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  1. WiredTiger
  2. WT-7185

Avoid aborting a transaction if it is force evicting and oldest

    • Type: Icon: Improvement Improvement
    • Resolution: Fixed
    • Priority: Icon: Major - P3 Major - P3
    • WT10.0.1, 4.4.7, 5.0.0-rc0
    • Affects Version/s: None
    • Component/s: None
    • Labels:
    • v4.4

      Given a large enough page is created a by a transaction running in WiredTiger, it may get force evicted causing a rollback to be returned to the transaction, the transaction also needs to be the oldest transaction in the system and meet the criteria to be "blocking", see: __wt_txn_is_blocking. However this behaviour is also impacting MongoDB transactions.

      The original change WT-6444 was intended to rollback batch operations and have the mongodb layer split them into smaller chunks, as a consequence of this change we are rolling back more than we intended.

      There are two possible fixes here:

      1. Improve the heuristic used to determine whether we should roll back a transaction.
      2. Have MongoDB supply a config to WiredTiger to identify that a MongoDB transaction is running, and skip the rollback logic.

      I've created a python reproducer for this scenario:

      import time, wiredtiger, wttest
      from wtscenario import make_scenarios
      def timestamp_str(t):
          return '%x' % t
      class test_repro01(wttest.WiredTigerTestCase):
          conn_config = 'cache_size=10000MB,eviction=(threads_max=1)'
          session_config = 'isolation=snapshot'
          def test_repro01(self):
              uri = 'table:test_repro01'
              self.session.create(uri, 'key_format=i,value_format=S')
              self.conn.set_timestamp('oldest_timestamp=' + timestamp_str(1))
              cursor = self.session.open_cursor(uri)
              doc_count = 30000
              value1 = 'x' * 1000
              for j in range(0, 9):
                  for i in range(j*doc_count, (j+1)*doc_count):
                      cursor[i] = value1
                  self.session.commit_transaction('commit_timestamp=' + timestamp_str(2))



            luke.chen@mongodb.com Luke Chen
            luke.pearson@mongodb.com Luke Pearson
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