stoneage7/VDI-shrink-trim.md

## VDI-shrink-trim.md

      
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              VDI-shrink-trim.md
            
          
    Motivation

The purpose of this gist is to set up a virtual machine in such a way that the on-disk image in the host machine automatically grows and shrinks as needed by the guest machine. This utilizes the (still undocumented) "--discard" and "--nonrotational" parameters in "VBoxManage storageattach" which make the attached image appear as an SSD to the guest. Guest OS will then issue TRIM commands to the virtual controller where such an image is attached. VirtualBox is then able to capture the commands and punch holes in the attached VDIs.
Although there is some initial setup needed, I think the time saved with babysitting the VDIs is worth it. Usually you would need to zero out the free space with zerofree or sdelete and then run "VBoxManage --compact" on your images. With this setup you can allocate a large dynamic VDI (1TB or so) and it will keep itself at minimum size for easy syncing, backup, etc. You can also set it up in a template machine if you use one for clones etc.
Requirements


Linux (TODO: btrfs / ext4 only?) or Windows 8.1 (7 doesn't work)
(TODO: windows 10?)
dynamically-sized VDI type disk and SATA type controller (TODO: support for others?)
backup of important data
tested on VirtualBox 4.3.* only

Set up

Linux


In host go to Settings -> Storage


Select your attached image and at the bottom select "Remove the attachment highlighted..."


Open a command prompt in the directory where your virtual machine is saved


Run (replacing machine name, VDI file name, and if you have more than one image attached to the controller, device and port):
  "C:\Program Files\Oracle\VirtualBox\VBoxManage" storageattach "[GuestOsMachineName]" --storagectl "SATA" --port 1 --device 0 --nonrotational on --discard on --medium "[file.vdi]" --type hdd


Boot your guest OS and add "discard" flag to your fstab (or systemd's .mount unit if you have it set up that way) if needed.


If the constant TRIM-ming takes too much power off your computer, mount the filesystems with nodiscard and set up a regular cron job (or systemd .timer unit) to run "fstrim /" (point it at the image's mount point)


Windows 8.1

I never had success using a discard-enabled filesystem under Windows as a guest. You may try to use the same steps as in Linux but my machine starts choking after the image grows above ~6GB in size. This is a way that works for me (tm):


In Windows guest, start up an admin prompt and disable TRIM completely using the following command.
  fsutil behavior set disabledeletenotify 1


Shut down the guest


In host machine open a command prompt in the directory where your virtual machine is saved


Run (replacing machine name, VDI file name, and if you have more than one image attached to the controller, device and port):
  "C:\Program Files\Oracle\VirtualBox\VBoxManage" storageattach "[GuestOsMachineName]" --storagectl "SATA" --port 1 --device 0 --nonrotational on --discard on --medium "[file.vdi]" --type hdd


Now boot back your guest machine and create a .bat file like this:
  fsutil behavior set disabledeletenotify 0
  defrag /L C:
  fsutil behavior set disabledeletenotify 1


Note that this needs to run with Admin privileges


Optionally set up a task in task scheduler (under Administrative Tools -> Task Scheduler) to run it regularly.


In conclusion

The VDIs themselves are organized in 1MB blocks, so this will only work if the space being TRIM-med is at least a 1MB contiguous block at a 1MB boundary. Occasional defrag (with "defrag.exe /D" under Windows or "btrfs filesystem defrag" under Linux) in the background may be beneficial. VirtualBox will only trim whole 1MB blocks.