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  1. Core Server
  2. SERVER-31679

Increase in disk i/o for writes to replica set

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: 3.6.0-rc0
    • Fix Version/s: 3.6.6, 3.7.3
    • Component/s: Replication, Storage
    • Labels:
    • Backwards Compatibility:
      Fully Compatible
    • Operating System:
      ALL
    • Backport Requested:
      v3.6
    • Sprint:
      Storage 2018-02-12, Storage NYC 2018-03-12
    • Case:

      Description

      Simple insert loop running against 1-node replica set

          db.c.insert({_id: 0, i: 0})
          for (i = 0; i < 100000; i++)
              db.c.update({_id: 0}, {$inc: {i: 1}})
      

      Shows a large increase in journal syncs, disk write requests, bytes written. 3.4.9 left, 3.6.0-rc0 right:

      It appears we're doing a flush on every update, even though there's no write concern. In the simple repro above this doesn't seem to affect performance (for reasons tbd) but with more complex workloads the additional journal i/o will impact performance.

      Does not reproduce with standalone mongod.

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              Assignee:
              geert.bosch Geert Bosch
              Reporter:
              bruce.lucas Bruce Lucas
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                Updated:
                Resolved: