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 13:29:01 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Aaron Straus <aaron@...finllc.com>
Cc:	Hans-Peter Jansen <hpj@...la.net>, linux-kernel@...r.kernel.org,
	Chuck Lever <chuck.lever@...cle.com>,
	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, 2008-09-22 at 10:04 -0700, Aaron Straus wrote:
> 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).

There is always the option of using syslog.

> 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?

You can never guarantee that in a multi-threaded environment.

Two threads may, for instance, force 2 competing fsync() calls: that
again may cause out-of-order writes.
...and even if the client doesn't reorder the writes, the _server_ may
do it, since multiple nfsd threads may race when processing writes to
the same file.

Anyway, the patch to force a single threaded nfs client to write out the
data in order is trivial. See attachment...

Trond

Download attachment "linux-2.6.27-ordered_writes.dif" of type "message/rfc822" (750 bytes)