In one scenario I observed very long bootstrap times of tablet servers (something between to 45 minutes and 60 minutes) even if tablet servers had relatively small amount of data under management (~80GByte). It turned out the time was spent on replaying WAL segments, with kudu cluster ksck reporting something like below all the time during bootstrap:
The workload I ran before shutting down the tablet servers consisted of many small UPSERT operations, but the cluster was idle after terminating the workload for long time (about few hours or so). The workload was generated by
The table that the UPSERT workload was running against had been pre-populated by the following:
As it turned out, tablet servers accumulated huge number of DMS which required flushing/compaction, but after the memory pressure subsided, the compaction policy was scheduling just one operation per tablet in every 120 seconds (the latter interval is controlled by --flush_threshold_secs). In fact, tablet servers could flush those rowsets non-stop since the maintenance threads were completely idle otherwise and there were no active workload running against the cluster. Those DMS has been around for long time (much more than 120 seconds) and were anchoring a lot of WAL segments. So, the operations from the WAL had to be replayed once I restarted the tablet servers.
It would be great to update the flushing/compaction policy to allow tablet servers run FlushDeltaMemStoresOp as soon as a DMS becomes older than specified by --flush_threshold_secs when the maintenance threads are not busy otherwise.