Read repair will only repair data that is read on the nodes that are up at that time, and does not guarantee that any changes it detects will be written back to the nodes. The diff mutations are async fire and forget messages which may go missing or be dropped or ignored by the recipient just like any other message.
Also getting hit with a bunch of read repair operations is pretty painful. The normal read runs, the coordinator detects the digest mis-match, the read runs again from all nodes and they all have to return their full data (no digests this time), the coordinator detects the diffs, mutations are sent back to each node that needs them. All this happens sync to the read request when the CL > ONE. Thats 2 reads with more network IO and up to RF mutations . The delete thing is important but repair also reduces the chance of reads getting hit with RR and gives me confidence when it's necessary to nuke a bad node. Your plan may work but it feels risky to me. You may end up with worse read performance and unpleasent emotions if you ever have to nuke a node. Others may disagree. Not ignoring the fact the repair can take a long time, fail, hurt performance etc. There are plans to improve it though. Cheers ----------------- Aaron Morton Freelance Cassandra Developer @aaronmorton http://www.thelastpickle.com On 22 Jul 2011, at 19:55, jonathan.co...@gmail.com wrote: > One of the main reasons for regularly running repair is to make sure deletes > are propagated in the cluster, i.e., data is not resurrected if a node never > received the delete call. > > And repair-on-read takes care of repairing inconsistencies "on-the-fly". > > So if I were to set a universal TTL on all columns - so everything would only > live for a certain age, would I be able to get away without having to do > regular repairs with nodetool? > > I realize this scenario would not be applicable for everyone, but our data > model would allow us to do this. > > So could this be an alternative to running the (resource-intensive, > long-running) repairs with nodetool? > > Thanks.
