1

I'm currently testing replication in our development environment.

Our current (primary) mongo instance has around 70-80 GB of data. I created a new instance and added it to the replica set.

It takes around 4 hours to sync the collections at which point it starts playing back the oplog to catch up with writes on the primary. But doing rs.printSlaveReplicationInfo() shows that the secondary keeps lagging more and more behind the master, and eventually fails with the following error:

OplogStartMissing: error fetching oplog during initial sync :: caused by :: Our last optime fetched: { ts: Timestamp(1581379449, 5879), t: 4 }. source's GTE: { ts: Timestamp(1581379449, 20317), t: 4 }

At which point the initial sync starts over again. Both the master and slave are using version 4.2

Improve this question

1 Answer 1

Reset to default
1

How big is your oplog?

Could it be that the oplog is filling up and overwriting the operations since the sync started before the initial sync has finished? If this is the case, the new secondary will never catch up. I had this error once before, but can’t recall the exact error now.

Check the oplog start / end points by using 

rs.printReplicationInfo()

If this is the case, either increase the oplog size or pause the application (if possible).

I’ll dig through an old ticket and check the error I had with a sync failing.

Improve this answer
2
  • This was the issue, the oplog was too small to hold all the operations that happened during the initial collection copying, increasing the size resolved the issue
    – Thiatt
    Feb 16, 2020 at 16:34
  • In my case I had a second problem beyond oplog size, which was heavy write load on primary.
    – rtrigo
    Aug 10, 2020 at 9:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.