Thanks for your answers, Rocko. Today I have installed the Karmic Koala Alpha 1 with Kernel 2.6.30-5-generic, and it seems that all the former problems with ext4 are gone. For testing purposes I have created 5 big dvd iso files (together about 30 GB of data), moved them around in the system, copyied and deleted them three or four times, and - as a final barrier - emptyied the trash with meanwhile around 120 GB of data in it. Everything went smoothly without the system tottering for even one second!! Great work, guys!!