Handling cloned disks with duplicated identifiers

A disk may be copied by creating a hardware snapshot (such as an EMC BCV™ or Hitachi ShadowCopy™) or clone, by using dd or a similar command to replicate the disk, or by building a new LUN from the space that was previously used by a deleted LUN. To avoid the duplicate disk ID condition, the default action of VxVM is to prevent such duplicated disks from being imported.

Advanced disk arrays provide hardware tools that you can use to create clones of existing disks outside the control of VxVM. For example, these disks may have been created as hardware snapshots or mirrors of existing disks in a disk group. As a result, the VxVM private region is also duplicated on the cloned disk. When the disk group containing the original disk is subsequently imported, VxVM detects multiple disks that have the same disk identifier that is defined in the private region. In releases prior to 5.0, if VxVM could not determine which disk was the original, it would not import such disks into the disk group. The duplicated disks would have to be re-initialized before they could be imported.

From release 5.0, a unique disk identifier (UDID) is added to the disk's private region when the disk is initialized or when the disk is imported into a disk group (if this identifier does not already exist). Whenever a disk is brought online, the current UDID value that is known to the Device Discovery Layer (DDL) is compared with the UDID that is set in the disk's private region. If the UDID values do not match, the udid_mismatch flag is set on the disk. This flag can be viewed with the vxdisk list command. This allows a LUN snapshot to be imported on the same host as the original LUN. It also allows multiple snapshots of the same LUN to be simultaneously imported on a single server, which can be useful for off-host backup and processing.

A new set of vxdisk and vxdg operations are provided to handle such disks; either by writing the DDL value of the UDID to a disk's private region, or by tagging a disk and specifying that it is a cloned disk to the vxdg import operation.

The following is sample output from the vxdisk list command showing that disks sdg, sdh and sdi are marked with the udid_mismatch flag:

# vxdisk list

DEVICE       TYPE          DISK     GROUP    STATUS
sda          auto:cdsdisk  -        -        online
sdb          auto:cdsdisk  -        -        online
sde          auto:cdsdisk  -        -        online
sdf          auto:cdsdisk  -        -        online
sdg          auto:cdsdisk  -        -        online udid_mismatch
sdh          auto:cdsdisk  -        -        online udid_mismatch
sdi          auto:cdsdisk  -        -        online udid_mismatch