Forcing periodic flushes can only be a workaround for some bug in the Linux kernel (losing control over I/O scheduling when lots of unwritten data is in the cache). So I'm not convinced at all that this is a good idea or the final word. VirtualBox does normal user land reads/writes from/to image files. No kernel level trickery involved at all. There have been other complaints about Linux becoming unresponsive when lots of data is written, e.g. https://bugzilla.kernel.org/show_bug.cgi?id=43094 - that one has been closed on formal grounds, but it's highly likely that the nvidia kernel module makes any difference. We'll see if we can provide a very very simple testcase for the Linux folks to reproduce.
