The image above is a sample diagram of a stretched SVC configuration. It meets the most basic requirements needed for the configuration. Some of the key items of note are…
3rd Site Quorum Disk
SVC uses a concept of a quorum disk for maintaining cluster availability and storing configuration data. A small portion (couple hundred MBs) of configured storage is reserved for quorum space. In a give SVC cluster there is one active quorum copy, and two candidate quorum copies. In a normal SVC configuration the quorum disks (as they are called) are automatically configured on storage available to the SVC cluster. In a stretched SVC configuration the quorum selection must be controlled manually to ensure availability, which brings me back to the 3rd site quorum…
A stretched SVC configuration provides volume access across two sites. However a third separate site is deployed to act as a “witness” in the event of failure. Since we can’t have one active quorum disk (remember there is only 1 active, 2 candidate or backup disks) in two sites we place it at a third site. Should one of our two sites fail, the third site is available to coordinate automatic fail over.
A third site quorum is an important availability feature and the primary enabling technology for having automatic fail over between sites. A similar product addition was announced to be added to the EMC VPLEX product http://searchstorage.techtarget.com/news/2240035694/EMC-Vplex-expanded-for-active-active-storage-at-EMC-World-2011.
Last item to mention on third site quorums is the requirements for it are minimal. A DS3400 stuck in a storage closet can suffice.
High Available volumes – Volume Mirroring
SVC can do some cool things with volumes: non-disruptive migrations, conversions from/to thick/thin volumes, replication and FlashCopy and volume mirroring. Volume mirroring is utilized in a stretch SVC configuration to ensure that each site has a consistent copy of data. Hosts see a single volume, SVC maintains two copies of the data that makes up the volume. The volume mirror copies are setup so each site has a copy. A loss of a volume copy does not impact volume access, so storage can be taken offline for maintenance if needed.
A Stretched SVC configuration still supports all of the same external storage systems, host types, and host multipath software. I think that’s important to note…with some changes in your SVC configuration only, you can have an storage setup which gives you read/write access to a single volume(s) in two data centers at the same time.
In Part 3 I’ll tell you what you can do when you combine stretched SVC with VMware vSphere.