linux-ext4 - Re: [dm-devel] Re: jbd2 inside a device mapper module

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Date:	Tue, 30 Dec 2008 11:51:47 -0200
From:	Alberto Bertogli <albertito@...tiri.com.ar>
To:	Alex Tomas <bzzz@....com>
Cc:	Shyam_Iyer@...l.com, dm-devel@...hat.com,
	linux-ext4@...r.kernel.org, tytso@....EDU,
	linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] Re: jbd2 inside a device mapper module

On Tue, Dec 30, 2008 at 09:55:57AM +0300, Alex Tomas wrote:
> one good thing about JBD is that you can't update target block and csum
> atomically. so, either you use some form of COW or you use journalling.
> given we already have JBD it'd make sense to use it?

I'm sorry, but I'm not following. Is that first sentence right?

The main disadvantage I see of using jbd at the moment is that I loose
the possibility of having checksums and data in a different device.

The only alternative to jbd that I have at the moment is the "two
metadatas" approach I explained in another email (but please let me know
if it wasn't clear).

They both provide what I need (atomicity in data and csum writes), one
is easier, more tested, but prevents a feature. The other is a bit more
difficult, untested and written my me, but allows a feature. I have no
idea, performance-wise, how they will behave (it is expected they suck,
according to the other emails).

At this moment I'm going with the two metadatas approach, because I
think it has less limitations and it'd be fun to write. If then it's
unfit for some reason, I can always go back and use jbd. But I'm
obviously open to suggestions and more alternatives.

Thanks a lot,
		Alberto

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