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 and later releases run 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


Such a disk group may still not be importable by VxVM 4.0 on Linux with a pre-2.6 kernel if it would increase the number of minor numbers on the system that are assigned to volumes to more than 4079, or if the number of available extended major numbers is smaller than 15.

You can use the following command to discover the maximum number of volumes that are supported by VxVM on a Linux host:

# cat /proc/sys/vxvm/vxio/vol_max_volumes

See the vxdg(1M) manual page.