linux-kernel - Re: [PATCH] improve the performance of large sequential write NFS workloads

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Date:	Wed, 23 Dec 2009 23:30:14 -0500
From:	Steve Rago <sar@...-labs.com>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	Jan Kara <jack@...e.cz>, Wu Fengguang <fengguang.wu@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe" <jens.axboe@...cle.com>,
	Peter Staubach <staubach@...hat.com>
Subject: Re: [PATCH] improve the performance of large sequential write NFS
	workloads


On Thu, 2009-12-24 at 00:44 +0100, Trond Myklebust wrote:

> > #2 is the difficult one.  If you wait for memory pressure, you could
> > have waited too long, because depending on the latency of the commit,
> > you could run into low-memory situations.  Then mayhem ensues, the
> > oom-killer gets cranky (if you haven't disabled it), and stuff starts
> > failing and/or hanging.  So you need to be careful about setting the
> > threshold for generating a commit so that the client doesn't run out of
> > memory before the server can respond.
> 
> Right, but this is why we have limits on the total number of dirty pages
> that can be kept in memory. The NFS unstable writes don't significantly
> change that model, they just add an extra step: once all the dirty data
> has been transmitted to the server, your COMMIT defines a
> synchronisation point after which you know that the data you just sent
> is all on disk. Given a reasonable NFS server implementation, it will
> already have started the write out of that data, and so hopefully the
> COMMIT operation itself will run reasonably quickly.

Right.  The trick is to do this with the best performance possible.

> 
> Any userland application with basic data integrity requirements will
> have the same expectations. It will write out the data and then fsync()
> at regular intervals. I've never heard of any expectations from
> filesystem and VM designers that applications should be required to
> fine-tune the length of those intervals in order to achieve decent
> performance.

Agreed, except that the more you call fsync(), the more you are stalling
the writing, so application designers must use fsync() judiciously.
Otherwise they'd just use synchronous writes.  (Apologies if I sound
like Captain Obvious.)

Thanks,

Steve

> 
> Cheers
>   Trond
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