On Mon, 14 Sep 2015 13:57:26 -0700 Gregory Farnum <gfarnum@xxxxxxxxxx> wrote: > The OSD is supposed to stay down if any of the networks are missing. > Ceph is a CP system in CAP parlance; there's no such thing as a CA > system. ;) I know I am being fussy, but within my team your email was sited that you cannot consider ceph as a CA system. Hence I make my argument in public so I can be humbled in public. Just to clarify your opinion I site http://www.infoq.com/articles/cap-twelve-years-later-how-the-rules-have-changed suggests: <quote> The CAP theorem states that any networked shared-data system can have at most two of three desirable properties. * consistency (C) equivalent to having a single up-to-date copy of the data; * high availability (A) of that data (for updates) * tolerance to network partitions (P). </quote> So I dispute that a CA system cannot exist. I think you are too absolute even in interpretation of this vague theory. A further quote from the author of said theorem from the same article: <quote> The "2 of 3" formulation was always misleading because it tended to oversimplify the tensions among properties. </quote> As I understand it: Ceph as a cluster always provides "Consistency". (or else you found a bug) If a ceph cluster is operating it will always provide acknowledgment (it may block) to the client if the operation has succeeded or failed hence provides "availability". if a ceph cluster is partitioned, only one partition will continue operation, hence you cannot consider the system "partition" tolerant as multiple parts of the system cannot operate when partitioned. Hence as a cluster ceph is CA. Alternatively if you look at it from an OSD rather than cluster perspective, you can get the perspective you take, OSD's are CP system in CAP parlance. I would argue it is all a matter of perspective, and believe that to call Brewer's theorem anything other than guidance without strong discussion of your understanding of consistency and its interaction with partitioning is to confuse and over simplify. Best regards Owen -- 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
