linux-ext4 - Re: [PATCH 8/9] e2scrub_all: refactor device probe loop

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Message-ID: <20190321174811.GF9434@mit.edu>
Date:   Thu, 21 Mar 2019 13:48:11 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        darrick.wong@...cle.com
Subject: Re: [PATCH 8/9] e2scrub_all: refactor device probe loop

On Thu, Mar 21, 2019 at 11:27:42AM +0100, Lukas Czerner wrote:
> 
> Snapshot of a thinspanshot is allowed though, so we might want to
> include those. Not sure if it's wise to do it by default, but regardless
> it's probably something for a separate change.

Yeah, it's definitely a separate change.  One potential design
question is that for a thin volume, you can do both a thin or a think
snapshot, and in some cases one might succeed while the other will
fail.  So do we make this choice be a parameter that we set in the
config file, or do we try to see if there is sufficient spare
freespace for a thick snapshot (and then do that), or a thin snapshot
(and then do that) --- and which should use prefer?

The other thing I'll note is that in order for us to tell whether
something is a thin or thick LV, we're going to to need to ask lvs to
return multiple parameters, so the optimization of using:

	for NAME in $(lvs -o lv_path --noheadings -S...) ; do
	    ...
	done

will no longer work.  (Or we end up calling lvs a second time, which
is less efficient.)

Just curious --- do we know how commonly thin LV's are being used by
customers of various distros?  I assume enterprise distro users will
be the most conservative, but how common is the uptake of thin LV's by
Fedora and OpenSuSE users?

						- Ted