Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

46 Understanding Veritas Volume Manager

Volume layouts in VxVM

Figure 1-23 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: Failure of more than one column in a RAID-5 plex detaches the volume. The

volume 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