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  1. Percona XtraDB Cluster
  2. PXC-3118

BF abort for MDL conflict stuck when Thread Pool used

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    Details

    • Type: Bug
    • Status: On Hold
    • Priority: High
    • Resolution: Unresolved
    • Affects Version/s: 5.7.28-31.41
    • Fix Version/s: None
    • Component/s: None
    • Labels:
      None

      Description

      A whole cluster gets effectively blocked for writes when a DDL query fails to successfully trigger brute force abort. And this is happening when

      thread_handling = pool-of-threads
      

      is used.
      So, as a result, we can observe long waiting for MDL lock, which should never happen in PXC/Galera due to priority nature of DDL handling, like:

                 Id: 9
               User: msandbox
               Host: localhost
                 db: test
            Command: Query
               Time: 332
              State: Waiting for table metadata lock
               Info: truncate table t1
      

      An example debug level log from affected node in attachment.

      How to reproduce

      Use PXC node member with

      thread_handling = pool-of-threads
      

      .
      In session one start a transaction on a simple table:

      node1 [localhost:26529] {msandbox} (test) > begin; select * from t1;
      Query OK, 0 rows affected (0.00 sec)
      
      Empty set (0.00 sec)
      

      In session 2 on the same node, try a DDL on the same table, like:

      node1 [localhost:26529] {msandbox} (test) > truncate table t1;
      

      Confirmed on PXC 5.7.26, .27 and .28.

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              Assignee:
              Unassigned
              Reporter:
              przemyslaw.malkowski@percona.com Przemyslaw Malkowski
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                Dates

                Created:
                Updated:

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