Wednesday, 30 August 2006

More on solid state disks

I'm working on a DMX-3 installation at the moment. For those who aren't aware, EMC moved from the multi-cabinet Symmetrix-5 hardware (8830/8730 type stuff) to the DMX which was fixed size - DMX1000, 2000 and 3000 models of 1, 2 and 3 cabinet installations respectively. I never really understood this move; yes it may have made life easier for EMC Engineering and CEs as the equipment shipped with its final footprint configuration, only cache/cards and disk could be added; but you had to think and commit upfront to the size of the array you wanted, which might not be financially attractive. Perhaps the idea was to create a "storage network" where every bit of data could be moved around with the minimum of effort; unfortunately that never happened and is unlikely to in the short term.

Anyway, I digress; back to DMX-3. So, doing the binfile (configuration) for the new arrays, I notice we lose a small amount of data on each of the first set of disks installed. This is for vaulting. Track back to previous models; if power was lost or likely to be lost, an array would destage all uncommitted tracks to disk after blocking further I/O on FAs. Unfortunately in a maximum configuration of 10 cabinets of 240 disks, it simply wouldn't be possible to provide battery backup for all the hard drives to destage the data. A quick calculation shows a standard HDD consumes 12.5 watts of power (300GB model), so that's 3000W per cabinet or 30,000W for a full configuration. Imagine the batteries needed for this, just on that rare off chance that power is lost. Vaulting simplifies the battery requirements by creating a save area on disk to which the pending tracks are written. When the array powers back up, the contents of cache, the vault and disk are compared to return the array back to the pre-power loss position.

This is a much better solution than simply shoving more batteries in.

So moving on from this, I thought more about exactly what 12.5W per drive means. Imagine a full configuration of 10 cabinets of 240 drives (an unlikely prospect, I know) which requires 30,000W of power. In fact the power requirements are almost double that. This is a significant amount of energy and cooling.

Going back to solid state disks I mentioned some time ago, I'd expect that the power usage would come down considerably depending on the percentage of writes that go to NAND cache. NAND devices are already up to 16GB (so could be 5% of a current 300GB drive) and growing. If power savings of 50% can be achieved, then this could have a dramatic effect on datacentre design. Come on Samsung, give us some more details of your flashon drives and we can see what hybrid drives can do......

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