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

From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Christoph Anton Mitterer <calestyo@scientia.net>,
	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 14:33:46 -0500	[thread overview]
Message-ID: <566F199A.5030704@gmail.com> (raw)
In-Reply-To: <1450120614.6629.34.camel@scientia.net>

On 2015-12-14 14:16, Christoph Anton Mitterer wrote:
> 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. :)
The traditional reasoning was that read-only meant that users couldn't 
change anything, not that the actual data on disk wouldn't change. 
That, and there's been some really brain-dead software over the years 
that depended on atimes being right (now, the only remaining software I 
know of that even uses them at all is Mutt).
>
> 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)
This should be 'Nothing on the backing device may change as a result of 
the FS', nitpicking I know, but we should be specific so that we 
hopefully avoid ending up in the same situation again.