Re: Testing Sandforce SSD

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On Mon, 26 Jul 2010, Greg Smith wrote:
Matthew Wakeling wrote:
Does your latency graph really have milliseconds as the y axis? If so, this device is really slow - some requests have a latency of more than a second!

Have you tried that yourself? If you generate one of those with standard hard drives and a BBWC under Linux, I expect you'll discover those latencies to be >5 seconds long. I recently saw >100 *seconds* running a large pgbench test due to latency flushing things to disk, on a system with 72GB of RAM. Takes a long time to flush >3GB of random I/O out to disk when the kernel will happily cache that many writes until checkpoint time.

Apologies, I was interpreting the graph as the latency of the device, not all the layers in-between as well. There isn't any indication in the email with the graph as to what the test conditions or software are. Obviously if you factor in checkpoints and the OS writing out everything, then you would have to expect some large latency operations. However, if the device itself behaved as in the graph, I would be most unhappy and send it back.

Yeb also made the point - there are far too many points on that graph to really tell what the average latency is. It'd be instructive to have a few figures, like "only x% of requests took longer than y".

Matthew

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