linux-kernel - Re: [PATCH v2] NFS: introduce writeback wait queue

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Date:	Mon, 5 Oct 2009 21:08:50 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH v2] NFS: introduce writeback wait queue

On Mon, Oct 05, 2009 at 06:55:54PM +0800, Myklebust, Trond wrote:
> On Oct 5, 2009, at 3:40, "Wu Fengguang" <fengguang.wu@...el.com> wrote:
> 
> > On Mon, Oct 05, 2009 at 03:35:51PM +0800, Wu Fengguang wrote:
> >> Trond, I see this trace on linux-next. There are no more dirty pages
> >> when `cp' aborts after filling up the partition:
> >>
> >>        cp: writing `/mnt/test/zero3': No space left on device
> >>
> >> I noticed that since then nr_writeback is decreased very slowly
> >> (~100 pages per second). Looks like an interesting behavior.
> >
> > In the mean time, there are constant 7-8MB/s writes in the NFS server.
> > The network flow is much smaller ~400K/s. How can I debug this issue?
> 
> Hi Fengguang
> 
> This is deliberate behaviour. When asynchronous writes start recieving  
> errors, then we switch to synchronous write mode until the error  
> condition clears.

Ah yes. After ENOSPC, with nfsstat I saw the client side write/commit
numbers remain constant, while the server side write number increases
~200 per-second, and commit number also remain static. When all client
side nr_writeback drops to 0, the server side write number also stops.

> The reason is for doing so is firstly because some filesystems (XFS)  
> perform very poorly under ENOSPC, and so it takes forever to write  
> back pages (we don't want to cancel all writebacks for temporary  
> conditions like ENOSPC). It also allows us to deliver the errors more  
> promptly to the application.

Thanks,
Fengguang
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