Understanding Veritas Volume Manager 47

Volume layouts in VxVM

parity stripe. Figure 1-21shows the row and column arrangement of a traditional RAID-5 array.

Figure 1-21 Traditional RAID-5 array

Row 0

Row 1

Stripe 1 Stripe 3

Stripe 2

Column 0 Column 1 Column 2 Column 3

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 38 for further information about stripe units.