linux-kernel - Re: periodic iowait spike

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Message-ID: <20150506140249.GN5099@twin.jikos.cz>
Date:	Wed, 6 May 2015 16:02:50 +0200
From:	David Sterba <dsterba@...e.cz>
To:	Toralf Förster <toralf.foerster@....de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: periodic iowait spike

On Sun, May 03, 2015 at 12:02:08PM +0200, Toralf Förster wrote:
> The picture in [1], created with isag from the sysstat package, shows
> a periodic spike in the iowait of my tinderbox server (hardened Gentoo
> 3.19.6-r1), correlating to write transactions per seconds from a
> similar graph.
> 
> I'm wondering what (kernel, btrfs or ?) is causing this.

Could be btrfs and the transaction commit, 30 seconds by default. The
plot roughly matches to the ~30 seconds delay, but not everywhere. Raw
numbers would could tell you more. Also, you could try to change the
commit interval (mount option 'commit') and see if the iowait spikes
change as well.
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