"Matthew" <matthew@xxxxxxxxxxx> writes: > On Tue, 29 Jan 2008, Gregory Stark wrote: >>> This was with 8192 random requests of size 8192 bytes from an 80GB test file. >>> Unsorted requests ranged from 1.8 MB/s with no prefetching to 28MB/s with lots >>> of prefetching. Sorted requests went from 2.4MB/s to 38MB/s. That's almost >>> exactly 16x improvement for both, and this is top of the line hardware. >> >> Neat. The curves look very similar to mine. I also like that with your >> hardware the benefit maxes out at pretty much exactly where I had >> mathematically predicted they would ((stripe size)^2 / 2). > > Why would that be the case? Does that mean that we can select a stripe size of > 100GB and get massive performance improvements? Doesn't seem logical to me. To > me, it maxes out at 16x speed because there are 16 discs. Sorry, I meant "number of drives in the array" not number of bytes. So with 16 drives you would need approximately 128 random pending i/o operations to expect all drives to be busy at all times. I got this from a back-of-the-envelope calculation which now that I'm trying to reproduce it seems to be wrong. Previously I thought it was n(n+1)/2 or about n^2/2. So at 16 I would have expected about 128 pending i/o requests before all the drives could be expected to be busy. Now that I'm working it out more carefully I'm getting that the expected number of pending i/o requests before all drives are busy is n + n/2 + n/3 + ... + n/n which is actually n * H(n) which is approximated closely by n * log(n). That would predict that 16 drives would actually max out at 44.4 pending i/o requests. It would predict that my three-drive array would max out well below that at 7.7 pending i/o requests. Empirically neither result seems to match reality. Other factors must be dominating. > Amusingly, there appears to be a spam filter preventing my message (with its > image) getting through to the performance mailing list. This has been plaguing us for a while. When we figure out who's misconfigured system is doing it I expect they'll be banned from the internet for life! -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's Slony Replication support! ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
