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From Stefan Richter <s.rich...@data-artisans.com>
Subject Re: Tuning RocksDB
Date Wed, 03 May 2017 15:54:50 GMT

typically, I would expect that the bottleneck with the RocksDB backend is not RocksDB itself,
but your TypeSerializers. I suggest to first run a profiler/sampling attached to the process
and check if the problematic methods are in serialization or the actual accesses to RocksDB.
The RocksDB backend has to go through de/serialize roundtrips on every single state access,
while the FSBackend works on heap objects immediately. For checkpoints, the RocksDB backend
can write bytes directly whereas the FSBackend has to use the serializers to get from objects
to bytes, so their actions w.r.t. how serializers are used are kind of inverted between operation
and checkpointing. For Flink 1.3 we also will introduce incremental checkpoints on RocksDB
that piggyback on the SST files. Flink 1.2 is writing checkpoints and savepoints fully and
in a custom format.


> Am 03.05.2017 um 16:46 schrieb Jason Brelloch <jb.bc.flk@gmail.com>:
> Hey all,
> I am looking for some advice on tuning rocksDB for better performance in Flink 1.2. 
I created a pretty simple job with a single kafka source and one flatmap function that just
stores 50000 events in a single key of managed keyed state and then drops everything else,
to test checkpoint performance.  Using a basic FsStateBackend configured as:
> val backend = new FsStateBackend("file:///home/jason/flink/checkpoint")
> env.setStateBackend(backend)
> With about 30MB of state we see the checkpoints completing in 151ms.  Using a RocksDBStateBackend
configured as:
> val backend = new RocksDBStateBackend("file:///home/jason/flink/checkpoint")
> backend.setDbStoragePath("file:///home/jason/flink/rocksdb")
> backend.setPredefinedOptions(PredefinedOptions.FLASH_SSD_OPTIMIZED)
> env.setStateBackend(backend)
> Running the same test the checkpoint takes 3 minutes 42 seconds.
> I expect it to be slower, but that seems excessive.  I am also a little confused as to
when rocksDB and flink decide to write to disk, because watching the database the .sst file
wasn't created until significantly after the checkpoint was completed, and the state had not
changed.  Is there anything I can do to increase the speed of the checkpoints, or anywhere
I can look to debug the issue?  (Nothing seems out of the ordinary in the flink logs or rocksDB
> Thanks!
> -- 
> Jason Brelloch | Product Developer
> 3405 Piedmont Rd. NE, Suite 325, Atlanta, GA 30305 
>  <http://www.bettercloud.com/>
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