I've been using MongoDB on CentOS with ext4 for a few months now with no problems. I'm potentially going to be moving to Joyentcloud, and their SmartOS is based off of OpenSolaris and uses ZFS. I'm wondering if there could be some downsides to switching to this for MongoDB?
10gen strongly recommends XFS if you know how to tune it, or EXT4 otherwise. With that said I would advise realistic (including load, if you can figure out how) testing with non-production data.
With that said, I would consider using the
--nojournal flag when starting MongoDB to avoid excessive disk thrashing that would be caused by ZFS Journaling watching MongoDB's Journaling.
No, it is a bad idea :
ZFS uses as much free memory as it can for caching
so, the RAM is not used efficiently.
ZFS uses CPU for each read an write, using an hardware raid with ext4 or xfs will free up some cpu cycles that would be available for MonGoDB (wich is cpu intensiv)
I would never use ZFS for a DB, i would rather use ZFS as a simple database of big files.