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

Chapter 1, Understanding VERITAS Volume Manager
Volume Layouts in VxVM
29
RAID-5 (Striping with Parity)
Note VxVM supports RAID-5 for private disk groups, but not for shareable disk groups
in a cluster environment. In addition, VxVM does not support the mirroring of
RAID-5 volumes that are configured using VERITAS Volume Manager software.
Disk devices that support RAID-5 in hardware may be mirrored.
You need a full license to use this feature.
Although both mirroring (RAID-1) and RAID-5 provide redundancy of data, they use
different methods. Mirroring provides data redundancy by maintaining multiple
complete copies of the data in a volume. Data being written to a mirrored volume is
reflected in all copies. If a portion of a mirrored volume fails, the system continues to use
the other copies of the data.
RAID-5 provides data redundancy by using parity. Parity is a calculated value used to
reconstruct data after a failure. While data is being written to a RAID-5 volume, parity is
calculated by doing an exclusive OR (XOR) procedure on the data. The resulting parity is
then written to the volume. The data and calculated parity are contained in a plex that is
“striped” across multiple disks. If a portion of a RAID-5 volume fails, the data that was on
that portion of the failed volume can be recreated from the remaining data and parity
information. It is also possible to mix concatenation and striping in the layout.
The figure, “Parity Locations in a RAID-5 Mode” on page 29, shows parity locations in a
RAID-5 array configuration. Every stripe has a column containing a parity stripe unit and
columns containing data. The parity is spread over all of the disks in the array, reducing
the write time for large independent writes because the writes do not have to wait until a
single parity disk can accept the data.
Parity Locations in a RAID-5 Mode
l
RAID-5 volumes can additionally perform logging to minimize recovery time. RAID-5
volumes use RAID-5 logs to keep a copy of the data and parity currently being written.
RAID-5 logging is optional and can be created along with RAID-5 volumes or added later.
Parity
Parity
Data
Data
Data
Data
Data
Parity
Data
Data
Parity
Data
Stripe 1
Stripe 4
Stripe 3
Stripe 2