From: Theodore Tso Subject: Re: jbd2 inside a device mapper module Date: Fri, 26 Dec 2008 13:06:42 -0500 Message-ID: <20081226180642.GO9871@mit.edu> References: <20081224211038.GT4127@blitiri.com.ar> <20081224234915.GA23723@mit.edu> <20081225143535.GA4127@blitiri.com.ar> <20081225155248.GJ9871@mit.edu> <20081226000005.GB4127@blitiri.com.ar> <20081226033736.GK9871@mit.edu> <20081226161708.GC4127@blitiri.com.ar> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dm-devel@redhat.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: Alberto Bertogli Return-path: Content-Disposition: inline In-Reply-To: <20081226161708.GC4127@blitiri.com.ar> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com List-Id: linux-ext4.vger.kernel.org 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