On 09/19/2013 09:17 AM, Gandalf Corvotempesta wrote:
Hi to all, increasing the total numbers of MONs available in a cluster, for example growing from 3 to 5, will also decrease the hardware requirements (i.e. RAM and CPU) for each mon instance ?
We have no benchmarks on that, that I am aware of. But the short and sweet answer should be "not really, highly unlikely".
If anything, increasing the number of mons should increase the response time, although for such low numbers that should also be virtually negligible.
Think of the monitors as the guys behind the counter at a the tourist desk giving maps for your city and providing information to people, and if you assume that for each one of these guys there's a single line. Having more of these guys helps with larger numbers of tourists.
But now imagine that your city is constantly under development, and for some reason those maps need to be updated rather frequently. Each time there's a new change, a courier delivers the latest change to the city map in a post-it to the tourist office. He gets in line with all the other people, and delivers said post-it to one of the guys behind the counter. Now this guy will have to give that post-it to his supervisor (it might even be himself), and the supervisor will make the appropriate change in the city map and give a map for review to every guy behind the counter, which in turn will check the change and tell the supervisor whether they accept said map or not -- when a majority accepts the map, then the supervisor will issue an order to start providing this new version of the map to whomever is in the queue.
So you can see how adding more guys behind the counter can easily increase the complexity of getting the maps ready for public consumption, as there are a few more hops in the process. Fortunately for us, in Ceph this is accomplished faster than in any tourist office I've ever been to.
I guess it is just a matter of how many clients (including OSDs, MDSs) you have in the cluster. More monitors will sort of load balance reads across monitors; but updates are centralized and overseen by a single monitor.
In any case, you should be safe running 3 or 5 monitors without any noticeable decrease in performance. 7 may also be just fine. More than that I have no idea, but you should feel free to test this out in your system and share results; I'm sure all of us would appreciate :)
-Joao
Another question: when a client write something to the cluster, the write ack is sent back to the client as soon as one OSD has wrote data to the journal, or after one replica is made? For example, could I use a 10GbE network for the public side and a bonded 1GbE for cluster side without performance bottleneck ? In this case , only replication will be made slower with no negative performance on clients (RGW and so on) I'll host some VM disk images so I need the maximum speed client-size and replication could also be slower (our VM has an ephemeral storage) _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
-- Joao Eduardo Luis Software Engineer | http://inktank.com | http://ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
