On Thu, Nov 27, 2008 at 08:08:32 -0800, Bill Moseley wrote: > Ketama: YES > Hit: 34248. Miss: 5752. Total: 40000 (connecting with 5 servers) > Hit: 29382. Miss: 10618. Total: 40000 (replace) > > In this case is looks like Ketama helps if you simply drop the server, > but if that server is the server is replaced it's worse.
If by "replace" you mean "add some other server, not necessary with the same address as the deleted one", then everything is clear: deleting one of six servers results in loss of 1/6 of keyspace. Adding another _brand new_ server results in remapping 1/6 of the remaining keyspace to it. Thus the hit rate is 4000 * 5/6 * 5/6 = 27777 ~= 29328. Only if you add the server with the same address as the deleted one (i.e. will bring it back) will you get the same hit rate as after deletion (4000 * 5/6 = 33333 ~= 34248). Ketama uses server address to compute server to keyspace mapping. -- Tomash Brechko
