[jira] [Created] (SOLR-10265) Overseer can become the bottleneck in very large clusters

["Varun Thacker (JIRA)" <ji...@apache.org>]

Varun Thacker created SOLR-10265:
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             Summary: Overseer can become the bottleneck in very large clusters
                 Key: SOLR-10265
                 URL: https://issues.apache.org/jira/browse/SOLR-10265
             Project: Solr
          Issue Type: Improvement
      Security Level: Public (Default Security Level. Issues are Public)
            Reporter: Varun Thacker


Let's say we have a large cluster. Some numbers:
- To ingest the data at the volume we want to I need roughly a 600 shard collection.
- Index into the collection for 1 hour and then create a new collection 
- For a 30 days retention window with these numbers we would end up wth  ~400k cores in the cluster
- Just a rolling restart of this cluster can take hours because the overseer queue gets backed up. If a few nodes looses connectivity to ZooKeeper then also we can end up with lots of messages in the Overseer queue

With some tests here are the two high level problems we have identified:

1> How fast can the overseer process operations:

The rate at which the overseer processes events is too slow at this scale. 

I ran {{OverseerTest#testPerformance}} which creates 10 collections ( 1 shard 1 replica ) and generates 20k state change events. The test took 119 seconds to run on my machine which means ~170 events a second. Let's say a server can process 5x of my machine so 1k events a second. 

Total events generated by a 400k replica cluster = 400k * 4 ( state changes till replica become active ) = 1.6M / 1k events a second will be 1600 minutes.

Second observation was that the rate at which the overseer can process events slows down when the number of items in the queue gets larger

I ran the same {{OverseerTest#testPerformance}} but changed the number of events generated to 2000 instead. The test took only 5 seconds to run. So it was a lot faster than the test run which generated 20k events


2> State changes overwhelming ZK:

For every state change Solr is writing out a big state.json to zookeeper. This can lead to the zookeeper transaction logs going out of control even with auto purging etc set . 

I haven't debugged why the transaction logs ran into terabytes without taking into snapshots but this was my assumption based on the other problems we observed



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