On 04/29/2011 06:42 AM, Scott Marlowe wrote:
I think you misunderstood. He's not storing 480GB on the drives, that's how much WAL is moving across it. It could easily be a single 80GB SSD drive or something like that.
Right; that's why you don't necessarily get saved by the fact that larger databases must go onto more flash cells, too. Sometimes, yes, but not always. The WAL is really close to a worst-case for flash: lots of redundant information that's constantly overwritten. It's the last thing you want to consider putting onto SSD. There's a good reason why so many of the "enterprise" SSDs try to distinguish themselves with redundancy and wear leveling advancements; it's so this sort of workload doesn't kill them.
Combine that workload possibility with the limitations of MLC flash, and you can see why the lifetimes actually are a serious concern in some situations. Not all of them, of course, but this is why I recommend things like directly measuring your WAL volume.
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