On Thu, 5 Dec 2013 20:24:54 +0100 Piergiorgio Sartor <piergiorgio.sartor@xxxxxxxx> wrote: > Hi all, > > I've a system, with an LSI 2308 SAS controller > and 5 2.5" HDD attached. > Each HDD can do around 100MB/sec read/write. > This was tested will all HDDs in parallel, to > make sure the controller can sustain them. > Single disk has same performance. > > I was testing RAID 0/5/6 perfomances and I found > something I could not clearly understand. > > The test was done with "dd", I wanted to know the > maximum possible performance. > Specifically, for reading: > > dd if=/dev/md127 of=/dev/null bs=4k > > For writing: > > dd if=/dev/zero of=/dev/md127 bs=4k conv=fdatasync > > Note than large block size did not change the > results. I guess the page size is quite optimal. > > I tested each RAID with 4 and 5 HDDs, with chunk > size of 512k, 64k and 16k. > The "stripe_cache_size" was set to the max 32768. > > The results were observed with "iostat -k 5", > taking care to consider variations and ramp up. > > The table, with MB/sec, the number are the HDDs > the "r" is read, "w" is write: > > chunk RAID 4r 4w 5r 5w > 512k 0 400 400 500 500 > 512k 5 260 300 360 400 > 512k 6 55 180 100 290 > > 64k 0 400 400 440 500 > 64k 5 150 300 160 400 > 64k 6 100 180 140 290 > > 16k 0 380 400 350 500 > 16k 5 100 300 130 390 > 16k 6 80 180 100 290 > > Now, RAID-0/5 seem to perform as expected, > depending on the number of HDDs. Expecially > with large chunk size. > Write performances are not a problem, even > if those are CPU intensive, with parity RAID. > RAID-0/5 do not react well with small chunk. > RAID-6, on the other hand, seems to have an > idea of its own. > First of all, it does not seem to respect > proportionality. I would think a 4 HDDs > RAID-6 should more or less read as fast as > 2 HDDs. I can understand some loss, due to > the parity skip, but not so much. In fact it > improves with smaller chunk. > With 5 HDDs, I would expect something better > than 100MB/sec. > > Any idea on this? Am I doing something wrong? > Some suggestion on tuning something in order > to try to improve RAID-6? > > Thanks, > > bye, > Does look strange. First thing I would check is the read-ahead size. md sets it for you but might be messing up some how. Have a look at /sys/block/mdX/bdi/read_ahead_kb for each configuration and see if making it some uniform large number has any effect. NeilBrown
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