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Date: Thu, 20 Jul 2017 14:59:58 -0700
From: Omar Sandoval <osandov@osandov.com>
To: Christoph Anton Mitterer <calestyo@scientia.net>
Cc: linux-btrfs <linux-btrfs@vger.kernel.org>,
        Martin Raiber <martin@urbackup.org>
Subject: Re: strange No space left on device issues
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On Thu, Jul 20, 2017 at 11:53:16PM +0200, Christoph Anton Mitterer wrote:
> On Thu, 2017-07-20 at 14:48 -0700, Omar Sandoval wrote:
> > Just to be sure, did you explicitly write 0 to these?
> Nope... that seemed to have been the default value, i.e. I used
> sysctl(8) in read (and not set) mode here.

Okay, all good then.

> > These sysctls are
> > really confusing, see https://www.kernel.org/doc/Documentation/sysctl
> > /vm.txt.
> > Basically, there are two ways to specify these, either as a ratio of
> > system memory (vm.dirty_ratio and vm.dirty_background_ratio) or a
> > static
> > number of bytes (vm.dirty_bytes and vm.dirty_background_bytes). If
> > you
> > set one, the other appears as 0, and the kernel sets the ratios by
> > default. But if you explicitly set them to 0, the kernel is going to
> > flush stuff extremely aggressively.
> I see,... not sure why both are 0 here... at least I didn't change it
> myself - must be something from the distro?

That's normal, the default is to have the ratio set instead:

$ sysctl vm.dirty_{,background_}{bytes,ratio}
vm.dirty_bytes = 0
vm.dirty_ratio = 20
vm.dirty_background_bytes = 0
vm.dirty_background_ratio = 10

> > Awesome, glad to hear it! I hadn't been able to reproduce the issue
> > outside of Facebook. Can I add your tested-by?
> Sure, but better use my other mail address for it, if you don't mind:
> Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

No problem. I'll resend the patch with that shortly.

> > > I assume you'll take care to get that patch into stable kernels?
> > > Is this patch alone enough to recommend the Debian maintainers to
> > > include it into their 4.9 long term stable kernels?
> > 
> > I'll mark it for stable, assuming Debian tracks the upstream LTS
> > releases it should get in.
> Okay :-)
> 
> Nevertheless I'll open a bug at their BTS, just to be safe.
> 
> 
> Thanks :)
> 
> Chris.