linux-kernel - Re: [NFS] blocks of zeros (NULLs) in NFS files in kernels >= 2.6.20

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Date:	Mon, 22 Sep 2008 13:26:27 -0400
From:	"Chuck Lever" <chuck.lever@...cle.com>
To:	"Aaron Straus" <aaron@...finllc.com>
Cc:	"Trond Myklebust" <trond.myklebust@....uio.no>,
	"Hans-Peter Jansen" <hpj@...la.net>, linux-kernel@...r.kernel.org,
	"Neil Brown" <neilb@...e.de>,
	"Linux NFS Mailing List" <linux-nfs@...r.kernel.org>
Subject: Re: [NFS] blocks of zeros (NULLs) in NFS files in kernels >= 2.6.20

On Mon, Sep 22, 2008 at 1:04 PM, Aaron Straus <aaron@...finllc.com> wrote:
> Hi,
>
> On Sep 22 12:35 PM, Trond Myklebust wrote:
>> Revert _what_ exactly?
>
> Yep.  I narrowed the problem down to an offending hunk in a particular
> patch.  Removing that hunk did eliminate the problem.   However,
> reverting that hunk is likely wrong and the code has changed _a lot_
> since that commit.
>
>> My understanding was that this is a consequence of unordered writes
>> causing the file to be extended while some other task is reading.
>
> Yes.  I added some debugging statements to look at the writeout path.
>
> I think the following happens:
>
>  - page 0 gets partially written to by app
>  - VM writes out partial dirty page 0
>  - page 0 gets fully written by app
>  - page 1 gets partially written by app
>  - VM writes out dirty page 1
>
>  At this point there is a hole in the file.  The tail end of page 0 is
> still not written to server.
>
>  - VM writes out dirty page 0
>  ...
>
>> AFAICS, this sort of behaviour has _always_ been possible. I can't see
>> how reverting anything will fix it.
>
> Here is the crux.  It was possible previously but unlikely e.g. our app
> never saw this behavior.  The new writeout semantics causes visible
> holes in files often.
>
> Anyway, I agree the new writeout semantics are allowed and possibly
> saner than the previous writeout path.  The problem is that it is
> __annoying__ for this use case (log files).
>
> I'm not sure if there is an easy solution.  We want the VM to writeout
> the address space in order.   Maybe we can start the scan for dirty
> pages at the last page we wrote out i.e. page 0 in the example above?

Why can't you use O_SYNC | O_APPEND ?

--
Chuck Lever
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