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Subject: Re: [PATCH 00/22 take 3] UBI: Unsorted Block Images
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: =?ISO-8859-1?Q?J=F6rn?= Engel <joern@lazybastard.org>
Cc: Matt Mackall <mpm@selenic.com>, Josh Boyer <jwboyer@linux.vnet.ibm.com>,
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Date: Wed, 21 Mar 2007 12:25:34 +0100
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On Wed, 2007-03-21 at 12:05 +0100, Jörn Engel wrote:
> On Tue, 20 March 2007 01:42:46 +0100, Thomas Gleixner wrote:
> > On Mon, 2007-03-19 at 17:32 -0500, Matt Mackall wrote:
> > 
> > > > > 4. JFFS2 has its own wear-leving scheme, as do several other
> > > > >    filesystems, so they probably want to bypass this piece of the stack.
> > > > 
> > > > JFFS2 on top of UBI delegates the wear levelling to UBI, as JFFS2s own
> > > > wear levelling sucks. 
> > > 
> > > Ok, fine. How about LogFS, then?
> > 
> > LogFS can easily leverage UBI's wear algorithm.
> 
> Ok, now we have reached the absurd.  UBI quite fundamentally cannot do
> wear leveling as good as LogFS can.  Simply because UBI has zero
> knowledge of the _contents_ of its blocks.  Knowing whether a block is
> 90% garbage or not makes a great difference.
> 
> Also LogFS currently requires erasesizes of 2^n.

Last time I talked to you about that, you said it would be possible and
fixable. We talked about several mechanisms, which would allow a
filesystem or other users to hint such things to UBI.

Even if the LogFS wear levelling is so superior, it CAN'T do across
device wear levelling.

	tglx