Re: [Qemu-devel] TRIM/DISCARD/UNMAP support on qemu-nbd

[Paolo Bonzini]

Il 08/01/2014 23:53, Richard W.M. Jones ha scritto:
> On Wed, Jan 08, 2014 at 11:45:39PM +0100, Paolo Bonzini wrote:
>> Il 08/01/2014 23:24, Richard W.M. Jones ha scritto:
>>> It's extremely difficult to know when it's safe to add this parameter.
>>> Qemu gives no indication of when using discard=.. is safe (ie. won't
>>> cause qemu to fail to start up or fail in some other way).  It's even
>>> worse when we have to go via libvirt which itself doesn't expose
>>> qemu's capabilities upwards.
>>
>> It is a bug that "-help" doesn't list discard=on, but QMP
>> query-command-line-options lists it correctly.
>>
>> libvirt could safely ignore discard if the underlying QEMU does not
>> support it, but that's not how it was implemented.  Currently,
>> explicitly specifying either discard='on' and discard='off' will cause
>> the VM to fail to start if QEMU does not support it.  There are
>> tradeoffs in both solutions...
> 
> That sucks .. for me ...
> 
> Can't we have an option like discard=ifpossible?  libguestfs would use
> this, since we'd always prefer to honour discard requests from our
> kernel.

That would be a libvirt option, not QEMU.

Paolo