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Benedict commented on CASSANDRA-6696: ------------------------------------- The problem here is packing vnodes fairly across the disks: either we need to ensure that all vnodes are of roughly equal size (very difficult), or we probably need to have a dynamic allocation strategy, and the problem with _that_ is that when the token range gets redistributed by node additions/removals, the whole cluster suddenly needs to start kicking off rebalancing of their local disks. We could support splitting the token range into M distinct chunks, where M is preferably some multiple of the number of disks, and split the total token range into M chunks, then allocate each chunk to a disk in round-robin fashion. This then remains deterministic, and it is I think easier to guarantee an even distribution within a given token range than it is to guarantee all vnodes are of equal size, whilst still supporting a dynamic cluster size. Even here, though, realistically I think we need the number of chunks to be quite a bit smaller than the number of vnodes to guarantee anything approaching balance of these chunks. > Drive replacement in JBOD can cause data to reappear. > ------------------------------------------------------ > > Key: CASSANDRA-6696 > URL: https://issues.apache.org/jira/browse/CASSANDRA-6696 > Project: Cassandra > Issue Type: Improvement > Components: Core > Reporter: sankalp kohli > Assignee: Marcus Eriksson > Fix For: 3.0 > > > In JBOD, when someone gets a bad drive, the bad drive is replaced with a new > empty one and repair is run. > This can cause deleted data to come back in some cases. Also this is true for > corrupt stables in which we delete the corrupt stable and run repair. > Here is an example: > Say we have 3 nodes A,B and C and RF=3 and GC grace=10days. > row=sankalp col=sankalp is written 20 days back and successfully went to all > three nodes. > Then a delete/tombstone was written successfully for the same row column 15 > days back. > Since this tombstone is more than gc grace, it got compacted in Nodes A and B > since it got compacted with the actual data. So there is no trace of this row > column in node A and B. > Now in node C, say the original data is in drive1 and tombstone is in drive2. > Compaction has not yet reclaimed the data and tombstone. > Drive2 becomes corrupt and was replaced with new empty drive. > Due to the replacement, the tombstone in now gone and row=sankalp col=sankalp > has come back to life. > Now after replacing the drive we run repair. This data will be propagated to > all nodes. > Note: This is still a problem even if we run repair every gc grace. > -- This message was sent by Atlassian JIRA (v6.2#6252)