linux-kernel - Re: [PATCH 4/7][TAKE5] support new modes in fallocate

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Message-ID: <20070626231431.GO31489@sgi.com>
Date:	Wed, 27 Jun 2007 09:14:31 +1000
From:	David Chinner <dgc@....com>
To:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, David Chinner <dgc@....com>,
	suparna@...ibm.com, cmm@...ibm.com, xfs@....sgi.com
Subject: Re: [PATCH 4/7][TAKE5] support new modes in fallocate

On Mon, Jun 25, 2007 at 03:46:26PM -0600, Andreas Dilger wrote:
> On Jun 25, 2007  20:33 +0530, Amit K. Arora wrote:
> > I have not implemented FA_FL_FREE_ENOSPC and FA_ZERO_SPACE flags yet, as
> > *suggested* by Andreas in http://lkml.org/lkml/2007/6/14/323  post.
> > If it is decided that these flags are also needed, I will update this
> > patch. Thanks!
> 
> Can you clarify - what is the current behaviour when ENOSPC (or some other
> error) is hit?  Does it keep the current fallocate() or does it free it?
> 
> For FA_ZERO_SPACE - I'd think this would (IMHO) be the default - we
> don't want to expose uninitialized disk blocks to userspace.  I'm not
> sure if this makes sense at all.

Someone on the XFs list had an interesting request - preallocated
swap files. You can't use unwritten extents for this because
of sys_swapon()s use of bmap() (XFS returns holes for reading
unwritten extents), so we need a method of preallocating that does
not zero or mark the extent unread. i.e. FA_MKSWAP.

I thinkthis would be:

#define FA_FL_NO_ZERO_SPACE	0x08	/* default is to zero space */

#define FA_MKSWAP 	(FA_ALLOCATE | FA_FL_NO_ZERO_SPACE)

That way we can allocate large swap files that don't need zeroing
in a single, fast operation, and hence potentially bring new
swap space online without needed very much memory at all (i.e.
should succeed in most near-OOM conditions).

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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