linux-kernel - Re: [PATCH 1/6] fs: add hole punching to fallocate

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Message-ID: <20101109044242.GH2715@dastard>
Date:	Tue, 9 Nov 2010 15:42:42 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Ted Ts'o <tytso@....edu>, Josef Bacik <josef@...hat.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com, joel.becker@...cle.com, cmm@...ibm.com,
	cluster-devel@...hat.com
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate

On Mon, Nov 08, 2010 at 10:30:38PM -0500, Ted Ts'o wrote:
> On Tue, Nov 09, 2010 at 12:12:22PM +1100, Dave Chinner wrote:
> > Hole punching was not included originally in fallocate() for a
> > variety of reasons. IIRC, they were along the lines of:
> > 
> > 	1 de-allocating of blocks in an allocation syscall is wrong.
> > 	  People wanted a new syscall for this functionality.
....
> > I guess that leaves #1 to be debated;
> > I don't think there is any problem with doing what you propose.
> 
> I don't have a problem either.
> 
> As a completely separate proposal, what do people think about an
> FALLOCATE_FL_ZEROIZE after which time the blocks are allocated, but
> reading from them returns zero.

That's exactly the new XFS_IOC_ZERO_RANGE ioctl in 2.6.36 does
(commit 447223520520b17d3b6d0631aa4838fbaf8eddb4 "xfs: Introduce
XFS_IOC_ZERO_RANGE") The git commit I pointed to in the last email
is the rudimentary fallocate() interface support I have for that
code which goes along with an xfs_io patch I have. Given that there
seems to be interest for this operation, I'll flesh it out into a
proper patch....

> This could be done either by (a)
> sending a discard in the case of devices where discard_zeros_data is
> true and discard_granularty is less than the fs block size, or (b) by
> setting the uninitialized flag in the extent tree.

Implementation is up to the filesystem. However, XFS does (b)
because:

	1) it was extremely simple to implement (one of the
	   advantages of having an exceedingly complex allocation
	   interface to begin with :P)
	2) conversion is atomic, fast and reliable
	3) it is independent of the underlying storage; and
	4) reads of unwritten extents operate at memory speed,
	   not disk speed.

Cheers,

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