Thats just a FUD question… to be preferred or not preferred?

ORIGINALLY POSTED 1st March 2013

27,043 views on developerworks

One of the internal, and IBM Business Partner mailing groups I subscribe and participate in ( Hi to all on Diskcore ) recently debated :

Want to know whether IBM V3700 are true symmetric active active controllers.

A response was made, but also asked :

Can you define what you or the customer means exactly by true symmetric active/active disk controller? 

The whole “preferred” status of a vdisk is an attempt to load balance.

So when you create volumes…

vdisk id = 0  - preferred to node 2
vdisk id = 1  - preferred to node 1
vdisk id = 2  - preferred to node 2
etc

The idea being, if a multipath driver understands ALUA (preferred) then overall you spread your load over both nodes. (control canisters) So load balances at a lun level. If a multipath driver does not understand or use ALUA then it will round robin I/O across nodes. So load balances I/O within a lun.

In both cases, all SVC derivatives will behave the same.

1. reads will be processed by which ever node received the read request.
2. writes will be acked by the node that received the write request after it is mirrored to the partner node
   • the “owner” node – i.e. the preferred node will be the one that owns doing the actual destage from cache to disk
   • so the default “ALUA” assignment still applies for writes, i.e. it load balances write to disk operations at a lun level.

All said and done…

It makes no difference, and, SVC, V7000, V3700, V3500, Flex System V7000… they are all ACTIVE/ACTIVE and don’t let any EMC propaganda tell you otherwise