Low-Cost DR with SVC Stretched Cluster

Ever since IBM’s SAN Volume Controller was first delivered back in 2003 folks have been looking at the 2n cluster nodes and wanting to separate them physically to achieve a degree of disaster recovery. This is often referred to as  ‘split i/o group’ or ‘split cluster’ but as of April 2013 I have been told the preferred nomenclature is now “SVC Stretched Cluster”. For this to be a good idea, two additional features were needed. Vdisk mirroring, which was delivered with SVC 4.3 and the ability to manually place the quorum data wherever you want, which was delivered in SVC 5.1.

The key advantage of stretched cluster is that if you lose one of your computer rooms, the SVC will keep processing and is designed to automatically switch to using the secondary copy of any vdisks in less than 30 seconds. All this without any additional replication licensing being required.

There are some design rules you need to pay attention to if you want to maximise the DR value from stretched cluster. Firstly, SVC is a cluster and as such it has a quorum. SVC maintains 3 copies of the quorum. In the following examples I will assume we have two SVC nodes (SVC can include 2, 4, 6 or 8 nodes, but the vast majority are 2 node systems). The principles are the same for all cluster sizes.

To build a robust DR solution using SVC stretched cluster you ideally want 3 computer rooms – two of them up to 10Kms apart and a third within OM3 shortwave distance of the second. The first is to house the SVC node1, the second is to house SVC node2 and the third is to house your primary quorum. In reality you don’t need three computer rooms, but you do need 3 separate power sources as a minimum.

The SVC is an inherently HA device, but if you lose half the nodes (1 in this case) plus the primary quorum, the SVC will stop processing. However, with the quorum in a 3rd location, on separate power, you would have to be the victim of a campus-wide disaster to lose the system.

Just to clarify – this allows you to build a localized DR solution using ‘free’ features of SVC. You can of course do SVC to SVC replication over any distance you desire using the licensed  Global Mirror function, and that would give you a much more complete DR solution because of the greater physical separation.

There are a few different ways to skin the stretched-cluster cat, but the one that seems to be the most practical is something like this:

Location 1:

• One SVC node
• At least one disk system
• At least 2 FC switches

Location 2:

• One SVC node
• At least one FC switch

Location 3:

• At least one disk system. Must be from a special list of qualified systems, but can be very small e.g. DS3500 with 3 drives
• At least one FC switch

The switches have been placed so as to avoid a fire in any one room taking out a second room. The fabric can of course be more complex with core/edge etc as required. You’ll note the longwave connections. The CF8 hardware nodes that have been shipping with SVC since 2009 have a standard option to add longwave SFPs. If you want to do stretched cluster on the 8A4 entry nodes you will need to source your own Finisar FTLF1424P2BCD longwave SFPs. You will need two supported longwave SFPs in each SVC node.

Remember the two key advantages of this solution:

1. Lower cost than replication between two completely separate systems
2. Automatic failover (<30 seconds) if a site goes down

Content revised for clarity 12th July 2010, 27th July 2010. “Stretched Cluster” terminology  clarified 23rd April 2013.