On 11.04.2012 21:19, Mike Christie wrote: > 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? I tried 3.3.1, it shows exactly the same very slow speed (about 3 MB/sec vs 60 MB/sec). > 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 This thread refers to buffered I/O as far as I can see. Note I especially used iflag=direct of dd to rule out all buffer operations. The I/O really is very very slow, the disk is 100% busy all this time (which is also not the situation described in the thread you referenced above - there, disk (SSD) does not have enough work to do). > // 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 I think this patch is included into 3.3 kernel, it was in 3.3-rc2 if my git-fu is right. If it is, I tried it (as 3.3.1) and it didn't help at all. Thank you! /mjt -- 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
