Re: [PATCH v3] btrfs: Introduce new mount option to disable tree log replay

From: Christoph Anton Mitterer <calestyo@scientia.net>
To: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>,
	dsterba@suse.cz, Qu Wenruo <quwenruo@cn.fujitsu.com>,
	linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v3] btrfs: Introduce new mount option to disable tree log replay
Date: Mon, 14 Dec 2015 20:16:54 +0100	[thread overview]
Message-ID: <1450120614.6629.34.camel@scientia.net> (raw)
In-Reply-To: <566F016D.8050403@gmail.com>

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On Mon, 2015-12-14 at 12:50 -0500, Austin S. Hemmelgarn wrote:
> It should also imply noatime.  I'm not sure how BTRFS handles atime
> when 
> mounted RO, but I know a lot of old UNIX systems updated atime even
> on 
> filesystems mounted RO, and I know that at least at one point Linux
> did too.
I stumbled over that recently myself, and haven't bothered to try it
out, yet.
But Duncan's argument, why at least ro-snapshots (yes I know, this may
not be exactly the same as RO mount option) would need to imply
noatime, is pretty convincing. :)

Anyway, if it "ro" wouldn't imply noatime, I would ask why, because the
atime is definitely something the fs exports normally to userland,...
and that's how I'd basically consider hard-ro vs. (soft-)ro:

soft-ro: data as visible by the mounted fs must not change (unless
         perhaps for necessary repair/replay operations to get the 
         filesystem back in a consistent state)
hard-ro: soft-ro + nothing on the backing devices may change (bitwise)

Cheers,
Chris.

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