VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Volume Layouts in VxVM
Chapter 136
complete. However, only the data write to disk A is complete. The parity
write to disk C is incomplete, which would cause the data on disk B to be
reconstructed incorrectly.
Figure 1-25 Incomplete Write
This failure can be avoided by logging all data and parity writes before
committing them to the array. In this way, the log can be replayed,
causing the data and parity updates to be completed before the
reconstruction of the failed drive takes place.
Logs are associated with a RAID-5 volume by being attached as log
plexes. More than one log plex can exist for each RAID-5 volume, in
which case the log areas are mirrored.
See “Adding a RAID-5 Log” on page 285 for information on how to add a
RAID-5 log to a RAID-5 volume.
Layered Volumes
A layered volume is a virtual Volume Manager object that is built on top
of other volumes. The layered volume structure tolerates failure better
and has greater redundancy than the standard volume structure. For
example, in a striped-mirror layered volume, each mirror (plex) covers a
smaller area of storage space, so recovery is quicker than with a
standard mirrored volume.
The figure, “Example of a Striped-Mirror Layered Volume,” illustrates
the structure of a typical layered volume. It shows subdisks with two
columns, built on underlying volumes with each volume internally
mirrored. The volume and striped plex in the “Managed by User” area
allow you to perform normal tasks in VxVM. User tasks can be
performed only on the top-level volume of a layered volume.
Completed
Corrupted Data
Incomplete
Disk A Disk B Disk C
Data Write Parity Write