When Zero isn't 0 (Crush weight mysteries)

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Hello,

I just (manually) added 1 OSD each to my 2 cache-tier nodes. 
The plan was/is to actually do the data-migration at the least busiest day
in Japan, New Years (the actual holiday is January 2nd this year). 

So I was going to have everything up and in but at weight 0 initially.

Alas at the "ceph osd crush add osd.x0 0 host=ceph-0x" steps Ceph happily
started to juggle a few PGs (about 7 total) around, despite of course no
weight in the cluster changing at all.
No harm done (this is the fast and not too busy cache-tier after all), but
very much unexpected.

So which part of the CRUSH algorithm goes around and pulls weights out of
thin air?

Christian
-- 
Christian Balzer        Network/Systems Engineer                
chibi@xxxxxxx   	Global OnLine Japan/Rakuten Communications
http://www.gol.com/
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