Comment 305 for bug 620074

Look at the difference in disk throughput when running with dirty_background_ratio=0 and dirty_ratio=0:
http://img9.imageshack.us/img9/811/fsyncgraph00.png

versus with dirty_background_ratio=40 and dirty_ratio=80:
http://img154.imageshack.us/img154/9427/fsyncgraph4080.png

Both images are graphs of vmstat output during this command:
dd if=/dev/zero of=bigfile bs=1M count=20k conv=fdatasync

I collected this data in single-user mode so no other processes were touching the disk.

Do note the fairly steady throughput in the first case, in stark contrast with the huge burst at the beginning and end of, and slowness throughout, the second case.

In case anyone missed the point, it took 55 seconds to write 20 GB with dbr=0,dr=0 and 593 seconds to write 20 GB with dbr=40,dr=80. For some reason, the page cache appears to be really gumming up the works.