Hi Scott, > On 06 Sep 2014, at 20:39, Scott Laird <scott at sigkill.org> wrote: > > IOPS are weird things with SSDs. In theory, you'd see 25% of the write IOPS when writing to a 4-way RAID5 device, since you write to all 4 devices in parallel. Except that's not actually true--unlike HDs where an IOP is an IOP, SSD IOPS limits are really just a function of request size. Because each operation would be ~1/3rd the size, you should see a net of about 3x the performance of one drive overall, or 75% of the sum of the drives. Which chunk size are you using? I presume this would only work if our writes are larger than the chuck size, which is normally around 128k, right?. In our cluster we are dominated by 4k writes so I don?t expect to get this IOPS boost you mention. Or did I miss something? Cheers, Dan > The CPU use will be higher, but it may or may not be a substantial hit for your use case. Journals are basically write-only, and 200G S3700s are supposed to be able to sustain around 360 MB/sec, so RAID 5 would give you somewhere around 1 GB/sec writing on paper. Depending on your access patterns, that may or may not be a win vs single SSDs; it should give you slightly lower latency for uncongested writes at the very least. It's probably worth benchmarking if you have the time. > > OTOH, S3700s seem to be pretty reliable, and if your cluster is big enough to handle the loss of 5 OSDs without a big hit, then the lack of complexity may be a bigger win all on its own. > > > Scott
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