Ops Mgr "Insufficient oplog size" is confusing and prevents backups
When using Ops Manager UI (I've not checked the API) to declare a MongoDB cluster to be backed up, Ops Manager tries to be a good citizen and check to see if the clusters oplogs are large enough, based on their recent usage to hold at least 3 hours worth of data based on the last 24 hours of usage patterns. if the check fails, the user is prevented from enabling backup and is shown the warning:
"Insufficient oplog size: The oplog window must be at least 3 hours over the last 24 hours for all members of replica set rsa. Please increase the oplog."
There are many use cases where using the last 24 hours of oplog use is not a good indication of future use. For example, a user may deploy a new MongoDB cluster to production, run a job to insert a set of reference data at a very high rate and then use Ops Manager to enable the cluster to be backed up, ready to go live, all in a short space of time (eg. an hour or so). They will likely be prevented from backing up due to the too restrictive and crude check that assumes wrongly that the oplog size is not sufficient. Changing the oplog size, that has already been accurately sized by the user for the expected production workload, just to enable backup, is not the right answer and could be detrimental to the clusters performance from then on.
The enhancement request has multiple parts:
1. Change the error check to just be a warning instead, to allow the user to be informed of a possible problem but allow them to evaluate/accept the risk and continue anyway.
2. Improve the oplog size/duration check algorithm (not sure how)
3. When displaying the oplog size warning saying that the oplog size should be increased, state the minimum size that it should be, based on the check algorithm's results. Otherwise it's a guessing game and the user may have to make multiple attempts to get it right, just to be able to enable backup.
This was removed in 4.2.14+ - https://docs.opsmanager.mongodb.com/v4.4/release-notes/application/#backup
We should also add a link to the docs on how to resolve this issue