linux-ext4 - Re: [PATCH] resize2fs: fix overly-pessimistic calculation of minimum size required

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Date:	Mon, 28 Apr 2014 11:34:54 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Theodore Ts'o <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] resize2fs: fix overly-pessimistic calculation of minimum size required

On Sun, 27 Apr 2014 00:35:37 -0400, "Theodore Ts'o" <tytso@....edu> wrote:
> On Sat, Apr 26, 2014 at 10:48:14PM -0400, Theodore Ts'o wrote:
> > For extent-mapped file systems, we need to reserve some extra space in
> > case we need to grow the extent tree.  Calculate the safety margin
> > more intelligently, so we don't overestimate the amount of space
> > required.
> > 
> > Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> > Reported-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> 
> I'm going to have to self-NACK this.  This patch causes the resize2fs
> regression tests to fail.  (In fact, Dmitry's original patch also
> causes the resize2fs regression tests to fail.)
Agree, regressions are not acceptable. Can you please spacify
which tests are failed. As far as i know xfstetsts has no tests
for resize2fs.
> The problem is kind of messy; when the file system starts at some
> insanely large size, and we shrink it very small, we end up releasing
> a lot of inode tables in the first block group (many for other block
> groups).  But until we're 100% sure the resize will be successful, we
> don't want to start overwriting those inode table blocks.
> 
> For this reason, if we try to constrain resize the file system down
> from 2TB to 512MB in one shot, we need to do this in multiple steps.
> I.e. by calling "resize2fs -M /dev/sdXX" multiple times.
> 
> There really isn't a good way around this, and in fact, if people are
> going to be doing silly things like take a file system from 16T down
> to 750MB, if they need to run resize2fs multiple times, that's fine.
> It would be nice if you could shrink the file system down in a single
> shot, but it's not high priority.
> 
> 					- Ted
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