linux-kernel - Re: Metadata in sys_sync_file

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Date:	Thu, 6 Nov 2008 15:20:54 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Chad Talbott <ctalbott@...gle.com>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...gle.com>,
	Michael Rubin <mrubin@...gle.com>
Subject: Re: Metadata in sys_sync_file_range and fadvise(DONTNEED)

On Wed, Nov 05, 2008 at 05:19:20PM -0800, Chad Talbott wrote:
> On Sun, Nov 2, 2008 at 2:45 PM, Dave Chinner <david@...morbit.com> wrote:
> > On Fri, Oct 31, 2008 at 01:54:14PM -0700, Chad Talbott wrote:
> >> Andrew suggests a new SYNC_FILE_RANGE_METADATA flag for
> >> sys_sync_file_range(), and leaving posix_fadvise() alone.
> >
> > What is the interface that a filesystem will see? No filesystem has
> > a "metadata sync" method - is this going to fall through to some new
> > convoluted combination of writeback flags to an inode/mapping
> > that more filesystems than not can get wrong?
> 
> Good point, coupled with metadata/data ordering and your argument
> below, a decent argument against exposing this interface.
> 
> > FWIW, sys_sync_file_range() is fundamentally broken for data
> > integrity writeback - at no time does it call a filesystem method
> > that can result in a barrier I/O being issued to disk after
> > writeback is complete. So, unlike fsync() or fdatasync(), the data
> > can still be lost after completion due to power failure on drives
> > with volatile write caches....
> 
> Seems to be true.  I'm not currently concerned with sync_file_range
> for data integrity, so I'm going to punt on this issue.

;)

> If the consensus is against exposing a "sync metadata" interface, I'm
> fine with ext2 silently updating metadata alongside neighboring data
> in *either* posix_fadvise() or sync_file_range.

I think that sync_file_range is the better choice for "correct"
behaviour. There is the assumption with syncing data explicitly
that the metadata needs to reference that data is written to disk
as well.

> Either way, does it
> seem reasonable for posix_fadvise(DONTNEED) to call
> __filemap_fdatawrite_range to do its work?

>From a kernel perspective, I don't think it really matters.  To an
application, it could. e.g. If you're calling posix_fadvise on a
large range, then the I/O patterns will be the same either way. If
you're calling posix_fadvise() on small, sparse ranges of the file,
then you'll turn one large, fast writeout into lots of small random
writes. i.e. upgrade the kernel and the application goes much
slower....

I guess this all depends on whether this would be considered a
regression or a stupid application ;)

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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