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Date: Tue, 20 Sep 2016 08:19:28 +0000
From: Hugo Mills <hugo@carfax.org.uk>
To: Duncan <1i5t5.duncan@cox.net>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: stability matrix (was: Is stability a joke?)
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On Tue, Sep 20, 2016 at 07:59:44AM +0000, Duncan wrote:
> Christoph Anton Mitterer posted on Mon, 19 Sep 2016 21:45:46 +0200 as
> excerpted:
>=20
> > On Mon, 2016-09-19 at 17:27 +0200, David Sterba wrote:
> >=20
> >> > For specific features:
> >> > - Autodefrag
> >> > =A0 - didn't that also cause reflinks to be broken up?
> >>=20
> >> No and never had.
> >=20
> > Absolutely sure? One year ago, I was told that at first too so I started
> > using it, but later on some (IIRC) developer said auto-defrag would also
> > suffer from it.
>=20
> AFAIK it was Hugo that said he looked into that, and that (if I'm=20
> representing it correctly) autodefrag breaks reflinks and triggers space-
> using duplication much as defrag does, but that it does it on a much=20
> smaller scale, since it (1) only triggers when some parts of a file are=
=20
> being rewritten anyway, thus breaking the reflink for those specific=20
> parts of the file due to COW (COW1 on otherwise NOCOW files) in any case,=
=20
> and (2) unlike defrag, doesn't rewrite and thus break the reflinks on=20
> entire files, just somewhat larger extents than the pure rewrite by=20
> itself without autodefrag would.
>=20
> Thus making the reflink-breaking and duplication effect of autodefrag=20
> there, but relatively quite small compared to on-demand per-file defrag.

   I didn't investigate it -- It was my firmly-stated misunderstanding
which caused someone (Filipe, I think) with much more actual knowledge
to correct me, thus making the actual behaviour much clearer. :)

   I think your description is accurate as far as my current
understanding goes.

   Hugo.

> --=20
> Duncan - List replies preferred.   No HTML msgs.
> "Every nonfree program has a lord, a master --
> and if you use the program, he is your master."  Richard Stallman
>=20
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--=20
Hugo Mills             | There isn't a noun that can't be verbed.
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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