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  1. Couchbase Server
  2. MB-41804

Ephemeral eviction can deadlock nodes



    • Untriaged
    • 1
    • Unknown
    • KV Sprint 2020-Oct


      Ephemeral buckets set to auto_delete with replicas configured can reach deadlock due to eviction.

      Ephemeral buckets set to auto_delete can only evict items from active vbuckets, as replica vbuckets must remain consistent with their actives.

      However, eviction is not coordinated across nodes.
      Considering the simple case of a three node cluster (nodes A, B, and C) with one or more replicas configured.
      When node A reaches the high watermark, it responds by deleting items from its active vbuckets. Once these deletions are replicated, this will lower the memory usage of B and C. As a result, B and C may now be further away from hitting the high watermark, and need not delete active items yet.

      As B and C receive more ops, their active vbuckets grow, as do the replica vbuckets on A. Now, A has a smaller fraction of its quota available for active items.
      This can occur repeatedly, driving the number of active items on A lower and lower.

      As the replicas on A continue to grow, replica memory usage can eventually exceed the high watermark. At this point, the eviction pager will run constantly and all active items on A will be immediately evicted.

      While this is a very poor situation, node A could eventually recover if the active vb memory usage on B and C are reduced by eviction, expiry, or deletions.

      Unfortunately node A can become deadlocked if the replica memory usage exceeds 99% of the quota. At this point, node A will already have evicted all active items, and will now back off on incoming replication. This will not recover without intervention, as even when B and C do evict A will not stream these deletions as replication is stalled waiting for memory usage to drop.

      Ephemeral buckets which typically delete or expire items at a sufficient rate to avoid reaching the high watermark will be unaffected by this issue. The full deadlock also requires 3 or more nodes as replica memory can not reach 99% of a node's quota with just two nodes; that would require the active memory on the other node to reach 99%, which eviction should prevent.

      A greater number of nodes, or a greater number of replicas may make this scenario more likely.

      Issue is easily reproduced with pillowfight and cluster_run.

      Issue presents as item count dropping on a single node

      and eventual temp ooms from that node


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              owend Daniel Owen
              james.harrison James Harrison (Inactive)
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