linux-kernel - Re: Metadata in sys_sync_file

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Date:	Wed, 5 Nov 2008 17:07:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chad Talbott <ctalbott@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mrubin@...gle.com
Subject: Re: Metadata in sys_sync_file_range and fadvise(DONTNEED)

On Wed, 5 Nov 2008 16:56:54 -0800
Chad Talbott <ctalbott@...gle.com> wrote:

> 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 would work.

Although Nick is threatening to make
sync_file_range(SYNC_FILE_RANGE_WRITE) all slow by using WB_SYNC_ALL,
probably unnecessarily, ho hum.

> > 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.
> 

Thanks.
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