Writes in idle/How to debug filesystem activity

Hallo list,

I use btrfs on a SD card. When my computer is running in idle (i.e. nothing running but wmii and a few background tasks that don't do any i/o; freshly synced), from time to time some [btrfs-*] tasks do writes (for 5 seconds or so). What are those processes doing on my disk, having nothing to write/read? That leads to the second question: How can I debug file system activity in inotify style?

Thanks,
Jonas