On Tue, 27 Jul 2010 16:10:36 +0000 (GMT) Marco <jjletho67-diar@xxxxxxxx> wrote: > > > > > >If you look at /proc/diskstats and particularly the 4th and 6th fields for > >the device that you are interested in, and then take the differences for each > >field between 'before' and 'after' running a test you will get > > - the number of IO requests > > - the number of sectors > > > >that were serviced during that time. Taking a ratio will get you the number > >of sectors per IO. Normally more is better. > > Hi Neil, > are you sure the right value are the 4th and the 6th fields ? I see strange > value in them (the 4th field is bigger then the 6th while i was expecting the > contrary) > looking in the iostats.txt file (kernel documentation) i suspect the fields you > are interested are the first and the third. No, the first field is the major device number, and the third is the device name .... so I guess we are taking the same ratio, but you start counting at a different place to me. > In this hypothesis (1st and 3rd fields) the ratio i obtain are: > > sda 84.02 > sdb 113,69 > md2 8,01 > > md2 and his meber have a very different ratio.... Strange isn't it. And the ratios are the other-way-around to what I get. I don't currently understand why ... but it might not be at all relevant to the speed difference. > > >It might be interesting to find out what the data offset is for your RAID1 > >(mdadm --examine will tell you if there is one), and compare the > > request/sector numbers and see if they show anything. > > this is the output of mdadm --examine /dev/sda3 > > /dev/sda3: > Magic : a92b4efc > Version : 0.90.00 OK, so there is no data offset with 0.90, so that rules out differing offsets being an issue. > > Do you have some hypothesis ? No. It might be worth exploring why the request sizes are different - I don't know if it will lead anywhere useful though. NeilBrown > > thank you all > > Marco > > > > -- 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
