Messages in this thread | | | Date | Tue, 10 Apr 2012 10:00:38 +0400 | From | Michael Tokarev <> | Subject | Re: dramatic I/O slowdown after upgrading 2.6.38->3.0+ |
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On 10.04.2012 06:26, Dave Chinner wrote:
> Barriers. Turn them off, and see if that fixes your problem.
Thank you Dave for a hint. And nope, that's not it, not at all... ;) While turning off barriers helps a tiny bit, to gain a few %% from the huge slowdown, it does not cure the issue.
Meanwhile, I observed the following:
1) the issue persists on more recent kernels too, I tried 3.3 and it is also as slow as 3.0.
2) at least 2.6.38 kernel works fine, as fast as 2.6.32, I'll try 2.6.39 next.
I updated $subject accordingly.
3) the most important thing I think: this is general I/O speed issue. Here's why:
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!!
Preparing for another bisect attempt, slowly.....
Thank you!
/mjt
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