On Thu, Oct 30, 2008 at 11:16:05PM -0400, Jeffrey J. Kosowsky wrote:

> I must be missing something on this whole compression, pooling, and
> checksum matter.
> 
> I found 2 files in my cpool that have the same checksum (one is _0)
> but 'cmp' to different values. However, when I zcat them, they have
> the same value. I thought that (lossless) compression was a 1-1
> mapping?

Please post the output of the following:
ls -l yourfile*
md5sum yourfile*
BackupPC_zcat yourfile | md5sum
BackupPC_zcat yourfile_0 | md5sum

> But here we seem to have two files that are identical (and thus have
> the same checksum) but compress to 2 *different* results?

Again: The file name ist NOT the checksum of the whole file's contents!
It's just an MD5 sum which incorporates the first 256k of the file and
the file's original length (as I learned this week from this list).

> This would seem to be going against the grain of pooling where two
> identical files share the same pool entry.
> 
> What am I missing?

That hash collisions are expected.

Tino.

-- 
"What we nourish flourishes." - "Was wir nähren erblüht."

www.lichtkreis-chemnitz.de
www.craniosacralzentrum.de

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