libvirt users - Nov 2014 - Re: fstrim support in libvirt/qemu guests

On Mon, Nov 3, 2014 at 4:24 PM, Michal Privoznik <mprivozn@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.

On 11/3/2014 8:43 AM, Andrey Korolyov wrote:
> On Mon, Nov 3, 2014 at 4:24 PM, Michal Privoznik
<mprivozn@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.

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@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.

Regards,
  Dennis