Hi,
as far as I understand it,
you get no real benefit with doing them one by one, as each osd add, can cause a lot of
data to be moved to a different osd, even tho you just rebalanced it.
Less than with older releases, but yeah.
I’ve known someone who advised against doing them in parallel because one would — for a
time — have PGs with multiple remaps in the acting set. The objection may have been
paranoia, I’m not sure.
One compromise is to upweight the new OSDs one node at a time, so the churn is limited to
one failure domain at a time.
— aad