Storing AppVMs on Secondary Drives
Suppose you have a fast but small primary SSD and a large but slow secondary HDD. You want to store a subset of your AppVMs on the HDD.
Qubes 4.0 is more flexible than earlier versions about placing different VMs on different disks. For example, you can keep templates on one disk and AppVMs on another, without messy symlinks.
sudo pvs sudo lvs
Take note of the VG and thin pool names for your HDD, then register it with Qubes:
# pool_name is a freely chosen pool name # vg_name is LVM volume group name # thin_pool_name is LVM thin pool name qvm-pool --add pool_name lvm_thin -o volume_group=vg_name,thin_pool=thin_pool_name
Now, you can create qubes in that pool:
qvm-create -P pool_name --label red vmname
It isn’t possible to directly migrate an existing qube to the new pool, but you can clone it there, then remove the old one:
qvm-clone -P pool_name old_name new_name qvm-remove old_name
If that was a template, or other qube referenced elsewhere (NetVM or such), you will need to adjust those references manually after moving. For example:
qvm-prefs appvm_based_on_old_name_template template new_name
In theory, you can still use file-based disk images (“file” pool driver), but it lacks some features such as you won’t be able to do backups without shutting down the qube.
mv /var/lib/qubes/appvms/my-new-appvm /path/to/secondary/drive/my-new-appvm ln -s /path/to/secondary/drive/my-new-appvm /var/lib/qubes/appvms/
my-new-appvm will behave as if it were still stored on the primary SSD (except that it will probably be slower, since it’s actually stored on the secondary HDD).
The above procedure does not interfere with Qubes Backup. However, attempting to symlink a
private.imgfile (rather than the whole AppVM directory) is known to prevent the
private.imgfile from being backed up. The same problem may occur if the above procedure is attempted on a TemplateVM. 
After implementing the above procedure, starting
my-new-appvmwill cause dom0 notifications to occur stating that loop devices have been attached to dom0. This is normal. (No untrusted devices are actually being mounted to dom0.) Do not attempt to detach these disks. (They will automatically be detached when you shut down the AppVM.)