> I'm a bit lost: I tried a repair yesterday with only one CF and that didn't > really work the way I expected but I thought that would be a bug which only > affects that special case. > > So I tried again for all CFs. > > I started with a nicely compacted machine with around 320GB of load. Total > disc space on this node was 1.1TB.
Did you do repairs simultaneously on all nodes? I have seen very significant disk space increases under some circumstances. While I haven't filed a ticket about it because there was never time to confirm, I believe two things were at play: (1) nodes were sufficiently out a sync in a sufficiently spread out fashion that the granularity of the merkle tree (IIRC, and if I read correctly, it divides the ring into up to 2^15 segments but no more) became ineffective so that repair effectively had to transfer all the data. at first I thought there was an outright bug, but after looking at the code I suspected it was just the merkle tree granularity. (2) I suspected at the time that a contributing factor was also that as one repair might cause a node to significantly increase it's live sstables temporarily until they are compacted, another repair on another node may start and start validating compaction and streaming of that data - leading to disk space bload essentially being "contagious"; the third node streaming from the node that was temporarily bloated, will receive even more data from that node than it normally would. We're making sure to only run one repair at a time between any hosts that are neighbors of each other (meaning that at RF=3, that's 1 concurrent repair per 6 nodes in the cluster). I'd be interested in hearing anyone confirm or deny whether my understanding of (1) in particular is correct. To connect it to reality: a 20 GB CF divided into 2^15 segments implies each segment is > 600 kbyte in size. For CF:s with tens or hundreds of millions of small rows and a fairly random (with respect to partitioner) update pattern, it's not very difficult to end up in a situation where most 600 kbyte chunks contain out-of-synch data. Particularly in a situation with lots of dropped messages. I'm getting the 2^15 from AntiEntropyService.Validator.Validator() which passes a maxsize of 2^15 to the MerkelTree constructor. -- / Peter Schuller
