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

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.
Figure 1-20 shows parity locations in a RAID-5 array configuration.
Figure 1-20
Parity locations in a RAID-5 model
Data
Data
Parity
Data
Stripe 1
Parity
Data
Data
Data
Data
Parity
Data
Data
Stripe 2
Stripe 3
Stripe 4
Parity
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.
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.
See “Veritas Volume Manager RAID-5 arrays” on page 46.
Note: VxVM supports RAID-5 for private disk groups, but not for shareable disk
groups in a CVM environment. In addition, VxVM does not support the mirroring
of RAID-5 volumes that are configured using Veritas Volume Manager software.
RAID-5 LUNs hardware may be mirrored.
Traditional RAID-5 arrays
A traditional RAID-5 array is several disks organized in rows and columns. A
column is a number of disks located in the same ordinal position in the array. A
45Understanding Veritas Volume Manager
Volume layouts in VxVM