Re: How to build a big file server

[Soeren Sonnenburg <reiserfs@nn7.de>]

On Thu, 2003-06-05 at 18:47, Carl-Daniel Hailfinger wrote:
> Heinz-Josef Claes wrote:
> >From the debian web page:
> >
> > http://packages.debian.org/testing/utils/storebackup.html
> >
> > File comparisons are done with MD5 checksums, so no changes go
> > unnoticed.
> 
> If you believe the last sentence, I have a bridge to sell.
> 
> To be more exact: MD5 is a 128=2^7 bit hash. Assuming a file length of 4kB
> = 2^8*4096=2^20 bits, approximately 2^(2^(20-7))= 2^8192= 10^2457
> different files have the same hash.
> 
> That's right: for a given MD5 hash, there are more different files with
> 4kB size sharing the same hash than the count of atoms in the whole
> universe. If the files are larger, it gets worse.
> 
> md5sum(1) is not diff(1). Most of the time, it will suffice as el cheapo
> replacement, but for backups it's definitely horrible. You don't store
> your backup tapes in the microwave, do you?

you forget one thing: how likely is it that a file with MD5SUM A turns
into a a file which has the same MD5SUM A. I would guess that that kind
of file corruption has a likelihood of very close to zero.

S.