On Tue, Mar 22, 2011 at 7:19 AM, Sasha Dolgy <sdo...@gmail.com> wrote: > More, I suppose the question I'm after is, can the snitch method be > adjusted adhoc (with node restart) or once it's changed from > SimpleSnitch to Ec2Snitch that's it?
You can change Snitches on a cluster with data on it, as long as you are very careful about what you are doing and you are in a particular case which you are probably not in if you want to change your Snitch. The snitch meaningfully determines replica placement strategy, and in general when changing snitches you need the replica placement strategy to stay exactly the same. Unfortunately the point of changing a snitch is usually.. changing your replica placement strategy. Simplest case is if the replica placement strategy actually stays the same, like for example when Digg replaced its custom version of the PropertyFileSnitch with SimpleSnitch in prep for going single-DC, because we weren't actually using the functionality of PFS. In that case, I simply generated a set of input which hashed correctly such that I had one piece of input per node. I then verified the topology based on this input before and after changing my snitch, and got the same results both times, confirming that my change of the Snitch was a no-op. A less simple, but still tractable case is if the topology changes such that one or more replicas is different but at least one is still the same. In this case, repair would be likely to repair.. most.. of your data. But honestly if you have to change strategy that much (and are not running IP-partitioned counts, which make this operation much more difficult) you probably just want to dump and reload your data into a new cluster which has the topology and snitch you want. =Rob
