Support passthrought of TRIM commands to raw partitions

[javispedro]

Since VirtualBox 4.2 it appears that the AHCI virtual controller supports the ATA TRIM command.

However, only the VDI image file backend actually implements it, where it is used to detect unused blocks so that the image file size can be reduced (if using a dynamic image). The rest of backends do not seem to use it for anything.

My request is for the RAW block device backend to send TRIM/DISCARD commands to the backing block device upon receiving a TRIM command from the guest.

This would enable e.g. Windows guests on raw partitions (via VMDK's partitionedDevice) sending TRIM commands to a backing SSD device, thereby increasing performance and SSD longevity.

Unlike discard support for VDI images, this should not involve any extra bookkeeping ("discard" commands can be sent to the raw block device mostly "as is", no need to cluster them).

[Joshua Naismith]

Just Cc-ing.

[jack lee]

This enhancement is very useful to save disk space.