VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Miscellaneous RAID-5 Operations
Chapter 8 383
Step 2. Parity is updated to reflect the contents of the new data region. First, the
contents of the old data undergo an exclusive OR (XOR) with the parity
(logically removing the old data). The new data is then XORed into the
parity (logically adding the new data). The new data and new parity are
written to a log.
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.
This process is known as a read-modify-write cycle, which is the
default type of write for RAID-5. If a disk fails, both data and parity
stripe units on that disk become unavailable. The disk array is then said
to be operating in a degraded mode. See Figure 8-2, Read-Modify-Write,