Cyclical / rhythmic disk IO: (activity keeps increasing)

I have got a unique problem with one of my servers. The disk IO stat is consistently increasing for last couple of weeks. See this graph from Munin:

http://paraschopra.com/temp/io-consolidated.png

From Linode's dashboard, I see more fine grained picture of disk IO. Here is the cyclical / rhythmic graph (a day's interval). But do note that even though it appears cyclical, over a period of weeks, avg disk IO is increasing consistently (see above graph):

http://paraschopra.com/temp/io-cyclical.png

Now, I did iotop and saw that kjournald is the only process doing writing for disk IO (apart from occassional rsyslogd – but frequency of disk IO of kjournald is much, much higher). In the graphs above, read component of IO is practically zero.

Why is kjournald writing even when there is no other process writing? Why is size of writes getting larger by the day? Please help!

PS: My disk is ext3