[libvirt] [libvirt PATCH] qemu: Support discard for disk

[Martin Kletzander]

On 05/15/2013 03:17 PM, Daniel P. Berrange wrote:
> On Wed, May 15, 2013 at 02:56:03PM +0200, Paolo Bonzini wrote:
>> Il 14/05/2013 14:44, Osier Yang ha scritto:
>>> QEMU introduced "discard" option for drive since commit a9384aff53,
>>>
>>> <...>
>>> @var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and
>>> controls whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
>>> requests are ignored or passed to the filesystem.  Some machine types
>>> may not support discard requests.
>>> </...>
>>>
>>> This patch exposes the support in libvirt.
>>>
>>> QEMU supported "discard" for "-drive" since v1.5.0-rc0:
>>
>> I would prefer to use ignore/unmap instead of off/on in libvirt.
> 
> If we think there's any chance QEMU might gain a 3rd possible
> value for this setting, then I tend to agree with you that
> avoiding off/on would be desirable.
> 

IMHO off/on is a bit more easier to understand, plus it gives us the
opportunity to shrink some of the code in the future (of course none of
this is needed neither a strong reason to use it).  ignore/unmap OTOH is
what we map it to in qemu.  I'd be ok with having ignore/unmap as a
alias for off/on, but that adds unnecessary code.  Adding a third option
would make the off/on option a bit clumsy afterwards, but it could work
as well.

Since we can still change it, is there any other reason why we should
prefer any of those?

I agree that basing it on whether there is a possibility in qemu to add
more options is probably the best way to go.

Martin