Thanks Anthony and Wesley for your input.

Let me explain in more detail why I'm interested in the somewhat obscure 
looking procedure in step 1.

Whats the difference between "ceph osd reweight" and "ceph osd crush reweight"? 
the difference is that command 1 only remaps shards within the same failure 
domain (as Anthony noted), while command 2 implies global changes to the crush 
map with rediúndant data movement. In other words, using

  ceph osd reweight osd.X 0

will only move the shards from osd.X to other OSDs (in the same failue domain) 
while

  ceph osd crush reweight osd.X 0

has a global effect and will move a lot more around. This "a lot more" is what 
I want to avoid. There is necessary data movement, namely the data on the OSDs 
I want to evacuate, and there is redundant data movement, which is everything 
else.

So, for evacuation, the first command is the command of choice if one wants to 
move exactly the shards that need to move.

If one re-creates OSDs with exactly the same IDs and weights that the evacuated 
OSDs had, which is the default when using "ceph osd destroy" as it preserves 
the crush weights, then, after adding the new OSDs, it will be exactly the 
shards that were evacuated in step 1 that will move back. That's the minimum 
possible data movement: data moved = data that needs to move.

I don't have balancer or anything enabled that could interfere with that 
procedure. Please don't bother commenting about things like that.

My actual question is, how dangerous is it to use

  ceph osd reweight osd.X 0

instead of

  ceph osd reweight osd.X 0.001

The first command will mark the OSD OUT while the second won't. The second 
command might leave 1-2 PGs on the OSDs, while the first one won't.

Does the OSD being formally UP+OUT make any difference compared with UP+IN for 
evacuation? My initial simplistic test says no, but I would like to be a bit 
more sure than that.

Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
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