Linux – kjournald reasons for high usage

filesystemskjournaldlinuxtuning

I'm trying to figure out why is kjournald going crazy on my machine. It's an 8-core box with loads of memory. It's got ~50% cpu load.

The iotop doesn't seem to point at any specific processes – some bursts of writes here and there (mostly cron starting, some monitoring stats generated, etc.) When I used sys/vm/block_dump to gather the write statistics, I got lists like this:

kjournald(1352): 1909
sendmail(28934): 13
cron(28910): 12
cron(28912): 11
munin-node(29015): 3
cron(28913): 3
check_asterisk_(28917): 3
sh(28917): 2
munin-node(29022): 2
munin-node(29021): 2

Where kjournald actions are just WRITEs.

Why is that happening? What else should I look at to limit the kjournald activity a bit? It seems disproportionate to what's actually being written.

Best Answer

kjournald is responsible for the journal of ext3 (journaling filesystem). It's known to use a lot of CPU under certain loads. There's not much to do except use another filesystem or disable journaling (effectively making the fs ext2).

Theoretically you can use one of the other modes of ext3 journaling and check if the CPU usage goes down, but remember that each method is a compromise on the safety of the data being written to the disk. You have ordered mode, writeback mode and 'everything' mode.

  1. Ordered: journal only metadata, but assures that data related to a metadata is saved before commiting the metadata changes to the journal.
  2. writeback: journal only metadata, but has no guarantee that the data is saved before the journal commit.
  3. journal: everything is journaled, data and metadata. It may be slow but YMMV.

You set the mode using the option data= when mounting the system, like data=ordered.

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