> Caution, using the method you described, the amount of data streamed at the > end with the full repair is not the amount of data written between stopping > the first node and the last node, but depends on the table size, the number > of partitions written, their distribution in the ring and the > 'repair_session_space' value. If the table is large, the writes touch a large > number of partitions scattered across the token ring, and the value of > 'repair_session_space' is small, you may end up with a very expensive > over-streaming.
Thanks for the warning. In our case it worked well (obviously we tested it on a test cluster before applying it on the production clusters), but it is good to know that this might not always be the case. Maybe I misunderstand how full and incremental repairs work in C* 4.x. I would appreciate if you could clarify this for me. So far, I assumed that a full repair on a cluster that is also using incremental repair pretty much works like on a cluster that is not using incremental repair at all, the only difference being that the set of repaired und unrepaired data is repaired separately, so the Merkle trees that are calculated for repaired and unrepaired data are completely separate. I also assumed that incremental repair only looks at unrepaired data, which is why it is so fast. Is either of these two assumptions wrong? If not, I do not quite understand how a lot of overstreaming might happen, as long as (I forgot to mention this step in my original e-mail) I run an incremental repair directly after restarting the nodes and marking all data as repaired. I understand that significant overstreaming might happen during this first repair (in the worst case streaming all the unrepaired data that a node stores), but due to the short amount of time between starting to mark data as repaired and running the incremental repair, the whole set of unrepaired data should be rather small, so this overstreaming should not cause any issues. From this point on, the unrepaired data on the different nodes should be in sync and discrepancies in the repaired data during the full repair should not be bigger than they had been if I had run a full repair without marking an data as repaired. I would really appreciate if you could point out the hole in this reasoning. Maybe I have a fundamentally wrong understanding of the repair process, and if I do I would like to correct this.
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