I noticed that from time to time one of my databases ( I use NT 4 SP 6 with 5 instance running on it Oracle server ver 8.0.4 ) keeps the server's hard disk working like hell even if there is no one connected to any other instance at all.

Could be this from the SMON ( defragmenting for instance ) , it does not return any error messages but it keeps working like this for 2 or 3 days continously then it gets back to its normal status.

Also performance degradation happens during this process.

What could be the cause ??

Thank You