In fact, we're spending a pretty penny on all this DAS. Each server's pair of redundant disks is around $1,300 of the $5,000 overall hardware cost or $26,000 across the entire group. That's more than a quarter of the cost of the whole room of servers. Even if you find ways to shave this by using smaller disks, and sub 10,000-rpm SAS disks for 15,000-rpm models, you're still going to spend around $20,000 for 20 servers' worth of storage. With RAID1 mirror sets, we're talking between $3.60 and $4.60 per gigabyte of usable storage, much of which we've already determined we're not going to be able to use. In reality, that per-gigabyte number ends up being perhaps two to three times that.
It just so happens that number is roughly equivalent to the cost per gigabyte of a solid SAN. This brings us to a typical SAN example: a dual-controller Dell EqualLogic Peer Storage 4000E with 16 SATA disks, at 500GB and 7,200 rpm each. That will yield about 5.4TB of usable space and provide somewhere in the neighborhood of 1,200 IOPS of transactional performance.
Sure, those specs seem to be worse than the total feeds and speeds off the DAS scenario. But the bottom line is that we had to buy all of that SAS performance to be able to cover the individual, transient performance and capacity requirements of 20 little islands. If we combine them into a single pool, the provisioning inefficiency drains away and we're left buying what we actually need for the entire organization in terms of capacity and performance. If we need 20GB for a server? No problem -- give it 20GB. In fact, if it actually needed 10GB right this second, give it that and increase it to 20GB on the fly four months from now when it actually needs it.
Better still, we're able to take advantage of all of the cool features SANs bring to the table: application-aware snapshots, direct-from-SAN backups, highly reliable dual controllers, lots of high-speed cache, usage trending, and the prospect of easy, block-level, site-to-site replication down the road, not to mention that shared storage boasts tremendous high-availability features, which just come along for the ride. In the end, we're trading wasted performance and capacity for a feature set that is impossible to find using DAS.
Sounds like a pretty good deal to me. Before you dismiss SAN technology as an expensive indulgence, do the math and see what it would really cost. It may fit your environment better than you think.