Purpose of --checksum-seed ?

[Andrew Gideon]

I'm trying to understand the point of the --checksum-seed option.  As I 
understand it from a little reading, checksums are not cached over 
executions of rsync.  So...what is the point of fixing the seed?

Is this in support of patches which *do* support caching of checksums?  
I've read about caching these in files and in xaddr.  Is there a "best 
solution" for caching checksums?

I'm asking because one particular volume, when I added the --checksum 
option, went from 11m37.520s to 290m22.423s.  If I could cut that down, 
this would be a good thing.

On the other hand, the reason I'm switching to checksum as opposed to the 
timestamp approach is to catch files that people are deliberately 
"backdating" to avoid the changes being seen.  I'm unclear how caching 
checksums doesn't cause exactly that problem (since a cached checksum 
means that the file isn't checked for changes each time rsync runs).

I guess I'm vague on a lot of the ideas around the of checksums.  
Enlightenment would be welcome.

Thanks...

	Andrew