Messages in this thread | | | Subject | Re: elevator algorithm bug in ll_rw_blk.c | From | "Michael O'Reilly" <> | Date | 17 Nov 1998 17:28:03 +0800 |
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"Stephen C. Tweedie" <sct@redhat.com> writes: > > The way it actually works, sequential writes tend to be kept in > sequential order in the buffer cache lists, so the typical cases work > pretty well. All the benchmarks show that sequential filesystem writes > work very well. >
Unfortunately, there's a case where it frequently works very very badly; That's when you have a LOT of disk cache (150 - 200meg) and a fast writer producing a lot of data).
In this case, the write performance frequently goes abysmal, even tho it's sequential writing. 'Abysmal' to the tune on 800K/sec on a 6 disk array.
(the actual performance case I saw this frequently on was a squid-1.1 server shutting down. It would write 6 x 200Meg file, one to each of the 6 disks).
I'm still not sure exactly what was happening, but the write performance would start of in the 4 or 5 meg/sec for the first 10 or so seconds, then drop to 800K/sec for the remainder of the data. Furious seeking occured during this time. I suspect the probem was that it was writing different points in the file, and because the window wasn't large enough, not noticing that it would have been better off just writing from the one point.
Michael.
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