Re: [libvirt] [PATCH v1 7/7] qemu: Enable memory-backend-file.discard-data whenever possible

On 04/17/2018 10:05 AM, Peter Krempa wrote: > On Thu, Apr 12, 2018 at 18:39:52 +0200, Michal Privoznik wrote: >> https://bugzilla.redhat.com/show_bug.cgi?id=1480668 >> >> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; >> --- >> src/qemu/qemu_command.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c >> index f864350bd5..67350719aa 100644 >> --- a/src/qemu/qemu_command.c >> +++ b/src/qemu/qemu_command.c >> @@ -3148,6 +3148,12 @@ qemuBuildMemoryBackendStr(virJSONValuePtr *backendProps, >> NULL) < 0) >> goto cleanup; >> >> + if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_FILE_DISCARD) && >> + virJSONValueObjectAdd(props, >> + "B:discard-data", true, > > This code path is used also for NVDIMMs, and since the documentaion for > the option states that: > > The new option can be used to indicate that the file contents can > be destroyed and don't need to be flushed to disk when QEMU exits > or when the memory backend object is removed. > > I'm not sure whether this is right. > Okay, I'll make it configurable via domain XML. However, I'm struggling to come up with useful design. Perhaps we can have <discardData/> element under <memoryBacking/>? <memoryBacking> <discardData/> </memoryBacking> and in order to be able to fine tune this per nvdimm/RAM modules we can have: <memory model='nvdimm'> <source ../> <target ../> <discardData/> </memory> However, this would allow the element even for cases when memory-backing-ram is used (or the old way of configuration without any memory-backend-* at all).