Painfully slow direct I/O on Linux guest

[Christian Holler]

Hello,

after experiencing quite slow I/O in general (Linux/Linux, compact vdi image), I did some experiments. In detail I created a file in the guest that I tried to overwrite with dd using direct I/O (dd if=/dev/zero of=/some.file bs=4M count=1000 conv=notrunc oflag=direct).

This command resulted in a writing speed of ~300 kb/s (yes, kb). The same thing in KVM resulted in a normal writing speed (30-50 mb/s) on a comparable host (although the second one has a slightly faster hdd setup, the hosts aren't the bottlenecks in this case).

Is this a known problem/expected behavior?

I am using Virtual Box 1.6.2 (closed) with the SATA controller.

I posted some more stats here: ​http://forums.virtualbox.org/viewtopic.php?p=27851#27851 if this is relevant.

If you need further details, feel free to contact me.

[Christian Holler]

-_- please close as invalid, additions were not properly loaded, direct i/o is now working (and much faster than normal i/o even ...)