[EMAIL PROTECTED] (Jeff Davis) writes: > On Wed, 2006-08-23 at 13:36 +0200, Markus Schiltknecht wrote: >> Hannu Krosing wrote: >> > But if you have very few writes, then there seems no reason to do sync >> > anyway. >> >> I think there is one: high-availability. A standby-server which can >> continue if your primary fails. Of course sync is only needed if you >> absolutely cannot effort loosing any committed transaction. >> > > I disagree about high-availability. In fact, I would say that sync > replication is trading availability and performance for synchronization > (which is a valid tradeoff, but costly). > > If you have an async system, all nodes must go down for the system to go > down. > > If you have a sync system, if any node goes down the system goes down. > If you plan on doing failover, consider this: what if it's not obvious > which system is still up? What if the network route between the two > systems goes down (or just becomes too slow to replicate over), but > clients can still connect to both servers? Then you have two systems > that both think that the other system went down, and both start > accepting transactions. Now you no longer have replication at all.
That is why for multimaster, there's a need for both automatic policy as well as some human intervention. - You need an automatic determination of "quorum", where, to be safe, it is only permissible for a set of $m$ servers to believe themselves to be active if they number more than 1/2 of the total of expected servers. Thus, if there are 13 servers in the cluster, then "quorum" is 7 servers. If a set of 6 servers get cut off from the rest of the network, they don't number at least 7, and thus know that they can't represent a quorum. - And if conditions change, a human may need to change the quorum number. If 4 new nodes get added, quorum moves up to 9. If 5 nodes get dropped, quorum moves down to 5. Deciding when to throw a node out of the quorum because it is responding too slowly is still not completely trivial, but having a quorum policy does address your issue. -- let name="cbbrowne" and tld="cbbrowne.com" in name ^ "@" ^ tld;; http://cbbrowne.com/info/linux.html "Be humble. A lot happened before you were born." - Life's Little Instruction Book ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
