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  1. Core Server
  2. SERVER-31767

Provide a window of snapshot history that is accessible for PIT reads

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    Details

    • Type: Task
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 4.1.1
    • Component/s: Storage
    • Labels:
    • Backwards Compatibility:
      Fully Compatible
    • Sprint:
      Storage NYC 2018-04-09, Storage NYC 2018-04-23, Storage NYC 2018-05-07, Storage NYC 2018-05-21, Storage NYC 2018-06-04

      Description

      In 3.6, we permit reads back to the majority commit point (approximately: a little additional slack is currently built in). Attempts to read at an older point will fail.

      For Global Point in Time Reads, a mongos will need to establish a common timestamp across all mongod nodes involved in a query. In order to do that, it would be helpful for each node to store some additional history.

      WiredTiger could monitor when it is under cache pressure from storing history, and update the oldest_timestamp in response, with the goal of allowing reads older than the majority commit point where possible without impacting performance.

      We should also consider whether to make the "comfort level" configurable, to maintain some history even on memory constrained nodes to increase the chance of finding a common point in time across a cluster.

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