About Tertiary Mirrors
If you have at least three storage servers in your SAN, you also have the option of creating a tertiary mirror. Individual tertiary volumes are, in effect, a duplicate image of a standard (two volume) mirror.
Follow the steps for creating a mirror, and then take the option to create a mirror on the existing mirror. This will in effect add a third volume to that original mirror. This new virtual volume will automatically be recognized as a tertiary mirror. To create a mirror from an existing mirror a valid iSCSI or Fibre Channel path from the primary storage server to the tertiary storage server is required. I/O is written from the primary storage server to the tertiary volume.
Once the tertiary mirror has initialized, the tertiary volume may be split off at any time (refer to Splitting a Tertiary Mirror). When the tertiary volume is removed from a tertiary mirror, the tertiary volume reverts to a linear virtual volume. Since this linear virtual volume contains an exact copy of the primary virtual volume data at the time of the removal from the mirror, it can be used for a wide variety of applications without impacting the performance or data integrity of the primary or secondary volumes. Tertiary mirrors can also be removed and replaced while mapped. Traveller reference volumes (a form of tertiary) cannot be replaced; they can only be deleted and recreated.
You can also create a tertiary mirror with Alternate Pathing (AP), MPIO, or Cluster Path (CP) attributes to provide High Availability capabilities on the primary and secondary volumes in a tertiary mirror. Mirrors are created in the standard manner reflected in this Help for AP, MPIO, and CP, but you must specify the mirror path type of your virtual volumes.
The use of tertiary mirrors is not supported when the primary and secondary volumes in a virtual volume are dual path.