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  1. Documentation
  2. DOCS-6485

Comment on: "manual/core/write-concern.txt"

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      Is this article correct if you're using WiredTiger?

      From what I've read about WiredTiger, there isn't a concept of a journal which is flushed to disk like a memory mapped file would be.

      See - http://www.slideshare.net/JonRangel/let-the-tiger-roar-mongodb-30-wiredtiger

      On slide 17 - It states "consistency without journalling":
      On slide 20 - It states "no journal commit interval: writes are written to the journal as they come in"

      So, in this case, is Acknowledged write concern the same as Journaled under WiredTiger?

      Also, in MMAPv1, when writes are Acknowledged, they sit in RAM before flushing to the journal, but in WiredTiger, if writes really are journaled "as they come in" according to the presentation, then the information on https://docs.mongodb.org/manual/faq/concurrency/ could be wrong.

      In particular, the section of "Can reads see changes that have not been committed to disk?" states that dirty reads (read uncommitted) are possible, but the presentation here seems to suggest that it's not possible under WiredTiger.

      See slide 13 - where it says "In-memory consistency" and slide 17 where it states that "Readers see the version that was committed before the operation started".

      http://www.slideshare.net/wiredtiger/mongo-db-worldwiredtiger?related=3

      Some clarity on the behaviour here is essential for me.

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              1 year, 14 weeks, 1 day ago