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

36 Understanding Veritas Volume Manager
Volume layouts in VxVM
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.
Caution: Striping a volume, or splitting a volume across multiple disks, increases the
chance that a disk failure will result in failure of that volume.
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 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 units (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 (or width) is 64 kilobytes.
For example, if there are three columns in a striped plex and six stripe units, data is striped
over the three columns, as illustrated in Figure 1-14.