Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

The blocks n, n+1, n+2 and n+3 (numbered relative to the start of the plex) are
contiguous on the plex, but actually come from two distinct subdisks on the same
physical disk.
The remaining free space in the subdisk, disk01-02, on VM disk, disk01, can be
put to other uses.
You can use concatenation with multiple subdisks when there is insufficient
contiguous space for the plex on any one disk. This form of concatenation can be
used for load balancing between disks, and for head movement optimization on
a particular disk.
Figure 1-13 shows data spread over two subdisks in a spanned plex.
Figure 1-13
Example of spanning
Plex with concatenated subdisks
Data blocks
Subdisks
VM disks
Physical disks
disk01-01
Data in
disk01-01
Data in
disk02-01
disk02-01
disk01-01 disk02-01
disk01
n n+1 n+2 n+3
devname1
n n+1 n+2
devname2
disk01-01
disk02
disk02-02disk02-01
n+3
The blocks n, n+1, n+2 and n+3 (numbered relative to the start of the plex) are
contiguous on the plex, but actually come from two distinct subdisks from two
distinct physical disks.
The remaining free space in the subdisk disk02-02 on VM disk disk02 can be put
to other uses.
Warning: Spanning a plex across multiple disks increases the chance that a disk
failure results in failure of the assigned volume. Use mirroring or RAID-5 to reduce
the risk that a single disk failure results in a volume failure.
37Understanding Veritas Volume Manager
Volume layouts in VxVM