HP Veritas Volume Manager 5.0 -UX 11i v3 manual Data Parity, Traditional RAID-5 arrays

46Understanding Veritas Volume Manager

Volume layouts in VxVM

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.

Figure 1-20shows 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.

Figure 1-20

l

Stripe 1

Stripe 2

Stripe 3

Stripe 4

Parity locations in a RAID-5 mode

Data	Data
Data	Parity
Parity	Data
Data	Data

Parity

Data

Data

Parity

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.

The implementation of RAID-5 in VxVM is described in “Veritas Volume Manager RAID-5 arrays” on page 47.

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 row is the minimal number of disks necessary to support the full width of a