We have a sharded cluster with 1 primary and 1 replica (2 total). I like keeping track of Mongo running a window with mongostat going. I'd like to know how many seconds behind the primary each of the shards is.

This data is already available in some statistics gathering mode, but it would be nice to be able to have this as a column in mongostat.

I know about how large my oplog is (15 hours) so this could be a percent of oplog size if that's more convenient to implement.

If you are reticent to change existing mongostat output, perhaps there could be a flag to show this extended data.

So, to reiterate, add a column (0 on primary boxes by definition) showing either:

• Seconds behind master, or
• Percent of oplog behind master, or
• both columns in case someone's really hyper-OCD about this.

Thanks your your attention.