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

179Creating and administering disk groups
Moving disk groups between systems
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.
For further information on minor number reservation, see the vxdg(1M) manual page.
Compatibility of disk groups between platforms
For disk groups that support the Cross-platform Data Sharing (CDS) feature, the upper
limit on the minor number range is restricted on AIX, HP-UX, Linux (with a 2.6 or later
kernel) and Solaris to 65,535 to ensure portability between these operating systems.
On a Linux platform with a pre-2.6 kernel, the number of minor numbers per major
number is limited to 256 with a base of 0. This has the effect of limiting the number of
volumes and disks that can be supported system-wide to a smaller value than is allowed on
other operating system platforms. The number of disks that are supported by a pre-2.6
Linux kernel is typically limited to a few hundred. With the extended major numbering
scheme that was implemented in VxVM 4.0 on Linux, a maximum of 4079 volumes could
be configured, provided that a contiguous block of 15 extended major numbers was
available.
VxVM 4.1 runs on a 2.6 version Linux kernel, which increases the number of minor
devices that are configurable from 256 to 65,536 per major device number. This allows a
large number of volumes and disk devices to be configured on a system. The theoretical
limit on the number of DMP and volume devices that can be configured on such a system
are 65,536 and 1,048,576 respectively. However, in practice, the number of VxVM
devices that can be configured in a single disk group is limited by the size of the private
region.
When a CDS-compatible disk group is imported on a Linux system with a pre-2.6 kernel,
VxVM attempts to reassign the minor numbers of the volumes, and fails if this is not
possible.
To help ensure that a CDS-compatible disk group is portable between operating systems,
including Linux with a pre-2.6 kernel, use the following command to set the maxdev
attribute on the disk group:
# vxdg -g diskgroup set maxdev=4079