On 04/11/2012 04:40 AM, Michael Tokarev wrote: > On 10.04.2012 19:13, Jan Kara wrote: >> > On Tue 10-04-12 10:00:38, Michael Tokarev wrote: > [] >>> >> 2.6.38: >>> >> # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100 >>> >> 100+0 records in >>> >> 100+0 records out >>> >> 104857600 bytes (105 MB) copied, 1.73126 s, 60.6 MB/s >>> >> >>> >> 3.0: >>> >> # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100 >>> >> 100+0 records in >>> >> 100+0 records out >>> >> 104857600 bytes (105 MB) copied, 29.4508 s, 3.6 MB/s >>> >> >>> >> That's about 20 times difference on direct read from the >>> >> same - idle - device!! >> > Huh, that's a huge difference for such a trivial load. So we can rule out >> > filesystems, writeback, mm. I also wouldn't think it's IO scheduler but >> > you can always check by comparing dd numbers after >> > echo none >/sys/block/sdb/queue/scheduler Did you try newer 3.X kernels or just 3.0? We were hitting a similar problem with iscsi. Same workload and it started with 2.6.38. I think it turned out to be this issue: // thread with issue like what we hit: http://thread.gmane.org/gmane.linux.kernel/1244680 // Patch that I think fixed issue: commit 3deaa7190a8da38453c4fabd9dec7f66d17fff67 Author: Shaohua Li <shaohua.li@xxxxxxxxx> Date: Fri Feb 3 15:37:17 2012 -0800 readahead: fix pipeline break caused by block plug -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
