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49Understanding 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.
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