Well, there is a setting for the minimum number of pgs per OSD (mon pg warn min per osd, see http://docs.ceph.com/docs/master/rados/configuration/pool-pg-config-ref/) and there will be a HEALTH_WARN state if you have too few. As far as I know having not enough PGs can cause trouble for CRUSH sometimes not being able to place an object. On Wed, Jan 3, 2018 at 11:10 PM, Marc Roos <M.Roos@xxxxxxxxxxxxxxxxx> wrote: > > > Is there a disadvantage to just always start pg_num and pgp_num with > something low like 8, and then later increase it when necessary? > > Question is then how to identify when necessary > > > > > > -----Original Message----- > From: Christian Wuerdig [mailto:christian.wuerdig@xxxxxxxxx] > Sent: dinsdag 2 januari 2018 19:40 > To: 于相洋 > Cc: Ceph-User > Subject: Re: Questions about pg num setting > > Have you had a look at http://ceph.com/pgcalc/? > > Generally if you have too many PGs per OSD you can get yourself into > trouble during recovery and backfilling operations consuming a lot more > RAM than you have and eventually making your cluster unusable (some more > info can be found here for example: > http://lists.ceph.com/pipermail/ceph-users-ceph.com/2016-October/013614.html > but there are other threads on the ML). > Also currently you cannot reduce the number of PGs for a pool so you are > much better of starting with a lower value and then gradually increasing > it. > > The fact that the ceph developers introduced a config option which > prevents users from increasing the number of PGs if it exceeds the > configured limit should be a tell-tale sign that having too many PGs per > OSD is considered a problem (see also > https://bugzilla.redhat.com/show_bug.cgi?id=1489064 and linked PRs) > > On Wed, Dec 27, 2017 at 3:15 PM, 于相洋 <penglaiyxy@xxxxxxxxx> wrote: >> Hi cephers, >> >> I have two questions about pg number setting. >> >> First : >> My storage informaiton is show as belows: >> HDD: 10 * 8TB >> CPU: Intel(R) Xeon(R) CPU E5645 @ 2.40GHz (24 cores) >> Memery: 64GB >> >> As my HDD capacity and my Mem is too large, so I want to set as many >> as 300 pgs to each OSD. Although 100 pgs per OSD is perferred. I want >> to know what is the disadvantage of setting too many pgs? >> >> >> Second: >> At begin ,I can not judge the capacity proportion of my workloads, so >> I can not set accurate pg numbers of different pools. How many pgs >> should I set for each pools first? >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" >> in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo >> info at http://vger.kernel.org/majordomo-info.html > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > > > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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