From: Theodore Tso <tytso@MIT.EDU>
Subject: Re: jbd2 inside a device mapper module
Date: Thu, 25 Dec 2008 22:37:36 -0500
Message-ID: <20081226033736.GK9871@mit.edu>
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To: Alberto Bertogli <albertito@blitiri.com.ar>
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On Thu, Dec 25, 2008 at 10:00:05PM -0200, Alberto Bertogli wrote:
> 
> I think I'm not explaining myself correctly. My code has _nothing_ to do
> with ext2/3/4 (or any other filesystem) whatsoever. I'm not using the
> journal as an external one for a filesystem. I want to use it to be able
> to do atomic writes in my own, filesystem independant, device-mapper
> code.

How many block writes are you batching into a single transaction?  If
you're not careful you may find that performance overhead will be
quite expensive.

> After what you told me (both this and the deprecation of
> jbd2_journal_create()), I took a look at e2fsprogs' source. From what I
> could see, "mke2fs -O journal_dev" creates the external journal inside
> some ext2/3/4 structures, which caused my journal-loading code to fail
> (because it doesn't know about ext stuff).

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.

					- Ted