With HEALTH_OK a mon data dir should be under 2GB for even such a large cluster.

During backfilling scenarios, the mons keep old maps and grow quite
quickly. So if you have balancing, pg splitting, etc. ongoing for
awhile, the mon stores will eventually trigger that 15GB alarm.
But the intended behavior is that once the PGs are all active+clean,
the old maps should be trimmed and the disk space freed.

However, several people have noted that (at least in luminous
releases) the old maps are not trimmed until after HEALTH_OK *and* all
mons are restarted. This ticket seems related:

(Over here we're restarting mons every ~2-3 weeks, resulting in the
mon stores dropping from >15GB to ~700MB each time).

-- Dan

On Wed, Feb 6, 2019 at 1:26 PM Sage Weil <s...@newdream.net> wrote:
> Hi Swami
> The limit is somewhat arbitrary, based on cluster sizes we had seen when
> we picked it.  In your case it should be perfectly safe to increase it.
> sage
> On Wed, 6 Feb 2019, M Ranga Swami Reddy wrote:
> > Hello -  Are the any limits for mon_data_size for cluster with 2PB
> > (with 2000+ OSDs)?
> >
> > Currently it set as 15G. What is logic behind this? Can we increase
> > when we get the mon_data_size_warn messages?
> >
> > I am getting the mon_data_size_warn message even though there a ample
> > of free space on the disk (around 300G free disk)
> >
> > Earlier thread on the same discusion:
> > https://www.spinics.net/lists/ceph-users/msg42456.html
> >
> > Thanks
> > Swami
> >
> >
> >
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