We often snapshot the primary database and spin up a new secondary to start replicating from it. Since the primary is so large (3TB +-) it takes about 45 minutes to start up. We do this every once in awhile and have noticed some interesting bugs that continue to happen.

1) When the new secondary is turned on for the first time, watching mongostat, you will see MAPPED and VSIZE climbing (should eventually get to 2800 gb) but it would stop at 196 gb. To get it to continue, you need to run mongo on that machine and run "show dbs" to get the database to think again. From then on it would be fine.

2) A different problem that happens, which doesn't happen the same time as the problem above, is that mongostat shows mongo mapping up to 2800 gb properly, but then never switching from rs status "UNK" to rs status "SEC". I had to manually go into the database and run show collections in a database to, again, get it to think. Once I did that it switched over to SEC and was fine.

Any input is appreciated, thanks!