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author | Pankaj Gupta <pagupta@redhat.com> | 2019-08-21 17:46:24 +0530 |
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committer | Michael S. Tsirkin <mst@redhat.com> | 2019-09-16 06:57:24 -0400 |
commit | 5dbf335968faa45e5a1ee3083fb6816d90dc6ff3 (patch) | |
tree | c1254d6305d20e0e09a0b32ecd31c9b973d12f14 /docs | |
parent | e7df189e19e86bf9f4d7aea4c6cf50ac0ebfce46 (diff) | |
download | qemu-5dbf335968faa45e5a1ee3083fb6816d90dc6ff3.zip qemu-5dbf335968faa45e5a1ee3083fb6816d90dc6ff3.tar.gz qemu-5dbf335968faa45e5a1ee3083fb6816d90dc6ff3.tar.bz2 |
virtio pmem: user document
This patch documents the steps to use virtio pmem.
It also documents other useful information about
virtio pmem e.g use-case, comparison with Qemu NVDIMM
backend and current limitations.
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Message-Id: <20190821121624.5382-1-pagupta@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Diffstat (limited to 'docs')
-rw-r--r-- | docs/virtio-pmem.rst | 75 |
1 files changed, 75 insertions, 0 deletions
diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst new file mode 100644 index 0000000..e77881b --- /dev/null +++ b/docs/virtio-pmem.rst @@ -0,0 +1,75 @@ + +======================== +QEMU virtio pmem +======================== + + This document explains the setup and usage of the virtio pmem device + which is available since QEMU v4.1.0. + + The virtio pmem device is a paravirtualized persistent memory device + on regular (i.e non-NVDIMM) storage. + +Usecase +-------- + + Virtio pmem allows to bypass the guest page cache and directly use + host page cache. This reduces guest memory footprint as the host can + make efficient memory reclaim decisions under memory pressure. + +o How does virtio-pmem compare to the nvdimm emulation supported by QEMU? + + NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not + persist the guest writes as there are no defined semantics in the device + specification. The virtio pmem device provides guest write persistence + on non-NVDIMM host storage. + +virtio pmem usage +----------------- + + A virtio pmem device backed by a memory-backend-file can be created on + the QEMU command line as in the following example: + + -object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G + -device virtio-pmem-pci,memdev=mem1,id=nv1 + + where: + - "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>" + creates a backend file with the specified size. + + - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem + pci device whose storage is provided by above memory backend device. + + Multiple virtio pmem devices can be created if multiple pairs of "-object" + and "-device" are provided. + +Hotplug +------- + +Virtio pmem devices can be hotplugged via the QEMU monitor. First, the +memory backing has to be added via 'object_add'; afterwards, the virtio +pmem device can be added via 'device_add'. + +For example, the following commands add another 4GB virtio pmem device to +the guest: + + (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G + (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2 + +Guest Data Persistence +---------------------- + + Guest data persistence on non-NVDIMM requires guest userspace applications + to perform fsync/msync. This is different from a real nvdimm backend where + no additional fsync/msync is required. This is to persist guest writes in + host backing file which otherwise remains in host page cache and there is + risk of losing the data in case of power failure. + + With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides + a hint to application to perform fsync for write persistence. + +Limitations +------------ +- Real nvdimm device backend is not supported. +- virtio pmem hotunplug is not supported. +- ACPI NVDIMM features like regions/namespaces are not supported. +- ndctl command is not supported. |