Tuesday, 4 July 2006

The Incredible Shrinking Storage Arrays

For those who don't relate to the title, check out IMDB....

You know how it is, you go to buy some more storage. You need, say 10TB. You get the vendor to quote - but how much to do you actually end up with? First of all, disk drives never have the capacity they purport to have even if you take into consideration the "binary" TB versus the "decimal" TB. Next there's the effect of RAID. That's a known quantity and expected, so I guess we can't complain about that one. But then there's the effect of carving up the physical storage into logical LUNs; this can easily result in 10% wastage. Plus there's more: EMC on the DMX-3 now uses the first 30 or so disks installed into the box to store a copy of cache memory in case of a power outage; OK, good feature but it carves into the host available space. Apologies to EMC there - you were the first vendor I thought to have a dig at.

Enterprise arrays are not alone in this attritious behaviour - Netapp filers for instance will "lose" 10% of their storage to inodes (the part which keeps track of your files) and another 20% reserved for snapshots - that's after RAID has been accounted for.

What we need is clear unambiguous statements from our vendors - "you want to buy 10TB of storage? Then it will cost you 20....."

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