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    SubjectRe: dramatic I/O slowdown after upgrading 2.6.38->3.0+
    On Wed 11-04-12 12:19:43, 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?
    >
    > 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@intel.com>
    > Date: Fri Feb 3 15:37:17 2012 -0800
    >
    > readahead: fix pipeline break caused by block plug
    I already asked about this but that doesn't seem to be the cause.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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