VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Volume Layouts in VxVM
26 VERITAS Volume Manager Administrator’s Guide
Traditional RAID-5 Arrays
A traditional RAID-5 array is several disks organized in rows and columns. A column is a
number of disks located in the same ordinal position in the array. A row is the minimal
number of disks necessary to support the full width of a parity stripe. The figure,
“Traditional RAID-5 Array”, shows the row and column arrangement of a traditional
RAID-5 array.
Traditional RAID-5 Array
This traditional array structure supports growth by adding more rows per column.
Striping is accomplished by applying the first stripe across the disks in Row 0, then the
second stripe across the disks in Row 1, then the third stripe across the Row 0 disks, and
so on. This type of array requires all disks columns, and rows to be of equal size.
VERITAS Volume Manager RAID-5 Arrays
The RAID-5 array structure in VERITAS Volume Manager differs from the traditional
structure. Due to the virtual nature of its disks and other objects, VxVM does not use
rows. Instead, VxVM uses columns consisting of variable length subdisks (as shown in
“VERITAS Volume ManagerRAID-5 Array” on page 27). Each subdisk represents a
specific area of a disk.
VxVM allows each column of a RAID-5 plex to consist of a different number of subdisks.
The subdisks in a given column can be derived from different physical disks. Additional
subdisks can be added to the columns as necessary. Striping is implemented by applying
the first stripe across each subdisk at the top of each column, then applying another stripe
below that, and so on for the length of the columns. Equal-sized stripe units are used for
each column. For RAID-5, the default stripe unit size is 16 kilobytes. See “Striping
(RAID-0)” on page 17 for further information about stripe units.
Column 0 Column 1 Column 2 Column 3
Row 0
Row 1
Stripe 3
Stripe 2
Stripe 1