linux-ext4 - Re: jbd2 inside a device mapper module

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Message-ID: <20081226180642.GO9871@mit.edu>
Date:	Fri, 26 Dec 2008 13:06:42 -0500
From:	Theodore Tso <tytso@....EDU>
To:	Alberto Bertogli <albertito@...tiri.com.ar>
Cc:	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com
Subject: Re: jbd2 inside a device mapper module

On Fri, Dec 26, 2008 at 02:17:08PM -0200, Alberto Bertogli wrote:
> 
> At this moment I'm trying to keep it simple, so I plan to batch two for
> each sector written to the device: one for the metadata and one for the
> data.
> 

I think I can pretty much guarantee that your performance will be so
horrible that it won't be worth using.

> > Yes, this is necessary because in a production system you need to be
> > able to identify the external journal by UUID, and the ext2/3/4
> > superblock makes it easy to add a label, UUID, et. al.  It also
> > significantly lowers the chance that an external journal will get
> > misidentified as some other filesystem based on the data stored in the
> > journal.
> 
> Yes, it makes sense. I've reserved the first sector for that purpose.

Why not just use the ext3/4 external journal format?

						- Ted
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