On Tue, Jan 05, 2021 at 12:39:00PM +0100, Michal Privoznik wrote:
Currently, we configure QEMU to prealloc memory almost by default. Well, by default for NVDIMMs, hugepages and if user asked us to (via memoryBacking <allocation mode="immediate"/>).
However, when guest's NVDIMM is backed by real life NVDIMM this approach is not the best. In this case users should put <pmem/> into the <memory/> device <source/>, like this:
<memory model='nvdimm' access='shared'> <source> <path>/dev/pmem0</path> <pmem/> </source> </memory>
Instructing QEMU to do prealloc in this case means that each page of the NVDIMM is "touched" (the first byte is read and written back - see QEMU commit v2.9.0-rc1~26^2) which cripples device wear.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1894053 Signed-off-by: Michal Privoznik <mprivozn@redhat.com> ---
v2 of:
https://www.redhat.com/archives/libvir-list/2020-November/msg01568.html
diff to v1: - I've dropped the first hunk of v1 which forbade prealloc even if user requested it explicitly.
src/qemu/qemu_command.c | 5 ++++- .../memory-hotplug-nvdimm-pmem.x86_64-latest.args | 2 +- 2 files changed, 5 insertions(+), 2 deletions(-)
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|