Re: [Xen-devel] Block-attach USB devices

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Aug 13, 2014 at 10:36 PM, Franz Flintzer <fflintzer@xxxxxxxxx> wrote:
> Hi,
>
> I am using xen 4.3.2 and trying to hotplug usb flash drives. I have a Fedora
> 20 host and a Windows 7 DomU (hvm).
>
> When using "xl block-attach" I can successfully attach a USB-device in write
> mode and make changes to it in DomU. They are only saved to the USB if I
> detach it from within the Dom0 ("xl block-detach"). In Windows there is no
> option to safely eject though because the device is seen as a hard drive.

How are you determining whether the data has reached the disk or not?

> I am thinking this has to do with Windows writing to the memory first for
> performance reasons. With flash drives imo Windows writes straight to the
> drive to prevent data loss.
>
> Is there a way to let the Windows DomU recognize the device as a USB flash
> drive?

You can pass the USB device in as a USB device in the config file when
you boot the VM; but I'm assuming you want to hot-plug the USB into a
running VM.  That functionality is still in the works for xl,
unfortunately.

If you really need synchronous writes, you can turn off file caching
on the disk within Windows:

https://www.google.co.uk/search?q=windows+turning+off+caching+on+a+drive

 -George

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