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

178 Creating and administering disk groups
Moving disk groups between systems
When you move a disk group between systems, it is possible for the minor numbers that it
used on its previous system to coincide (or collide) with those of objects known to VxVM
on the new system. To get around this potential problem, you can allocate separate ranges
of minor numbers for each disk group. VxVM uses the specified range of minor numbers
when it creates volume objects from the disks in the disk group. This guarantees that each
volume has the same minor number across reboots or reconfigurations. Disk groups may
then be moved between machines without causing device number collisions.
VxVM chooses minor device numbers for objects created from this disk group starting at
the base minor number base_minor. Minor numbers can range from this value up to
16,777,215. Try to leave a reasonable number of unallocated minor numbers near the top
of this range to allow for temporary device number remapping in the event that a device
minor number collision may still occur.
VxVM reserves the range of minor numbers from 0 to 999 for use with volumes in the
boot disk group. For example, the rootvol volume is always assigned minor number 0.
If you do not specify the base of the minor number range for a disk group, VxVM chooses
one at random. The number chosen is at least 1000, is a multiple of 1000, and yields a
usable range of 1000 device numbers. The chosen number also does not overlap within a
range of 1000 of any currently imported disk groups, and it does not overlap any currently
allocated volume device numbers.
Note: The default policy ensures that a small number of disk groups can be merged
successfully between a set of machines. However, where disk groups are merged
automatically using failover mechanisms, select ranges that avoid overlap.
To view the base minor number for an existing disk group, use the
vxprint command as
shown in the following examples for the disk group, mydg:
# vxprint -l mydg | egrep minors
minors: >=45000
# vxprint -g mydg -m | egrep base_minor
base_minor=45000
To set a base volume device minor number for a disk group that is being created, use the
following command:
# vxdg init diskgroup minor=base_minor disk_access_name ...
For example, the following command creates the disk group, newdg, that includes the
specified disks, and has a base minor number of 30000:
# xvdg init newdg minor=30000 c1d0t0 c1t1d0
If a disk group already exists, you can use the vxdg reminor command to change its base
minor number:
# vxdg -g diskgroup reminor new_base_minor
For example, the following command changes the base minor number to 30000 for the
disk group, mydg:
# vxprint -g mydg reminor 30000