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martinsumner 2017-02-08 17:11:51 +00:00
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@ -54,7 +54,9 @@ Compared to the figures at the time of the LSM-Tree paper, the actual delta in t
The availability of SDDs is not a silver bullet to disk i/o problems when cost is considered, as although they eliminate the additional costs of random page access through the removal of the disk head movement overhead (of about 6.5ms per shift), this benefit is at an order of magnitude difference in cost compared to spinning disks, and at a cost greater than half the price of DRAM. SSDs have not taken the problem of managing the overheads of disk persistence away, they've simply added another dimension to the economic profiling problem.
In physical on-premise server environments there is also commonly the cost of disk controllers. Disk controllers bend the economics of persistence through the presence of flash-backed write caches. However, disk controllers also fail - within the NHS environment disk controller failures are the second most common device failure after individual disks. Failures of disk controllers are also expensive to resolve, not being hot-pluggable like disks, and carrying greater risk of node data-loss due to either bad luck or bad process during the change. It is noticeable that EC2 does not have disk controllers and given their failure rate and cost of recovery, this appears to be a sensible trade-off.
In physical on-premise server environments there is also commonly the cost of disk controllers. Disk controllers bend the economics of persistence through the presence of flash-backed write caches. However, disk controllers also fail - within the NHS environment disk controller failures are the second most common device failure after individual disks. Failures of disk controllers are also expensive to resolve, not being hot-pluggable like disks, and carrying greater risk of node data-loss due to either bad luck or bad process during the change.
It is noticeable that EC2 does not have disk controllers and given their failure rate and cost of recovery, this appears to be a sensible trade-off. However, software-only RAID has drawbacks, include the time to setup RAID (24 hours on a d2.2xlarge node), recover from a disk failure and the time to run [scheduled checks](https://www.thomas-krenn.com/en/wiki/Mdadm_checkarray).
Making cost-driven decisions about storage design remains as relevant now as it was two decades ago when the LSM-Tree paper was published, especially as we can now directly see those costs reflected in hourly resource charges.