Virtual disk mirroring

Virtual disk mirroring allows a VDisk to have two physical copies. Each VDisk copy can belong to a different managed disk (MDisk) group, and each copy has the same virtual capacity as the VDisk.

When a server writes to a mirrored VDisk, the SAN Volume Controller cluster writes the data to both copies. When a server reads a mirrored VDisk, the SAN Volume Controller cluster picks one of the copies to read. If one of the mirrored VDisk copies is temporarily unavailable; for example, because the RAID controller that provides the MDisk group is unavailable, the VDisk remains accessible to servers. The SAN Volume Controller cluster remembers which areas of the VDisk are written and resynchronizes these areas when both copies are available.

You can create a VDisk with one or two copies and convert a non-mirrored VDisk into a mirrored VDisk by adding a copy. When a copy is added in this way, the SAN Volume Controller cluster synchronizes the new copy so that it is the same as the existing VDisk. Servers can access the VDisk during this synchronization process.

You can convert a mirrored VDisk into a non-mirrored VDisk by deleting one copy or by splitting one copy to create a new non-mirrored VDisk.

The VDisk copy can be any type: image, striped, sequential, and space-efficient or not. The two copies can be of completely different types.

VDisk Mirroring can be used for the following applications:

Improving availability of VDisks by protecting them from a single storage controller failure.
Allowing concurrent maintenance of a storage controller that does not natively support concurrent maintenance.
Providing an alternative method of data migration with better availability characteristics. While a VDisk is being migrated using the data migration feature, it is vulnerable to failures on both the source and target MDisk group. VDisk Mirroring provides an alternative because you can start with a non-mirrored VDisk in the source MDisk group and then add a copy to that VDisk in the destination MDisk group. When the VDisk is synchronized, you can delete the original copy that is in the source MDisk group. During the synchronization process, the VDisk remains available even if there is a problem with the destination MDisk group.
Maintaining access to data that is stored on SSDs if one of the nodes in an I/O group is being serviced or fails.
Converting between fully allocated VDisks and space-efficient VDisks.

When you use VDisk mirroring, consider how quorum candidate disks are allocated. VDisk mirroring maintains some state data on the quorum disks. If a quorum disk is not accessible and VDisk mirroring is unable to update the state information, a mirrored VDisk might need to be taken offline to maintain data integrity. To ensure the high availability of the system, ensure that multiple quorum candidate disks, allocated on different controllers, are configured.

Attention: Mirrored VDisks may be taken offline if there is no quorum disk available. This behavior occurs because synchronization status for mirrored VDisks is recorded on the quorum disk. To protect against mirrored VDisks being taken offline, follow the guidelines for setting up quorum disks.