[[!toc levels=2]]

# Rationale

For [[automated testing|todo/automated_builds_and_tests]] purposes, we
need to use removable USB mass storage devices, e.g. to test USB-only
features such as the USB installer or persistence.

On a headless test server in a datacenter, one does not want to rely
on a set of physical USB sticks being plugged in.

Therefore, these USB mass storage devices must be emulated in
software, and should probably use files as underlying storage.

# Tools

## g-file-storage

* obsoleted by `g-mass-storage`
* not in Debian kernel as of 3.2.21-3

## g-mass-storage

Works fine with qemu-kvm (1.0+dfsg-11) and libvirt (0.9.12-3):

	modprobe dummy-hcd is_high_speed=1
	modprobe g_mass_storage file=/tmp/test.usb removable=1

Then I use qemu-kvm + libvirt USB 2.0 passthrough to lend that virtual
USB mass storage device to a qemu-kvm VM, which I boot from a Tails
0.12 ISO, that I clone onto the virtual USB device using the Tails
USB installer.

Then I unplug the virtual CD drive, and boot the same VM from the
virtual USB mass storage device, onto which I successfully setup
a Tails persistent volume. Rebooting, enabling the persistent volume
does work.

### Drawbacks

* **Not in Debian** kernel as of 3.2.21-3, asked in [[!debbug 678731]]
* **All underlying files must be listed at module load time**; this
  should not be a problem, but still a limitation to be kept in mind.

## qemu

qemu (1.1.0+dfsg-1) and qemu-kvm (1.0+dfsg-11) know how to emulate
a USB mass storage device, using `-usbdevice` (deprecated) or
(roughly) a combination of `-drive file=path_to_test_file.usb,if=none`
and `-device usb-storage,drive=DRIVE-ID`. At least with qemu-kvm:

* one can boot from such a USB device
* virt-manager (0.9.1-4) and libvirt (0.9.12-3) allow to add and
  configure such a device, and plug it into a virtual USB 2.0
  controller

### Drawbacks

* **removable property**: by default the emulated USB disk is *not*
  marked as removable; qemu and qemu-kvm from Wheezy support emulating
  removable USB devices, but libvirt has no way to pass this option.
  - [upstream ticket](https://bugzilla.redhat.com/show_bug.cgi?id=922495)
  - The qemu part was [discussed on
    Launchpad](https://bugs.launchpad.net/qemu/+bug/686613) already.
  - A patch that adds the missing feature is being upstream'd, see
    [[contribute/release_process/test/setup]] for details.
