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

[aderumier at odiso.com]

Le vendredi 04 juin 2021 à 15:23 +0200, Dominik Csapak a écrit :
> On 6/4/21 04:47, aderumier at 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)
> 
yes, Sute. I could be great to be able to change size/min_size from the
gui too.



> > 
> > 
> > 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
> 
Well, afaik, they are no "quorum" on pg consistency for repair
currently.
if a pg is corrupt, ceph is simply copy data from a pg copy where
checksum is ok.
and if no checksum is available, it take a random copy. (maybe it need
a manual pg_repair in this case).
But They are not something like "theses 2 copies have the more majority
(quorum) of checksum.

(Maybe I'm wrong, but 1 or 2 year ago, Sage have confirmed this on the
ceph mailing)