Thank you both Michel and Christian.

Looks like I will have to do the rebalancing eventually.
From past experience with Ceph 16 the rebalance will likely take at least a 
month with my 500 M objects.

It seems like a good idea to upgrade to Ceph 17 first as Michel suggests.

Unless:

I was hoping that Ceph might have a way to reduce the rebalancing, given that 
all constraints about failure domains are already fulfilled.

In particular, I was wondering whether I could play with the names of the 
"datacenter"s, to bring them in the same (alphabetical?) order as the hosts 
were so far.
I suspect that this is what avoided the reshuffling on my my mini test cluster.
I think it would be in alignment with Table 1 from the CRUSH paper: 
https://ceph.com/assets/pdfs/weil-crush-sc06.pdf

E.g. perhaps

take(root)
select(1, row)
select(3, cabinet)
emit

yields the same result as

take(root)
select(3, row)
select(1, cabinet)
emit

?


Niklas
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