Hi Janne
Am 20.11.24 um 11:30 schrieb Janne Johansson:
This post seem to show that, except they have their root named "nvme"
and they split on rack and not dc, but that is not important.
https://unix.stackexchange.com/questions/781250/ceph-crush-rules-explanation-for-multiroom-racks-setup
This is indeed a good example, thanks.
Let me put some thoughts/questions here:
step choose firstn 2 type rack
This choses 2 racks out of all available racks. As there are 2 racks
available, all are chosen.
step chooseleaf firstn 2 type host
For each selected rack from the previous step, 2 hosts are chosen. But
as the action is "chooseleaf", in fact not the hosts are picked, but one
random (?) OSD in each of the 2 selected hosts.
In the end we have 4 OSDs in 4 different hosts, 2 in each rack.
Is this understanding correct?
Shouldn't we note this one additionally:
min_size 4
max_size 4
Reason: If we wanted to place more ore less than 4 replicas, the rule
won't work. Or what would happen if we don't specify min/max_size?
Should lead to an error in case the pool is e.g. size=5, shouldn't it?
One last question: if we edit a crush map after a pool was created on
it, what happens? In my understanding, this lead to massive data
shifting so that the placements comply with the new rules. That right?
Thanks again
--
Andre Tann
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