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

[Richard W.M. Jones]

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.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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