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I am having some trouble with poor I/O running a Windows 7 VM on VirtualBox on FreeBSD 9.0.

I would like to change my virtual disk type to immutable, and then change the location of the differencing disk to a 5gb ramdisk using mdmfs.

Does that seem like it would provide a performance improvement for disk I/O? I am already using virtio for the network so I believe the problem has been isolated to the disk.

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You would probably see an improvement for writes - you would have to benchmark it to see if it's substantial or not.

Honestly I doubt it would be worth it - a ramdisk still needs to pass through the VFS layer (which is quite a bit of overhead), and VirtualBox will need to check its lookup table to see if the sector you're reading has changed and therefore needs to be read from the ramdisk (more overhead). If your system changes so little that you can use an immutable base image and a differencing disk most of what you need off the disk will be sucked into FreeBSD's buffer cache anyway.

There is also the problem of what happens if your system reboots/loses power (your ramdisk contents go away -- If that's not acceptable to you this solution won't work).

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