Slow MongoDB start time when data is replicated to a new machine

Question

We have a 3.2.13 Mongodb replica set with WiredTiger on AWS.

We are observing a strange behaviour: when stopping and starting the mongod service, the start time is about 10 minutes, but when stopping the mongod service, then replicating the data disk to a new AWS instance, and restarting on that machine, the start time is about 2 hours.

What may be the cause for this?

Answer 1

It looks like the initial slowness is because the EBS volume (which is restored from snapshot) is being initialized during the service startup.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-initialize.html