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

[Dominik Csapak <d.csapak@proxmox.com>]

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


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