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  1. Samza
  2. SAMZA-957

Avoid unnecessary KV Store flushes (part 3)



    • Bug
    • Status: Resolved
    • Major
    • Resolution: Fixed
    • None
    • 0.10.1
    • None
    • None


      We had an issue where RocksDB performance severely degraded for 23 hours and then resolved itself. To troubleshoot the issue I gathered some samples of the compaction stats from the RocksDB log and engaged with the RocksDB team via an existing, related issue: https://github.com/facebook/rocksdb/issues/696#issuecomment-222549220

      They pointed out that the job was flushing excessively:

      If you overload RocksDB with work (i.e. do bunch of writes really fast, or in your case, bunch of small flushes), it will begin stalling writes while the compactions (deferred work) completes. An interesting thing with RocksDB and LSM architecture is that the more behind you are on compactions, the more expensive the compactions are (due to increased write amplifications and single-threadness of L0->L1 compaction). So our write stalls have to be tuned exactly right for RocksDB to behave well with extremely high write rate.

      Looking through our commit history I see that SAMZA-812 and SAMZA-873 have both intended to address this issue, by reducing the amount of flushes in CachedStore.

      To be fair, the job in question did not have the SAMZA-873 patch, but I see even more room for improvement. Namely, CachedStore should never flush the underlying store unless its flush() was called. It can purge its dirty items to trade off performance for correctness, but flushing is excessive. So, this patch will remove the flushes from the all() and range() methods, simplify the LRU logic, and add a good unit test to verify and explain the proper LRU behavior.


        1. SAMZA-957_1.patch
          6 kB
          Jake Maes



            jmakes Jake Maes
            jmakes Jake Maes
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