Date: Fri, 24 Oct 2014 11:18:07 -0400
From: Zygo Blaxell <zblaxell@furryterror.org>
To: Duncan <1i5t5.duncan@cox.net>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: device balance times
Message-ID: <20141024151807.GF17395@hungrycats.org>
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On Fri, Oct 24, 2014 at 05:13:27AM +0000, Duncan wrote:
> Zygo Blaxell posted on Thu, 23 Oct 2014 22:35:29 -0400 as excerpted:
>=20
> > My pet peeve:  if balance is converting profiles from RAID1 to single,
> > the conversion should be *instantaneous* (or at least small_constant *
> > number_of_block_groups).  Pick one mirror, keep all the chunks on that
> > mirror, delete all the corresponding chunks on the other mirror.
>=20
> That would argue for either a third balance mode, --convert-only, or a=20
> different tool, avoiding a rewrite of existing chunks entirely, simply=20
> replicating them if adding redundancy or deleting a copy if reducing it,=
=20
> as necessary.

Isn't that what soft does?  [reading noises]  OK, maybe not.
'soft' leaves a chunk alone if it already fits all the target profile
requirements; however, in this case the profile (and only the profile,
no data) is changing.

I think just two modes are sufficient:  one that does everything the most
thorough way (throw scrub and defrag in there too, so we can do a single
pass over the filesystem that does all the maintenance tasks at once),
and one that takes advantage of every special-case shortcut available
to achieve specific goals in the shortest time.

I also think it's a little odd that conversion and balance are the
same tool.  Traditional RAID conversions don't care about filesystem
layout, because they work on a completely separate layer (i.e. at the
block level).  It's certainly possible to perform a RAID conversion
by reallocating all the filesystem-level objects, but just because
you can doesn't mean you should.  ;)


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