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

44 Understanding Veritas Volume Manager
Volume layouts in VxVM
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. Figure 1-21 shows
the row and column arrangement of a traditional RAID-5 array.
Figure 1-21 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 Figure 1-
22. 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 35 for further information about stripe units.
Column 0 Column 1 Column 2 Column 3
Row 0
Row 1
Stripe 3
Stripe 2
Stripe 1