Buy an 'expensive' SAN and save money

Think you can't afford a SAN? Run the numbers, weigh the advantages, and think again

The benefits of SANs (storage area networks) sound tantalizing: efficient centralized storage, high performance, and rock-solid reliability. But SANs are too expensive if you're a small business. Or are they? Let's do the numbers -- you may be surprised.

Take a typical, good-sized small business running 20 servers with an average investment per server of approximately $5,000. The ever popular HP Proliant DL380 G6 or Dell PowerEdge R710 falls in that price range. The flexibility to jam two quad-core processors, tons of RAM, and eight or more hard disks into a neat little space in your rack makes these servers great building blocks for an enterprise of just about any size.

[ InfoWorld blogger Paul Venezia proposes another storage alternative in "Out, damned disks! Servers don't need 'em" | Looking to revise your storage strategy? See InfoWorld's iGuide on the Enterprise Data Explosion. ]

For the purposes of our example, let's say each server has a pair of 300GB SAS disks in a RAID1 set for redundancy. That delivers about 278GB of usable, redundant capacity per server and will kick out around 300 IOPS of transactional performance -- enough to run a Microsoft Exchange implementation for a couple hundred people without bogging down the disks down too much.

Of course, some servers will need more capacity or performance, and some will need less. That's precisely the problem that plagues DAS (direct-attached storage) environments. Using DAS, each server is a storage island unto itself. If Server A is a domain controller and needs only 20GB of usable capacity and Server B is a file server that needs 600GB of usable capacity, they can't natively borrow capacity from one another. Moreover, if I have a 50GB SQL database that backs a critical line-of-business application, I might need five or six disks just to provide enough transactional performance to run it -- and end up buying maybe 10 times the amount of capacity. The result: massive amounts of inefficiency and overspecification, both characteristics of DAS environments.

If it's true that we're overbuying, why can't we just purchase less? That's easier said than done. First off, there's a floor to the size of SAS disk that we can buy; the smallest commonly available is 146GB (73GB disks are still around, but not for long). Thus, our domain controller is massively overspecified from the get-go. Sure, you could buy SATA disks -- inexpensive options in both HP and Dell's product lines -- but neither manufacturer will warrant these disks for more than one year. Add the fact that 7,200-rpm SATA disks provide less than half the transactional performance of a 15,000-rpm SAS drives and you can see why SATA disks are rarely a great choice.

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