From: Theodore Tso <tytso@MIT.EDU>
Subject: Re: jbd2 inside a device mapper module
Date: Fri, 26 Dec 2008 13:06:42 -0500
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To: Alberto Bertogli <albertito@blitiri.com.ar>
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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