Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

unit is located in the next stripe, shifted left one column from the previous parity

stripe unit location. If there are more stripes than columns, the parity stripe unit

placement begins in the rightmost column again.

Figure 1-23 shows a left-symmetric parity layout with five disks (one per column).

Figure 1-23

Left-symmetric layout

Data stripe unit

Parity stripe unit

Column

Stripe

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.

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.

Understanding Veritas Volume Manager

Volume layouts in VxVM