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  1. Core Server
  2. SERVER-51108

Performance impact when enabling FCV 4.0

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    • Icon: Question Question
    • Resolution: Duplicate
    • Icon: Minor - P4 Minor - P4
    • None
    • 4.0.15
    • None
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    Description

      Hello,

      We have a quite large cluster (over 20 shards / multiple databases and collections / config database having over 1M documents in chunk collection) . We've observed impact during setting FCV to 4.0 on such a cluster while the whole cluster "freezes" when cache chunks are refreshed on each shard.

      ============

      My understanding of the setting of FCV in case of 4.0 is:

      1. FCV is triggered on mongos : https://github.com/mongodb/mongo/blob/91e3352a1aa717674575fce3cc6edb2f279a4479/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp
      2. mongos sends the command to config servers: https://github.com/mongodb/mongo/blob/91e3352a1aa717674575fce3cc6edb2f279a4479/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp#L94
      3. config replicaset executes https://github.com/mongodb/mongo/blob/v4.0/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
      4. it does that through the block dedicated to config server: https://github.com/mongodb/mongo/blob/v4.0/src/mongo/db/commands/set_feature_compatibility_version_command.cpp#L184
      5. Within this block config will update to new FCV and trigger upgradeChunksHistory for each collection.
      6. It will trigger the upgrade of FCV on each shard: https://github.com/mongodb/mongo/blob/v4.0/src/mongo/db/commands/set_feature_compatibility_version_command.cpp#L226.
      7. setting FCV on config replicaset.
        ===============================

      Upgrade was performed from 3.6.10 to 4.0.15.

      Is there any way to mitigate the impact of enabling FCV on large cluster with lots of chunks?

      Thank you,

      Cristian

       

       

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            dmitry.agranat@mongodb.com Dmitry Agranat
            radan@adobe.com Cristian Radan
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