Q: How Does Direct-Attached Storage Provide Resiliency?
A: Resiliency, redundancy, performance, value…
Last time, we talked about what direct-attached storage (DAS) is and how to configure it on an IBM Storwize V7000 system. We explained that the easiest configuration is to connect fibre ports on the Power server directly to fibre ports on the V7000’s node canisters.
Added resiliency is provided by this setup with two paths to each LUN on the V7000: one path through each of the fibre adapter/node canister combinations. This is called multipath, whereby there is an active and a passive path to each LUN. If a fibre adapter fails, the active path will go through the other fibre adapter. Likewise, if a node canister fails, the active path will go through the other canister. IBM i supports up to 8 paths to a LUN. This can be achieved with additional fibre adapters (and fibre switches). Laboratory performance testing has also shown performance improvements with two or more paths compared to a single path. Two paths tend to provide the best balance of price and performance.
DAS is best suited for systems with up to two logical partitions. Each node canister on the V7000 has four fibre ports. With two fibre adapters per logical partition, it would be maxed out at two partitions. You could go with one fibre adapter in a partition but you would lose the multipath capability. However, you still have the option of creating hosted client partitions.