VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Miscellaneous RAID-5 Operations
Chapter 8386
Step 2. The new data is XORed with the old, unaffected data to generate a new
parity stripe unit. The new data and resulting parity are logged.
Step 3. The new parity is written to the parity stripe unit. The new data is
written to the data stripe units. All stripe units are written in a single
write.
See Figure 8-4, Reconstruct-Write,. A reconstruct-write is preferable to a
read-modify-write in this case because it reads only the necessary data
disks, rather than reading the disks and the parity disk.
Figure 8-4 Reconstruct-Write
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
Parityfor
Disk 5
Datafor
Disk 3
Datafor
Disk 4
= Step 1: Reads data from unaffected data stripe unit 3.
= Step 2: Performs XORs between old, unaffected data and new data. Logs
new data
and new parity.
= Step 3: Writes new parity (resulting from XOR) to parity stripe unit P0
SU = Stripe Unit