VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Miscellaneous RAID-5 Operations
Chapter 8 385
written to the data stripe units. The entire stripe is written in a single
write. See Figure 8-3, Full-Stripe Write,
Figure 8-3 Full-Stripe Write
Reconstruct-Writes
When 50 percent or more of the data disks are undergoing writes in a
single I/O, a reconstruct-write can be used. A reconstruct-write saves
I/O time by XORing. XORing does not require a read of the parity region
and only requires a read of the unaffected data. Unaffected data
amounts to less than 50 percent of the stripe units in the stripe.
A reconstruct-write procedure consists of the following steps:
Step 1. Unaffected data is read from the unchanged data stripe unit(s).
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: Performs XORs between data and parity to calculate new parity.
Logs new data and new parity.
= Step 2: Writes new parity (resulting from XOR) to parity stripe unit P0
SU = Stripe Unit
Parityfor
Disk 5
Datafor
Disk 3
Datafor
Disk 4