From: Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH 0/5 v2] add extent status tree caching
Date: Fri, 19 Jul 2013 13:33:09 +1000
Message-ID: <20130719033309.GQ11674@dastard>
References: <1373987883-4466-1-git-send-email-tytso@mit.edu>
 <51E8356C.9030603@redhat.com>
 <20130718185310.GA17548@thunk.org>
 <51E88ECD.3040806@redhat.com>
 <20130719025934.GE17938@thunk.org>
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Cc: Eric Sandeen <sandeen@redhat.com>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	Zheng Liu <gnehzuil.liu@gmail.com>
To: Theodore Ts'o <tytso@mit.edu>
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In-Reply-To: <20130719025934.GE17938@thunk.org>
Sender: linux-ext4-owner@vger.kernel.org

On Thu, Jul 18, 2013 at 10:59:34PM -0400, Theodore Ts'o wrote:
> On Thu, Jul 18, 2013 at 07:56:45PM -0500, Eric Sandeen wrote:
> > > The problem is we don't know that we're doing AIO until we see the
> > > first io_submit(2) call.  With this patch series, we'll pull the
> > > contents of the entire leaf tree block into extent cache, but if the
> > > extent tree is larger than that, if we read in the entire extent tree
> > > on the first AIO request, then that first request will delayed even
> > > more, and it's not clear that's a good thing.
> > 
> > Is blocking on a pre-AIO ioctl better than blocking on the
> > first AIO?
> 
> The precache ioctl is something which the application is expecting to
> block.  The question is, if we have a heavily fragmented extent tree,
> is it better for the first AIO to block long enough to read in one
> metadata block --- and then never block again, or to have that first
> AIO request take a long, LONG time?  Especially if the application
> isn't expecting it?
> 
> Also there are use cases for the precache ioctl even if you are not
> using AIO.  If you've taken care to make sure the file is as
> contiguous as possible, having the extents be cached will save a lot
> of memory compared to if the buffer heads are always entering the
> buffer cache.  So reading in all of the metadata can be a good thing
> to do, especially if you can do this *before* you declare that the
> server is healthy and is ready to start receiving traffic.

An ioctl is kinda silly for this. Just use O_NONBLOCK when calling
open() and do the prefetch right in the open call. The open() can
block, anyway, and what you are trying to do is non-blocking IO with
AIO, so it seems like we've already got a sensible, generic
interface for triggering this sort of prefetch operation.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com