[libvirt-users] fstrim support in libvirt/qemu guests

Dennis Jacobfeuerborn dennisml at conversis.de
Mon Nov 3 22:29:49 UTC 2014

On 03.11.2014 15:04, Brian Rak wrote:
> On 11/3/2014 8:43 AM, Andrey Korolyov wrote:
>> On Mon, Nov 3, 2014 at 4:24 PM, Michal Privoznik <mprivozn at redhat.com>
>> wrote:
>>> On 01.11.2014 05:30, Dennis Jacobfeuerborn wrote:
>>>> Hi,
>>>> I'm trying to find out how to enable fstrim support for libvirt/qemu
>>>> guests. Specifically I'm trying to enable this on a CentOS 7 host (qemu
>>>> 1.5). I searched the web but only found very vague information and no
>>>> clear documentation on the requirements or how to modify the guest xml
>>>> to enable discard/fstrim support.
>>>> Can someone point me in the right direction?
>>> One thing that you'll need is the guest agent:
>>>    http://wiki.libvirt.org/page/Qemu_guest_agent
>>> Then you can just use:
>>>    # virsh domfstrim $dom
>>> Michal
>> ... and probably discard-aware block driver (although I cannot name
>> anything except virtio-blk which is not supporting trim or will not
>> support in the near future). Not sure to which part, guest-host
>> interaction or driver abilities, the initial question belongs.
> The virtio-scsi driver supports it, though you take a minor performance
> hit for switching.

Yes, after a bit of experimentation it seems you need to use virtio-scsi
and raw images to make this work. Interesting enough then you can even
make use of fstrim in a non-SSD context and shrink the used space of the
raw image.


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