On Fri, Jan 24, 2014 at 05:47:57AM -0500, George Spelvin wrote: > I was doing some bulk reads on an ext4 file system on > a simple mirrored device (3.13 x86_64 kernel): > > md2 : active raid1 sdd3[0] sde3[1] > 1932514496 blocks [2/2] [UU] > bitmap: 0/15 pages [0KB], 65536KB chunk > > And I noticed that the reads were all hitting the first disk. > The second was basically idle. Here's "dstat -d -D md2,sdd,sde": > > --dsk/md2-----dsk/sdd-----dsk/sde-- > read writ: read writ: read writ > 510k 55k: 484k 58k: 27k 58k > 129M 0 : 129M 0 : 48k 0 > 108M 48k: 108M 60k: 576k 60k > 123M 0 : 123M 8192B: 0 8192B > 133M 0 : 133M 0 : 360k 0 > 132M 0 : 132M 0 : 20k 0 > 138M 0 : 138M 0 : 304k 0 > 128M 0 : 128M 0 : 896k 0 > 129M 0 : 129M 4096B: 64k 4096B > 135M 0 : 135M 0 : 36k 0 > 116M 12k: 116M 24k: 36k 28k > 117M 0 : 116M 4096B: 632k 0 > 127M 0 : 127M 0 : 288k 0 > 130M 0 : 130M 0 : 336k 0 > 133M 0 : 133M 0 : 212k 0 > 134M 0 : 134M 0 : 304k 0 > 130M 0 : 129M 4096B: 100k 4096B > 128M 0 : 127M 0 : 280k 0 > 106M 12k: 106M 28k: 372k 28k > 129M 0 : 129M 0 : 344k 0 > 134M 0 : 134M 0 : 196k 0 > 134M 0 : 134M 0 : 384k 0 > 129M 0 : 129M 0 : 304k 0 > > I thought (drivers/md/raid1.c:read_balance()) the driver was > supposed to do some striping on large reads. > > While 125M/s is nice, more than that would be nicer. > > The drives are identical, but are plugged in to different controllers. > sdd is on an AMD SB600 controller, while sde is on a PDC42819. > > Is there some knob I need to adjust to make read balancing happen? The reading is not balanced because it does not make sense to do balanced reads for sequential reading. In RAID-1 the disk sectors are consequitive. So if you would read one sector from one disk, and the following sector from the other disk, then the next read from disk 1 would need to skip a full resolvation of the disk, which may cost something like 8 ms. So better read contigously from the same disk, and hope for some other IO request that can use disk 2. For sequential reading RAID-10 in the "far" layout make a much more balanced reading scheme. You will get somethng like RAID-0 reading speeds here. 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
