Hi, I've tested with 3.5 (it's still running). Took another one than the one you linked, as you linked -rc7 and the official release was on the site too (so just 3.5.0 no rc/prerelease). It seems somewhat resolved (note - I'm no kernel dev ;). With that I mean the values are much higher now, however, the max_hw_sector_kb has a value of 16383 which is an odd number and is 16MiB - 1 kiB. The minus 1kiB does not make sense to me whatsoever. Have done some simple tests with values like 512 (default after boot), 2048 and most notably 12288 seems to perform best (64k chunk size, 7 disk RAID-5 (6 data disks thus) * 32 (makes 64*6*32)). That has doubled my write performance w/o touching any other settings whatsoever. This is not very extensive, just a copy/paste from my console. Had a 'while (true); do killall -USR1 dd; sleep 20; done' running in another terminal to have dd print statistics in between. At the time of testing nothing else was accessing any of the disks in the RAID array. There was sufficient time between the dd's for the kernel to flush it's buffers. Didn't copy the output for free, but checked the size of the buffers with that before starting the next dd. Wasn't more than a couple of MiB's before starting with over 3G RAM free, which explains the initial (way) higher speeds. root@datavault:/sys/block/md4/queue# cat max_sectors_kb 512 root@datavault:/sys/block/md4/queue# cat max_hw_sectors_kb 16383 root@datavault:/sys/block/md4/queue# dd if=/dev/zero of=/dev/md4p2 bs=1M count=20480 2482+0 records in 2482+0 records out 2602565632 bytes (2.6 GB) copied, 27.5633 s, 94.4 MB/s 3795+0 records in 3795+0 records out 3979345920 bytes (4.0 GB) copied, 47.6073 s, 83.6 MB/s 4980+0 records in 4980+0 records out 5221908480 bytes (5.2 GB) copied, 67.7112 s, 77.1 MB/s 5383+0 records in 5383+0 records out 5644484608 bytes (5.6 GB) copied, 87.6912 s, 64.4 MB/s 5931+0 records in 5931+0 records out 6219104256 bytes (6.2 GB) copied, 107.683 s, 57.8 MB/s 6827+0 records in 6827+0 records out 7158628352 bytes (7.2 GB) copied, 127.683 s, 56.1 MB/s 7920+0 records in 7920+0 records out 8304721920 bytes (8.3 GB) copied, 147.699 s, 56.2 MB/s 8594+0 records in 8594+0 records out 9011462144 bytes (9.0 GB) copied, 167.704 s, 53.7 MB/s 9157+0 records in 9157+0 records out 9601810432 bytes (9.6 GB) copied, 187.723 s, 51.1 MB/s 9709+0 records in 9709+0 records out 10180624384 bytes (10 GB) copied, 207.755 s, 49.0 MB/s 11159+0 records in 11159+0 records out 11701059584 bytes (12 GB) copied, 227.724 s, 51.4 MB/s 12307+0 records in 12307+0 records out 12904824832 bytes (13 GB) copied, 247.744 s, 52.1 MB/s 12867+0 records in 12867+0 records out 13492027392 bytes (13 GB) copied, 267.759 s, 50.4 MB/s 13634+0 records in 13634+0 records out 14296285184 bytes (14 GB) copied, 287.759 s, 49.7 MB/s 14287+0 records in 14287+0 records out 14981005312 bytes (15 GB) copied, 307.784 s, 48.7 MB/s 15118+0 records in 15118+0 records out 15852371968 bytes (16 GB) copied, 327.795 s, 48.4 MB/s 15699+0 records in 15699+0 records out 16461594624 bytes (16 GB) copied, 347.775 s, 47.3 MB/s 16325+0 records in 16325+0 records out 17118003200 bytes (17 GB) copied, 367.835 s, 46.5 MB/s 16938+0 records in 16938+0 records out 17760780288 bytes (18 GB) copied, 387.863 s, 45.8 MB/s 17476+0 records in 17476+0 records out 18324914176 bytes (18 GB) copied, 407.855 s, 44.9 MB/s 18234+0 records in 18234+0 records out 19119734784 bytes (19 GB) copied, 427.859 s, 44.7 MB/s 18887+0 records in 18887+0 records out 19804454912 bytes (20 GB) copied, 447.867 s, 44.2 MB/s 19393+0 records in 19393+0 records out 20335034368 bytes (20 GB) copied, 467.911 s, 43.5 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 511.516 s, 42.0 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 511.516 s, 42.0 MB/s root@datavault:/sys/block/md4/queue# mdadm --detail /dev/md4 /dev/md4: Version : 0.90 Creation Time : Thu Oct 20 23:48:39 2011 Raid Level : raid5 Array Size : 11721086976 (11178.10 GiB 12002.39 GB) Used Dev Size : 1953514496 (1863.02 GiB 2000.40 GB) Raid Devices : 7 Total Devices : 8 Preferred Minor : 4 Persistence : Superblock is persistent Update Time : Wed Aug 1 15:32:48 2012 State : clean Active Devices : 7 Working Devices : 8 Failed Devices : 0 Spare Devices : 1 Layout : left-symmetric Chunk Size : 64K UUID : 2213e62a:723f32ab:c4e8ec2d:2a80eff0 (local to host datavault) Events : 0.3449563 Number