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
