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Re: Writeback Stalls

To: Markus Stockhausen <stockhausen@xxxxxxxxxxx>
Subject: Re: Writeback Stalls
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 20 Sep 2012 16:05:43 +1000
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
In-reply-to: <12EF8D94C6F8734FB2FF37B9FBEDD1735831B7C9@xxxxxxxxxxxxxxxxxxxx>
References: <12EF8D94C6F8734FB2FF37B9FBEDD1735831B7A3@xxxxxxxxxxxxxxxxxxxx> <20120919213427.GD31501@dastard> <12EF8D94C6F8734FB2FF37B9FBEDD1735831B7C9@xxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Sep 20, 2012 at 04:33:09AM +0000, Markus Stockhausen wrote:
> > Fundamentally, you are doing it all wrong. High throughput, low
> > latency NFS servers write dirty data to disk fast, not leave it
> > memory until you run out of clean memory because that causes
> > everything to block waiting for writeback IO completion to be ale to
> > free memory...
> 
> Maybe I did not make it clear enough.

Plenty clear enough. Maybe I did not make it clear enough:

Nobody has the time to try to diagnose a problem on a configuration
that is obviously broken and pessimal for writeback behaviour. The
first step is to report your actual problem, not an artificial
behaviour you *think* demonstrates the same problem....

> The above setup is only
> for demonstration purposes. To expose the problem better. In
> real life we can see stalls tat thange from 0.5-1 seconds. Even
> with all caches active, small dirty writeback settings and unlimited
> bandwidth.

So describe the application, etc that you see this problem. Start
with:

http://xfs.org/index.php/XFS_FAQ#Q:_What_information_should_I_include_when_reporting_a_problem.3F

and we can go from there.

> In between I found others complaining about the same problem:
> 
> http://lwn.net/Articles/486313/
> http://oss.sgi.com/archives/xfs/2011-09/msg00189.html

How do you know they are the same problem?

Indeed, they aren't even writeback problems - they are application
IO latency issues caused by the introduction of stable pages during
writeback.

> So just one last question: Can I savely revert the the mentioned 
> commit d76ee18a8551e33ad7dbd55cac38bc7b094f3abb if I only 
> write data to a battery backed up hardware raid controller on a 
> server that is attached to an UPS?

NFS servers don't use mmap, so that patch is not causing your
writeback problems.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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