Major Minor RaidDevice State 0 8 64 0 active sync /dev/sde 1 8 80 1 active sync /dev/sdf 2 8 96 2 active sync /dev/sdg 3 8 112 3 active sync /dev/sdh 4 8 128 4 active sync /dev/sdi 5 8 176 5 active sync /dev/sdl 6 8 160 6 active sync /dev/sdk 7 8 144 - spare /dev/sdj root@datavault:/sys/block/md4/queue# echo 12288 > max_sectors_kb root@datavault:/sys/block/md4/queue# dd if=/dev/zero of=/dev/md4p2 bs=1M count=20480 381+0 records in 381+0 records out 399507456 bytes (400 MB) copied, 0.255935 s, 1.6 GB/s 1946+0 records in 1946+0 records out 2040528896 bytes (2.0 GB) copied, 20.2837 s, 101 MB/s 3318+0 records in 3318+0 records out 3479175168 bytes (3.5 GB) copied, 40.2759 s, 86.4 MB/s 4853+0 records in 4853+0 records out 5088739328 bytes (5.1 GB) copied, 60.2838 s, 84.4 MB/s 6282+0 records in 6282+0 records out 6587154432 bytes (6.6 GB) copied, 80.3035 s, 82.0 MB/s 7811+0 records in 7811+0 records out 8190427136 bytes (8.2 GB) copied, 100.3 s, 81.7 MB/s 9402+0 records in 9402+0 records out 9858711552 bytes (9.9 GB) copied, 120.324 s, 81.9 MB/s 10800+0 records in 10800+0 records out 11324620800 bytes (11 GB) copied, 140.324 s, 80.7 MB/s 12399+0 records in 12399+0 records out 13001293824 bytes (13 GB) copied, 160.336 s, 81.1 MB/s 13904+0 records in 13904+0 records out 14579400704 bytes (15 GB) copied, 180.352 s, 80.8 MB/s 15215+0 records in 15215+0 records out 15954083840 bytes (16 GB) copied, 200.356 s, 79.6 MB/s 16250+0 records in 16250+0 records out 17039360000 bytes (17 GB) copied, 220.36 s, 77.3 MB/s 17082+0 records in 17082+0 records out 17911775232 bytes (18 GB) copied, 240.376 s, 74.5 MB/s 17559+0 records in 17559+0 records out 18411945984 bytes (18 GB) copied, 260.412 s, 70.7 MB/s 18276+0 records in 18276+0 records out 19163774976 bytes (19 GB) copied, 280.404 s, 68.3 MB/s 19474+0 records in 19474+0 records out 20419969024 bytes (20 GB) copied, 300.404 s, 68.0 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 333.692 s, 64.4 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 333.692 s, 64.4 MB/s root@datavault:/sys/block/md4/queue# echo 2048 > max_sectors_kb root@datavault:/sys/block/md4/queue# dd if=/dev/zero of=/dev/md4p2 bs=1M count=20480 1465+0 records in 1465+0 records out 1536163840 bytes (1.5 GB) copied, 12.9843 s, 118 MB/s 2763+0 records in 2763+0 records out 2897215488 bytes (2.9 GB) copied, 33.0076 s, 87.8 MB/s 4138+0 records in 4138+0 records out 4339007488 bytes (4.3 GB) copied, 53.0197 s, 81.8 MB/s 5228+0 records in 5228+0 records out 5481955328 bytes (5.5 GB) copied, 73.0554 s, 75.0 MB/s 5958+0 records in 5958+0 records out 6247415808 bytes (6.2 GB) copied, 93.0753 s, 67.1 MB/s 6699+0 records in 6699+0 records out 7024410624 bytes (7.0 GB) copied, 113.08 s, 62.1 MB/s 8030+0 records in 8030+0 records out 8420065280 bytes (8.4 GB) copied, 133.083 s, 63.3 MB/s 8791+0 records in 8791+0 records out 9218031616 bytes (9.2 GB) copied, 153.131 s, 60.2 MB/s 9313+0 records in 9313+0 records out 9765388288 bytes (9.8 GB) copied, 173.183 s, 56.4 MB/s 9741+0 records in 9741+0 records out 10214178816 bytes (10 GB) copied, 193.195 s, 52.9 MB/s 10929+0 records in 10929+0 records out 11459887104 bytes (11 GB) copied, 213.208 s, 53.7 MB/s 11848+0 records in 11848+0 records out 12423528448 bytes (12 GB) copied, 233.231 s, 53.3 MB/s 12383+0 records in 12383+0 records out 12984516608 bytes (13 GB) copied, 253.235 s, 51.3 MB/s 12915+0 records in 12915+0 records out 13542359040 bytes (14 GB) copied, 273.255 s, 49.6 MB/s 13693+0 records in 13693+0 records out 14358151168 bytes (14 GB) copied, 293.249 s, 49.0 MB/s 15156+0 records in 15156+0 records out 15892217856 bytes (16 GB) copied, 313.276 s, 50.7 MB/s 16215+0 records in 16215+0 records out 17002659840 bytes (17 GB) copied, 333.315 s, 51.0 MB/s 16827+0 records in 16827+0 records out 17644388352 bytes (18 GB) copied, 353.283 s, 49.9 MB/s 17899+0 records in 17899+0 records out 18768461824 bytes (19 GB) copied, 373.315 s, 50.3 MB/s 19061+0 records in 19061+0 records out 19986907136 bytes (20 GB) copied, 393.359 s, 50.8 MB/s 19847+0 records in 19847+0 records out 20811087872 bytes (21 GB) copied, 413.375 s, 50.3 MB/s 20437+0 records in 20437+0 records out 21429747712 bytes (21 GB) copied, 433.379 s, 49.4 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 456.783 s, 47.0 MB/s 20480+0 records in 20480+0 records out 21474836480 bytes (21 GB) copied, 456.783 s, 47.0 MB/s