There are several layouts for data and parity that can be used in the setup of a RAID-5 array. The implementation of RAID-5 in VxVM uses a left-symmetric layout. This provides optimal performance for both random I/O operations and large sequential I/O operations. However, the layout selection is not as critical for performance as are the number of columns and the stripe unit size.

Left-symmetric layout stripes both data and parity across columns, placing the parity in a different column for every stripe of data. The first parity stripe unit is located in the rightmost column of the first stripe. Each successive parity stripe 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: Left-symmetric layout shows a left-symmetric parity layout with five disks (one per column).

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.