Severe slowdown caused by jbd2 process

[Jon Leighton <j@jonathanleighton.com>]

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Hi there,

I have been experiencing some slowness with an ext4 filesystem. I will
try to explain and hopefully somebody can identify whether this is
"normal" or not. Sorry if I am in any sense unscientific - filesystems
are somewhere near the edge of my computer science knowledge :)

Basically I am involved with doing some development on the Ruby on Rails
web app framework, and the automated tests for one component (Active
Record) does a lot of reading/writing from a database.

I realised that the test suite was running significantly slower for me
than for another developer, so I started to investigate. First I created
an unencrypted partition and put my databases on it, as I had previously
had everything encrypted.

This made it somewhat faster, but not massively.

I then used iotop to see what was going on when I ran the tests. I
discovered that the process jbd2/sda3-8 was doing *lots* of IO when I
run these tests.

I did some googling and tried a few things. Removing the journal solved
the problem (as would be expected, I guess), but also recreating the
partition as ext3 rather than ext4 solved it too (which perhaps
indicates a regression?) When I say 'solved', I mean it took a single
run of this particular test suite from say 4.5 minutes to more like
60-80 seconds.

I found some other people reporting a similar problem:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/607560

They complain of the jbd2 process running every few seconds. This is
something I had not noticed before, but I can observe this on my system
too. It runs and uses a lot of IO for a short period of time maybe every
2 seconds. So I think I am experiencing the same problem.

FWIW, using the noatime option does not help at all. Also, I have tried
using a very recent kernel build with no success. And I have run iotop
on another laptop (which also has an ext4 partition) and I cannot
observe this frequent running of jbd2.

So: does this sound like a bug, and if so, what can be done? I'm very
happy to provide any additional information as needed.

Many thanks,

Jon

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