We got a few size=4 pools, but most of them are metadata pools paired with m=3 or m=4 erasure coded pools for the actual data.
Goal is to provide the same availability and durability guarantees for the metadata as the data.
But we got some older odd setup with replicated size=4 for that reason (setup predates nautilus so no ec overrides originally).
I'd prefer erasure coding over a size=4 setup for most scenarios nowadays.
Paul
--
Paul Emmerich
Looking for help with your Ceph cluster? Contact us at https://croit.io
croit GmbH
Freseniusstr. 31h
81247 München
www.croit.io
Tel: +49 89 1896585 90
Paul Emmerich
Looking for help with your Ceph cluster? Contact us at https://croit.io
croit GmbH
Freseniusstr. 31h
81247 München
www.croit.io
Tel: +49 89 1896585 90
On Wed, Jul 24, 2019 at 9:22 PM Wido den Hollander <wido@xxxxxxxx> wrote:
Hi,
Is anybody using 4x (size=4, min_size=2) replication with Ceph?
The reason I'm asking is that a customer of mine asked me for a solution
to prevent a situation which occurred:
A cluster running with size=3 and replication over different racks was
being upgraded from 13.2.5 to 13.2.6.
During the upgrade, which involved patching the OS as well, they
rebooted one of the nodes. During that reboot suddenly a node in a
different rack rebooted. It was unclear why this happened, but the node
was gone.
While the upgraded node was rebooting and the other node crashed about
120 PGs were inactive due to min_size=2
Waiting for the nodes to come back, recovery to finish it took about 15
minutes before all VMs running inside OpenStack were back again.
As you are upgraded or performing any maintenance with size=3 you can't
tolerate a failure of a node as that will cause PGs to go inactive.
This made me think about using size=4 and min_size=2 to prevent this
situation.
This obviously has implications on write latency and cost, but it would
prevent such a situation.
Is anybody here running a Ceph cluster with size=4 and min_size=2 for
this reason?
Thank you,
Wido
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