ext4 jbd2 journaling active even on empty filesystem

Solution 1:

It seems that ext4 does its 'quick format' by postposting a lot of its initialisation until it is mounted (the uninit_bg option, perhaps?) and I suspect that jbd2 is doing this after you first format the drive. One of the comments here suggested that jbd2 writes to the partition after creation for a time proportional to its size, and suggested 10 minutes per 200 GB, which would indicate 50 minutes per TB. I formatted a 3TB ext4 drive yesterday and jbd2 was writing away for ages, but it has now stopped.

btrfs doesn't do this, btw, but I don't trust it yet for external drives - eg you need a recent kernel (3.4+, I think) or the kernel will oops (and possibly lose data) if you remove the drive without unmounting it first (which of course happens if it loses power).