linux-kernel - Re: Understanding I/O behaviour

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Message-Id: <1189693057.21778.234.camel@twins>
Date:	Thu, 13 Sep 2007 16:17:37 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	spamtrap@...bisoft.de
Cc:	Fengguang Wu <wfg@...l.ustc.edu.cn>, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: Understanding I/O behaviour - next try

On Wed, 2007-08-29 at 01:15 -0700, Martin Knoblauch wrote:

> > >  Another thing I saw during my tests is that when writing to NFS, the
> > > "dirty" or "nr_dirty" numbers are always 0. Is this a conceptual thing,
> > > or a bug?
> > 
> > What are the nr_unstable numbers?

NFS has the concept of unstable storage, that is a state where it is
agreed the page has been transferred to the remote server, but has not
yet been written to disk.

>  Ahh. Yes, they go up to 80-90k pages. Comparable to the nr_dirty
> numbers for the disk case. Good to know.
> 
>  For NFS, the nr_writeback numbers seem surprisingly high. They also go
> to 80-90k (pages ?). In the disk case they rarely go over 12k.

see: /proc/sys/fs/nfs/nfs_congestion_kb

That is the limit for when the nfs BDI is marked congested, so
nfs_writeout + nfs_unstable <= nfs_congestion_kb

The nfs_dirty always being 0 just means that pages very quickly start
their writeout cycle.

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