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

From: Christoph Anton Mitterer <calestyo@scientia.net>
To: Duncan <1i5t5.duncan@cox.net>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v3] btrfs: Introduce new mount option to disable tree log replay
Date: Thu, 17 Dec 2015 02:46:33 +0100	[thread overview]
Message-ID: <1450316793.6242.154.camel@scientia.net> (raw)
In-Reply-To: <pan$5604e$2cc633fa$ad91aa30$7832938f@cox.net>

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On Thu, 2015-12-17 at 01:09 +0000, Duncan wrote:
> Well, "don't load the journal on mounting" is exactly what the option

> would do.  The journal (aka log) of course has a slightly different 
> meaning, it's only the fsync log, but loading it is exactly what the 
> option would prevent, here.
That's not the point.
What David asked for was an option, that has the meaning "do what ever
is necessary to mount the fs in such a way, that the device isn't
changed".
At least that's how I've understood him.

*Right now* this is for btrfs the "nologreplay" option, so
"nodevwrites" or whatever you call it, would simply imply
"nologreply"... and in the future any other options that are necessary
to get the above defined semantics.

For ext, "nodevwrites" would imply "noload" (AFAIU).

If you now make "noload" an alias for "nodevwrites" in btrfs you
clearly break semantics here:
"noload" from ext4 hasn't the same meaning as "nodevwrites" from
btrfs.. it has *just now* while ext doesn't need any other possible
future options.

Maybe in 10 years, ext has a dozens new features (because btrfs still
hasn't stabilised yet, as it misses snapshot-aware defrag and checksums
for noCoWed data >:-D O:-D ... sorry, couldn't resist ;) ), one of that
new features needs to be disabled for "hard ro mounts", thus ext's
"nodevwrites" would in addition imply "noSuperNewExtFeature".

Then "noload" from ext4 isn't even *effectively* the same anymore as
"nodevwrites" from btrfs.
Therefore, it shouldn't be an alias.

Cheers,
Chris.

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