Veritas™ Volume Manager Administrator's Guide

Striping (RAID-0)

Striping (RAID-0) is useful if you need large amounts of data written to or read from physical disks, and performance is important. Striping is also helpful in balancing the I/O load from multi-user applications across multiple disks. By using parallel data transfer to and from multiple disks, striping significantly improves data-access performance.

Striping maps data so that the data is interleaved among two or more physical disks. A striped plex contains two or more subdisks, spread out over two or more physical disks. Data is allocated alternately and evenly to the subdisks of a striped plex.

The subdisks are grouped into "columns," with each physical disk limited to one column. Each column contains one or more subdisks and can be derived from one or more physical disks. The number and sizes of subdisks per column can vary. Additional subdisks can be added to columns, as necessary.

If five volumes are striped across the same five disks, then failure of any one of the five disks will require that all five volumes be restored from a backup. If each volume is on a separate disk, only one volume has to be restored. (As an alternative to or in conjunction with striping, use mirroring or RAID-5 to substantially reduce the chance that a single disk failure results in failure of a large number of volumes.)

Data is allocated in equal-sized stripe units that are interleaved between the columns. Each stripe unit is a set of contiguous blocks on a disk. The default stripe unit size is 64 kilobytes.

Figure: Striping across three columns shows an example with three columns in a striped plex, six stripe units, and data striped over the three columns.

Figure: Striping across three columns

A stripe consists of the set of stripe units at the same positions across all columns. In the figure, stripe units 1, 2, and 3 constitute a single stripe.

Striping continues for the length of the columns (if all columns are the same length), or until the end of the shortest column is reached. Any space remaining at the end of subdisks in longer columns becomes unused space.

Figure: Example of a striped plex with one subdisk per column shows a striped plex with three equal sized, single-subdisk columns.

Figure: Example of a striped plex with one subdisk per column

There is one column per physical disk. This example shows three subdisks that occupy all of the space on the VM disks. It is also possible for each subdisk in a striped plex to occupy only a portion of the VM disk, which leaves free space for other disk management tasks.

Figure: Example of a striped plex with concatenated subdisks per column shows a striped plex with three columns containing subdisks of different sizes.

Figure: Example of a striped plex with concatenated subdisks per column

Each column contains a different number of subdisks. There is one column per physical disk. Striped plexes can be created by using a single subdisk from each of the VM disks being striped across. It is also possible to allocate space from different regions of the same disk or from another disk (for example, if the size of the plex is increased). Columns can also contain subdisks from different VM disks.