linux-kernel - Re: Metadata in sys_sync_file

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Date:	Wed, 5 Nov 2008 16:56:54 -0800
From:	Chad Talbott <ctalbott@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Michael Rubin <mrubin@...gle.com>
Subject: Re: Metadata in sys_sync_file_range and fadvise(DONTNEED)

On Sat, Nov 1, 2008 at 1:21 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> And fadvise(FADV_DONTNEED) is just that: "I won't be using that data
> again".  Implementing specific writeback behaviour underneath that hint
> is unobvious and a bit weird.  It's a bit of a fluke that it does
> writeout at all!
>
> We have much more flexibility with sync_file_range(), and it is more
> explicit.

So in the new world, an application should call sync_file_range
(solving my problem by including metadata) to initiate writeout, and
then call posix_fadvise(DONTNEED) to drop the pages from page cache?
I think this would work for me.

> That being said, I don't understand why the IO scheduling problems
> which you're seeing are occurring.  There is code in fs/mpage.c
> specifically to handle this case (search for "write_boundary_block").
> It will spot that 4k indirect block in the middle of two 4MB data
> blocks and will schedule it for writeout at the right time.
>
> So why isn't that working?

Very good question, I'll look into why it's not helping here.

Chad
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