virtualbox.org

I am using virtualbox 4.1.2.

When I configure storage for VMs, I see the check box "solid-state drive" for the vdi attributes.

I know what SSD is. I am just curious what this check box makes difference.

If you hover the mouse over the option it tells you what it does. It causes VirtualBox to tell the guest OS that this drive is an SSD. What the guest OS does with that info is up to the guest OS, for that you would need to consult, say, the Win7 technical docs. Typically only the most modern guest OS will recognize the feature with native drivers.

And the advantage of that switch with W7 is that background defrag is automatically disabled to save read/writes on the disk. It's also not required to defragment an SSD drive, because there is no access time (or virtually none). So it's useful to set it if the VM is running on an SSD so the Guest will take proper actions to prevent unneeded read/write operations and does some other optimisations.

Does it also honor TRIM commands when using dynamically allocated VDI files?

Beli wrote:Does it also honor TRIM commands when using dynamically allocated VDI files?

That's supposedly a feature of the new v4.2 release, currently in beta. I say supposedly because the docs are sparse and I don't recall anyone reporting the results of tests.

I personally wouldn't hold out a huge amount of hope for the performance of this feature. The only practical way I can think of doing it would be to silently shuffle VDI blocks in the background, finishing off by truncating the file length as the VM shuts down. That would mean that the drive is being deliberately fragmented - not a problem for a real SSD, but it could be a problem for virtual disk images hosted on normal type drives.

mpack wrote:

Beli wrote:Does it also honor TRIM commands when using dynamically allocated VDI files?

That's supposedly a feature of the new v4.2 release ... I personally wouldn't hold out a huge amount of hope for the performance of this feature. ...

Unless the performance becomes abysmal, I'd like this feature. For me, keeping the VDI files as small as possible is far more important than the VMs having high-performance file systems. Any system that can auto-purge free space would be great, but there must obviously be a way for the guest to tell VBox when a block is free. TRIM may not be the best mechanism for this, but it should (as far as I know) work and won't require changing the guest additions when the guest is a modern OS.

I realize that others may have different requirements, but these people can always leave the SSD box unchecked. If there are users who need the guest to see an SSD but don't want VBox shuffling/purging VDI block (for performance reasons, I assume), I suppose it wouldn't be too difficult to add another checkbox under SSD to configure this behavior on a per-drive basis.