Message-Id: <200401162037.i0GKbgWY005453@turing-police.cc.vt.edu>
To: Timothy Miller <miller@techsource.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [OT] Redundancy eliminating file systems, breaking MD5, donating money to OSDL 
In-Reply-To: Your message of "Fri, 16 Jan 2004 15:22:39 EST."
             <4008480F.70206@techsource.com> 
From: Valdis.Kletnieks@vt.edu
References: <4008480F.70206@techsource.com>
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On Fri, 16 Jan 2004 15:22:39 EST, Timothy Miller <miller@techsource.com>  said:

> Think about it!  If we had a filesystem that actually DID this, and it 
> was in the Linux kernel, it would spread far and wide.  It's bound to 
> happen that someone will identify a collision.  We then report that to 
> the committee offering the reward and then donate it to OSDL to help 
> Linux development.

Actually, it's *not* "bound to happen".  Figure out the number of blocks you'd
need to have even a 1% chance of a birthday collision in a 2**128 space.

And you'd need that many disk blocks on *a single system*.

Then figure out the chances of a collision on a small machine that only has 20
or 30 terabytes (yes, in this case terabytes is small).

The whole reason "use MD5 as a check for identical blocks" is useful is because
the chances of *that* going wrong are vanishingly small compared to the chances
that a memory stick will throw an undetected multiple-bit error, or that a RAID
controller will write blocks to the wrong disks, or any number of other things
that *do* happen in real life, but rarely enough that we don't bother writing
code to defend against them.


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