linux-ext4 - Re: [PATCH 1/6] fs: add hole punching to fallocate

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Date:	Tue, 16 Nov 2010 21:28:14 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andreas Dilger <adilger@...ger.ca>, Jan Kara <jack@...e.cz>,
	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, cmm@...ibm.com,
	cluster-devel@...hat.com, ocfs2-devel@....oracle.com
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate

On Wed, Nov 17, 2010 at 01:11:50PM +1100, Dave Chinner wrote:
> On Tue, Nov 16, 2010 at 06:22:47PM -0600, Andreas Dilger wrote:
> > On 2010-11-16, at 07:14, Jan Kara wrote:
> > >> Yeah I went back and forth on this.  KEEP_SIZE won't change the
> > >> behavior of PUNCH_HOLE since PUNCH_HOLE implicitly means keep
> > >> the size.  I figured since its "mode" and not "flags" it would
> > >> be ok to make either way accepted, but if you prefer PUNCH_HOLE
> > >> means you have to have KEEP_SIZE set then I'm cool with that,
> > >> just let me know one way or the other.
> > > 
> > > So we call it "mode" but speak about "flags"? Seems a bit
> > > inconsistent.  I'd maybe lean a bit at the "flags" side and just
> > > make sure that only one of FALLOC_FL_KEEP_SIZE,
> > > FALLOC_FL_PUNCH_HOLE is set (interpreting FALLOC_FL_KEEP_SIZE as
> > > allocate blocks beyond i_size). But I'm not sure what others
> > > think.
> > 
> > IMHO, it makes more sense for consistency and "get what users
> > expect" that these be treated as flags.  Some users will want
> > KEEP_SIZE, but in other cases it may make sense that a hole punch
> > at the end of a file should shrink the file (i.e. the opposite of
> > an append).
> 
> What's wrong with ftruncate() for this?
> 
> There's plenty of open questions about the interface if we allow
> hole punching to change the file size. e.g. where do we set the EOF
> (offset or offset+len)?  What do we do with the rest of the blocks
> that are now beyond EOF?  We weren't asked to punch them out, so do
> we leave them behind? What if we are leaving written blocks beyond
> EOF - does any filesystem other than XFS support that (i.e. are we
> introducing different behaviour on different filesystems)?  And what
> happens if the offset is beyond EOF? Do we extend the file, and if
> so why wouldn't you just use ftruncate() instead?
> 
> IMO, allowing hole punching to change the file size makes it much
> more complicated and hence less likely to simply do what the user
> expects. It also is harder to implement and testing becomes much
> more intricate. From that perspective, it does not seem desirable to
> me...
> 

FWIW I agree with Dave, the only question at this point is do we force users to
specify KEEP_SIZE with PUNCH_HOLE?  On one hand it makes the interface a bit
more consistent, on the other hand it makes the documentation a little weird

"We have mode here, but if you want to use PUNCH_HOLE you also have to specify
KEEP_SIZE, so really it's like a flags field it's just named poorly"

I have no strong opinions the other way so if nobody else does then I'll just do
it Jan's way.  Thanks,

Josef
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