VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Volume Layouts in VxVM
28 VERITAS Volume Manager Administrator’s Guide
The figure, “Left-Symmetric Layout,” shows a left-symmetric parity layout with five
disks (one per column).
Left-Symmetric Layout
For each stripe, data is organized starting to the right of the parity stripe unit. In the
figure, data organization for the first stripe begins at P0 and continues to stripe units 0-3.
Data organization for the second stripe begins at P1, then continues to stripe unit 4, and
on to stripe units 5-7. Data organization proceeds in this manner for the remaining stripes.
Each parity stripe unit contains the result of an exclusive OR (XOR) operation performed
on the data in the data stripe units within the same stripe. If one column’s data is
inaccessible due to hardware or software failure, the data for each stripe can be restored
by XORing the contents of the remaining columns data stripe units against their
respective parity stripe units.
For example, if a disk corresponding to the whole or part of the far left column fails, the
volume is placed in a degraded mode. While in degraded mode, the data from the failed
column can be recreated by XORing stripe units 1-3 against parity stripe unit P0 to
recreate stripe unit 0, then XORing stripe units 4, 6, and 7 against parity stripe unit P1 to
recreate stripe unit 5, and so on.
Note Failureof morethanone columnina RAID-5 plexdetachesthe volume. Thevolume
is no longer allowed to satisfy read or write requests. Once the failed columns have
been recovered, it may be necessary to recover user data from backups.
1 2 3 P0
6 7 P1 4
11 P2 8 9
P3 12 13 14
16 17 18 19
Column
Stripe
Parity Stripe Unit
(Data)
Stripe Unit
0
5
10
15
P4