linux-ext4 - RE: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT

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Date:	Wed, 04 Jun 2014 13:57:37 +0900
From:	Namjae Jeon <namjae.jeon@...sung.com>
To:	'Theodore Ts'o' <tytso@....edu>,
	'Dave Chinner' <david@...morbit.com>
Cc:	'linux-ext4' <linux-ext4@...r.kernel.org>, xfs@....sgi.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	'Ashish Sangwan' <a.sangwan@...sung.com>,
	'Lukáš Czerner' <lczerner@...hat.com>
Subject: RE: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT_RANGE for fallocate

> 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

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