> Op 23 september 2016 om 9:11 schreef Tomasz Kuzemko <tomasz.kuzemko@xxxxxxxxxxxx>: > > > Hi, > > biggest issue with replica size 2 is that if you find an inconsistent > object you will not be able to tell which copy is the correct one. With > replica size 3 you could assume that those 2 copies that are the same > are correct. > > Until Ceph guarantees stored data integrity (that is - until we have > production-ready Bluestore), I would not go with replica size 2. > Not only that, but the same could happen if you have flapping OSDs. OSD 0 and 1 share a PG. 0 goes down, 1 is up and acting and accept writes. Now 1 goes down and 0 comes up. 0 becomes primary, but the PG is 'down' because 1 had the last data. You really need 1 to come back in this case before the PG will work again. I have seen this happen multiple times in systems which got overloaded. If you care about your data you run with size = 3 and min_size = 2. Wido > On 23.09.2016 09:02, Götz Reinicke - IT Koordinator wrote: > > Hi, > > > > Am 23.09.16 um 05:55 schrieb Zhongyan Gu: > >> Hi there, > >> the default rbd pool replica size is 3. However, I found that in our > >> all ssd environment, capacity become a cost issue. We want to save > >> more capacity. So one option is change the replica size from 3 to 2. > >> anyone can share the experience of pros vs cons regarding replica size > >> 2 vs 3? > > from my (still limited) POV, one main aspect is: how reliabel is your > > hardware if you think off this? How often will a disk break, a server > > crash, a datacenter burn down, a networkswitch fail? And if there is a > > failure, how fast could that broken part be replaced or how fast is your > > availabel hardware to replicate the lost OSD to the remaining system. > > > > I dont have numbers, but for our first initial cluster we go as well > > with a repl size of 2 and I dont have bad feelings yet when i look at > > the server and network infrastrukture we got. > > > > Others with more experiacne will give some other hints and may be > > numbers. I never found some sort of calculator which can say "Oh you get > > this hardware? Than a repl size of x y z is what you need." > > > > HTH a bit . Regards . Götz > > > > > > > > > > _______________________________________________ > > ceph-users mailing list > > ceph-users@xxxxxxxxxxxxxx > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > > > > -- > Tomasz Kuzemko > tomasz.kuzemko@xxxxxxxxxxxx > > _______________________________________________ > 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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