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

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Message-ID: <20080922174525.GF12483@merfinllc.com>
Date:	Mon, 22 Sep 2008 10:45:26 -0700
From:	Aaron Straus <aaron@...finllc.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
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

Hi,

On Sep 22 01:29 PM, Trond Myklebust wrote:
> > 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.

Definitely.  Everything in our control we can work around.... there are
a few applications we cannot easily change... see the follow-up in
another e-mail.

> > 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.

Definitely.  This is a single writer, single reader case though.

> Two threads may, for instance, force 2 competing fsync() calls: that
> again may cause out-of-order writes.

Yup.

> ...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.

Yup.  We're definitely not asking for anything like that.

> Anyway, the patch to force a single threaded nfs client to write out the
> data in order is trivial. See attachment...
>
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index 3229e21..eb6b211 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -1428,7 +1428,8 @@ static int nfs_write_mapping(struct address_space *mapping, int how)
>  		.sync_mode = WB_SYNC_NONE,
>  		.nr_to_write = LONG_MAX,
>  		.for_writepages = 1,
> -		.range_cyclic = 1,
> +		.range_start = 0,
> +		.range_end = LLONG_MAX,
>  	};
>  	int ret;
>  

Yeah I was looking at that while debugging.  Would that change have
chance to make it into mainline?  I assume it makes the normal writeout
path more expensive, by forcing a scan of the entire address space.

Also, I should test this, but I thought the VM was calling
nfs_writepages directly i.e. not going through nfs_write_mapping.  Let
me test with this patch.

					Thanks,
					=a=



-- 
===================
Aaron Straus
aaron@...finllc.com
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