Performance virtualbox diskfiles vs rawdisk

Question

Pros and cons with this two ways to make disk for virtual machines on virtualbox

$ vboxmanage createhd --format VDI --size ${SIZE} --filename /path/to/disk.vdi

and

# vboxmanage createrawvmdk -filename /path/to/disk.vmdk -rawdisk /dev/sdX

Answer 1

File-pro (which are raw cons):

• Will be cached by the operating system if they fit into ram
• Easier to handle in VirtualBox (i.e. to resize raw you have to recreate the vmdk)
• You might be able to use sparse-files (not sure)

Raw-pro (which are file cons):

• You can use LVs here, giving you full flexibilty
• Access should be faster, since it does not have to pass the filesystem-layer
• Easy to handle outside VirtualBox (e.g. you could use LV-snapshots)

Answer 2

These are my hdparm -tT timings on a 4x1TB software raid setup running Ubuntu 12.04.1 as the host.

VM with VDI format:

Timing cached reads: 23442 MB in 1.97 seconds = 11890.47 MB/sec

Timing buffered disk reads: 346 MB in 3.00 seconds = 115.33 MB/sec

VM with rawdisk format (50G LV on the same raid device):

Timing cached reads: 21602 MB in 2.00 seconds = 10812.49 MB/sec

Timing buffered disk reads: 186 MB in 3.02 seconds = 61.69 MB/sec

So rawdisk is about 50% worse performance that VDI, unless I'm doing something completely wrong. Probably going to stick with rawdisk though as the backup option with lvm snapshots seems sane.