On Thu, Feb 28, 2013 at 11:39 PM, Hiller, Dean <dean.hil...@nrel.gov> wrote:
> Isn't it true if I have 6 nodes, I could run nodetool repair on just 2 > nodes(RF=3) instead of using nodetool repair –pr??? > Yes, it is true. And to precise further, in your case you have 2 options: 1) doing repair *without* -pr on 2 nodes (assuming you pick the correct 2 nodes, it's *not* any 2 nodes) 2) doing a repair *with* -pr on the 6 nodes Both of those cases would 1) repair the full ring and 2) do the same amount of work. > What is the advantage of –pr then? As it happens, your case is a special case. You have a number of node that is a multiple of your replication factor. Now if that wasn't the case (say 5, 7 or 8 nodes with RF=3), then there is *no way* you can repair *without* -pr the whole cluster without doing *more* work than by doing a repair *with* -pr on all nodes. So the advantages of --pr (which btw, should be use for repair the whole cluster, not when you want to rebuild a specific node) are: 1) it always do the minimum of work, while repair without --pr is wasteful if the number of nodes is not a multiple of the replication factor (no matter how smart you are at scheduling the repairs). 2) even if your number of nodes is a multiple of the replication factor, you still have to make sure you pick the right N/RF nodes to repair if you don't use -pr. If you don't pick the correct ones, you will not repair the full ring. Using -pr is much more shoot-footing free: you have to run it on every node, period. -- Sylvain
