformation, online relayout retains data
redundancy by mirroring any temporary space used. Read and write access to data is not
interrupted during the transformation.
Data is not corrupted if the system fails during a transformation. The transformation
continues after the system is restored and both read and write access are maintained.
You can reverse the layout transformation process at any time, but the data may not be
returned to the exact previous storage location. Any existing transformation in the
volume must be stopped before doing a reversal.
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. Ifeither
of these conditions occurs, online relayout uses the vxresize(1M) command to shrink
or grow a file system as described in “Resizing a Volume” on page 200.
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 normalconfiguration changes(such asdetaching and reattaching aplex), thiscan only
occur when a system crashes while data is being written to a volume.
Data is written to the mirrors of a volume inparallel, as is the data and parity in a RAID-5
volume. If asystemcrash occursbeforeall the individual writescomplete,it ispossiblefor
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.