Truncating MongoDB oplog before EBS snapshot

I currently have a MongoDB (on AWS) backup strategy involving automating daily snapshots of the data/journal volume on one node. The process is:

1. fsyncLock()
2. trigger AWS snapshot on volume (and wait for success)
3. fsyncUnlock()

When replacing a RS member, I launch a new instance from the snapshot and add it to the replica set. The sole issue I have is on mongo process launch there is a very long delay (~20-30 minutes for ~500GB of data) before the new node jumps into SECONDARY and replays from another member's oplog. All I see is something like this in the mongodb.log file:

[initandlisten] Starting WiredTigerRecordStoreThread local.oplog.rs [initandlisten] The size storer reports that the oplog contains 659681 records totaling to 4196461664 bytes [initandlisten] Sampling from the oplog between Dec 20 05:29:14:2 and Jan 18 06:00:00:8 to determine where to place markers for truncation [initandlisten] Taking 335 samples and assuming that each section of oplog contains approximately 19672 records totaling to 125140475 bytes 

1. What is Mongo doing here exactly
2. Is the oplog useful to MongoDB on launch (from snapshot) other than determining when its last optime was (I have journaling enabled)?
3. Can I remedy this by just truncating everything from the oplog except the most recent entry before taking a snapshot?

0 Answers

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