With the introduction of our Active-Active Synchronous Replication (called ActiveCluster) I have been getting more and more questions around multiple-subnet iSCSI access. Some customers have their two arrays in different datacenters, and also different subnets (no stretched layer 2).
With ActiveCluster, a volume exists on both arrays, so essentially the iSCSI targets on the 2nd array just look like additional paths to that volume–as far as the host knows it is not a two arrays, it just has more paths.
Consequently, this discussion is the same as if you happen to have a single array using more than one subnet for its iSCSI targets or if you are using active-active across two arrays.
Though there are some different considerations which I will talk about later.
First off, should you use more than one subnet? Well keeping things simple is good, and for a single FlashArray I would probably do that. Chris Wahl wrote a great post on this awhile back that explains the ins and out of this well: