On Thu, Apr 03, 2008 at 09:20:37PM +0100, Peter Grandi wrote: > >>> On Wed, 2 Apr 2008 23:13:15 +0200, Keld Jørn Simonsen > >>> <keld@xxxxxxxx> said: > > [ ... slow RAID reading ... ] > > >> That could be the usual issue with apparent pauses in the > >> stream of IO requests to the array component devices, with > >> the usual workaround of trying 'blockdev --setra 65536 > >> /dev/mdN' and see if sequential reads improve. > > keld> Yes, that did it! > > But that's as usual very wrong. Such a large readhead has > negative consequences, and most likely is the result of both > some terrible misdesign in the Linux block IO subsystem (from > some further experiments it is most likely related to "plugging") > and integration of MD into it. > > However I have found that on relatively fast machines (I think) > much lower values of read-ahead still give reasonable speed, > with some values being much better than others. For example with > another RAID10 I get pretty decent speed with a read-ahead of > 128 on '/dev/md0' (but much worse with say 64 or 256). On others > 1000 sectors read-ahead is good. > > The read-ahead needed also depends a bit on the file system > type, don't trust tests done on the block device itself. > > So please experiment a bit to try and reduce it, at least until > I find the time to figure out the (surely embarrasing) reason > why it is needed and how to avoid it, or the Linux block IO and > MD maintainers confess (they almost surely already know why) > and/or fix it already. I did experiment and I noted that a 16 MiB readahead was sufficient. And then I was wondering if this had negative consequences, eg on random reads. I then had a test with reading 1000 files concurrently, and Some strange things happened. Each drive was doing about 2000 transactions per second (tps). Why? I thought a drive could only do about 150 tps, given t5hat it is a 7200 rpm drive. What is tps measuring? Why is the fs not reading the chunk size for every IO operation? Best regards keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
