[pve-devel] ceph create pool with min_size=1 not possible anymore with last gui wizard

Dominik Csapak d.csapak at proxmox.com
Fri Jun 4 15:23:09 CEST 2021

On 6/4/21 04:47, aderumier at odiso.com wrote:
> Hi,


> I was doing a training week with students,
> and I see that the new ceph wizard to create pool don't allow to set
> min_size=1 anymore.
> It's currently displaying a warning "min_size <= size/2 can lead to
> data loss, incomplete PGs or unfound objects",
> that's ok  ,  but It's also blocking the validation button.

yes, in our experience, setting min_size to 1 is always a bad idea
and most likely not what you want

what is possible though is to either create the pool on the cli,
or changing the min_size after creation to 1 (this is not blocked)

> Some users with small cluster/budgets want to do only size=2,
> so with min_size=2, the cluster will go read only in case of any osd
> down.
> It could be great to allow at least min_size=1 when size=2 is used.

"great" but very dangerous

> also,
> Other setup like size=4, min_size=2, also display the warning, but
> allow to validate the form.
> I'm not sure this warning is correct in this case , as since octopus,
> min_size
> is auto compute when pool is created, and a simple
> ceph osd pool create mypool 128 --size=4  , create a pool with
> min_size=2 by default.

the rationale behind this decision was (i think) because
if you have exactly 50% min_size of size (e.g. 4/2)
you can get inconsistent pgs, with no quorum as to
which pg is correct?
(though don't quote me on that)

so i think its always better to have > 50% min_size of size

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