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@xxxxxxxxxx> > --- > > 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@xxxxxxxxxx> 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 :|
