RE: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT_RANGE for fallocate

From: Namjae Jeon <namjae.jeon@samsung.com>
To: 'Theodore Ts'o' <tytso@mit.edu>, 'Dave Chinner' <david@fromorbit.com>
Cc: linux-kernel@vger.kernel.org, xfs@oss.sgi.com,
	"'Ashish Sangwan'" <a.sangwan@samsung.com>,
	linux-fsdevel@vger.kernel.org,
	"'Lukáš Czerner'" <lczerner@redhat.com>,
	'linux-ext4' <linux-ext4@vger.kernel.org>
Subject: RE: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT_RANGE for fallocate
Date: Wed, 04 Jun 2014 13:57:37 +0900	[thread overview]
Message-ID: <000e01cf7fb1$814ab6d0$83e02470$@samsung.com> (raw)
In-Reply-To: <20140602150258.GG30598@thunk.org>

> On Mon, Jun 02, 2014 at 03:06:13PM +0200, Lukáš Czerner wrote:
> > > > So what will happen when there is not enough space when "inserting a
> > > > range" ? And how should user proceed from there ?
> > > If insert range fails with an ENOSPC error, user could use collapse
> > > range on the same range to remove the hole.
> > > And after freeing more space, he can again try inserting range.
> > > Ofcourse, this type of guidance should be properly documented in
> > > manpage. When updating fallocate(2) manpage, I will keep  in mind to
> > > describe ENOSPC handling.
> >
> > Why collapse ? The hole is already there right ? Why not just use
> > fallocate to allocate the space for the hole. And that's my point
> > actually. Why not do it this way in the first place, because this is
> > really counterintuitive.
> 
> It's worse than that.  It's possible that the reason why you got the
> ENOSPC warning was because the operation to move the extents down
> required allocating a block, and it was *that* block allocation which
> failed.  So it's not deterministic whether or not the file's extent
> mappings were modified after a ENOSPC error, and so it's not clear
> whether or not a collapse_range function will undo the range that had
> been inserted --- or whether it ends up deleting existing data blocks.
> 
> In generally, you really want system calls to have all-or-nothing
> effects, where if the system call returns an error, the state of the
> file has not been changed.  And for that reason, I agree with Lukáš
> that it is really a good idea to decouple moving the blocks down, and
> allocating space --- and to make sure that if there is any failure
> while inserting the range, the state of the file is not modified at all.
Okay, I will remove allocating space part in insert range patch.
But renaming flags as FALLOC_FL_INSERT_HOLE is needed to concent with
XFS people. Because Dave prefered to call it FALLOC_FL_INSERT_RANGE
so that it looks like it is related to collapse range.

Hi Dave.
Do you have any objection about renaming as insert hole ?

Thanks for opinions!

> 
> Cheers,
> 
> 					- Ted

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs