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  1. Core Server
  2. SERVER-48398

Writing config document to "local.system.replset" should not acquire PBWM lock.

    • Type: Icon: Improvement Improvement
    • Resolution: Unresolved
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: None
    • Component/s: Replication
    • Replication
    • 144

      It seems when the node persists new config in collection "local.system.replset", it takes "local" database lock in stronger mode (X) and PBWM lock in IS mode. This can lead to 2 major side effects.
      1) Since PBWM lock is taken in IS mode, this can block the secondary oplog applier which requires PBWM in X mode. This can result in replication lag. 
      2) Since it takes "local" database lock in X mode, this can block other local database readers and writers. This will be addressed by SERVER-48399

      • Mainly, if this node X is the sync source for node Y, then the oplog fetcher of the node Y can be blocked behind the the reconfig via heartbeat thread due to database lock conflict, leading to replication lag.

      Generally, Upserts on non-replicated collection doesn't need to acquire pbwm lock. Basically, storeLocalConfigDocument  should acquire DB lock under ShouldNotConflictWithSecondaryBatchApplicationBlock.

            backlog-server-repl Backlog - Replication Team
            suganthi.mani@mongodb.com Suganthi Mani
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