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(-)
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index b06a086e18..f572ed64c1 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -3085,7 +3085,10 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps,
if (mem->nvdimmPath) {
memPath = g_strdup(mem->nvdimmPath);
- prealloc = true;
+ /* If the NVDIMM is a real device then there's nothing to prealloc.
+ * If anyhing, we would be only wearing off the device. */
+ if (!mem->nvdimmPmem)
+ prealloc = true;
} else if (useHugepage) {
if (qemuGetDomainHupageMemPath(priv->driver, def, pagesize, &memPath) < 0)
return -1;
diff --git a/tests/qemuxml2argvdata/memory-hotplug-nvdimm-pmem.x86_64-latest.args b/tests/qemuxml2argvdata/memory-hotplug-nvdimm-pmem.x86_64-latest.args
index cac02a6f6d..fb4ae4b518 100644
--- a/tests/qemuxml2argvdata/memory-hotplug-nvdimm-pmem.x86_64-latest.args
+++ b/tests/qemuxml2argvdata/memory-hotplug-nvdimm-pmem.x86_64-latest.args
@@ -20,7 +20,7 @@ file=/tmp/lib/domain--1-QEMUGuest1/master-key.aes \
-object memory-backend-ram,id=ram-node0,size=224395264 \
-numa node,nodeid=0,cpus=0-1,memdev=ram-node0 \
-object memory-backend-file,id=memnvdimm0,mem-path=/tmp/nvdimm,share=no,\
-prealloc=yes,size=536870912,pmem=yes \
+size=536870912,pmem=yes \
-device nvdimm,node=0,memdev=memnvdimm0,id=nvdimm0,slot=0 \
-uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \
-display none \
--
2.26.2
