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Reserving minor numbers for disk groups

A device minor number uniquely identifies some characteristic of a device to the device driver that controls that device. It is often used to identify some characteristic mode of an individual device, or to identify separate devices that are all under the control of a single controller. VxVM assigns unique device minor numbers to each object (volume, plex, subdisk, disk, or disk group) that it controls.

When you move a disk group between systems, it is possible for the minor numbers that it used on its previous system to coincide 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 65,535. 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


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 hdisk10 hdisk11

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

If a volume is open, its old device number remains in effect until the system is rebooted or until the disk group is deported and re-imported. If you close the open volume, you can run vxdg reminor again to allow the renumbering to take effect without rebooting or re-importing.

An example of where it is necessary to change the base minor number is for a cluster-shareable disk group. The volumes in a shared disk group must have the same minor number on all the nodes. If there is a conflict between the minor numbers when a node attempts to join the cluster, the join fails. You can use the reminor operation on the nodes that are in the cluster to resolve the conflict. In a cluster where more than one node is joined, use a base minor number which does not conflict on any node.

See the vxdg(1M) manual page.