VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Miscellaneous RAID-5 Operations
Chapter 8384
Figure 8-2 Read-Modify-Write
Full-Stripe Writes
When large writes (writes that cover an entire data stripe) are issued,
the read-modify-write procedure can be bypassed in favor of a
full-stripe write. A full-stripe write is faster than a read-modify-write
because it does not require the read process to take place. Eliminating
the read cycle reduces the I/O time necessary to write to the disk.
A full-stripe write procedure consists of the following steps:
Step 1. All the new data stripe units are XORed together, generating a new
parity value. The new data and new parity is written to a log.
Step 2. The new parity is written to the parity stripe unit. The new data is
New Data
Datafor
Disk 1
Datafor
Disk 2
XO
Disk 1
Column
Disk 2
Column
Disk 3
Column
Disk 4
Column
Disk 5
Column
4
Lo
Data
and
= Step 1: Reads data (from parity stripe unit P0 and data stripe units 0 &
1).
= Step 2: Performs XORs between data and parity to calculate new parity.
Logs new data and new parity.
SU = Stripe Unit
Parityfor
Disk 5