Can I remove rbd pool and re-create it?

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Hi Christian,

Thanks for your reply, and since I do really don't like the HEALTH_WARN and it
not allowed to decrease pg_num of a pool.

So can I just remove the default **rbd** pool and re-create it by using `ceph osd
pool create`?

If so, is there anything I have to pay attention to?

Thanks in advance!

On Fri, Jul 29, 2016 at 11:47:59AM +0900, Christian Balzer wrote:
> On Fri, 29 Jul 2016 09:59:38 +0800 Chengwei Yang wrote:
> 
> > Hi list,
> > 
> > I just followed the placement group guide to set pg_num for the rbd pool.
> > 
> How many other pools do you have, or is that the only pool?
> 
> The numbers mentioned are for all pools, not per pool, something that
> isn't abundantly clear from the documentation either.
> 
> >   "
> >   Less than 5 OSDs set pg_num to 128
> >   Between 5 and 10 OSDs set pg_num to 512
> >   Between 10 and 50 OSDs set pg_num to 4096
> >   If you have more than 50 OSDs, you need to understand the tradeoffs and how to
> >   calculate the pg_num value by yourself
> >   For calculating pg_num value by yourself please take help of pgcalc tool
> >   "
> > 
> You should have headed the hint about pgcalc, which is by far the best
> thing to do.
> The above numbers are an (imprecise) attempt to give a quick answer to a
> complex question.
> 
> > Since I have 40 OSDs, so I set pg_num to 4096 according to the above
> > recommendation.
> > 
> > However, after configured pg_num and pgp_num both to 4096, I found that my
> > ceph cluster in **HEALTH_WARN** status, which does surprised me and still
> > confusing me.
> > 
> PGcalc would recommend 2048 PGs at most (for a single pool) with 40 OSDs.
> 
> I assume the above high number of 4096 stems from the wisdom that with
> small clusters more PGs than normally recommended (100 per OSD) can be
> helpful. 
> It was also probably written before those WARN calculations were added to
> Ceph.
> 
> The above would better read:
> ---
> Use PGcalc!
> [...]
> Between 10 and 20 OSDs set pg_num to 1024
> Between 20 and 40 OSDs set pg_num to 2048
> 
> Over 40 definitely use and understand PGcalc.
> ---
> 
> > >   cluster bf6fa9e4-56db-481e-8585-29f0c8317773
> >      health HEALTH_WARN
> >             too many PGs per OSD (307 > max 300)
> > 
> > I see the cluster also says "4096 active+clean" so it's safe, but I do not like
> > the HEALTH_WARN in anyway.
> >
> You can ignore it, but yes, it is annoying.
>  
> > As I know(from ceph -s output), the recommended pg_num per OSD is [30, 300], any
> > other pg_num out of this range with bring cluster to HEALTH_WARN.
> > 
> > So what I would like to say: is the document misleading? Should we fix it?
> > 
> Definitely.
> 
> Christian
> -- 
> Christian Balzer        Network/Systems Engineer                
> chibi@xxxxxxx   	Global OnLine Japan/Rakuten Communications
> http://www.gol.com/

-- 
Thanks,
Chengwei

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