| Message ID | 20190821073630.2561-1-pagupta@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] virtio pmem: user document | expand |
On Wed, 21 Aug 2019 13:06:30 +0530 Pankaj Gupta <pagupta@redhat.com> wrote: > 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> > --- > v1->v2 > - Fixes on text format and 'Guest Data persistence' > section - Cornelia > > docs/virtio-pmem.rst | 75 ++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 75 insertions(+) > create mode 100644 docs/virtio-pmem.rst > > diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst > new file mode 100644 > index 0000000000..0346e61674 > --- /dev/null > +++ b/docs/virtio-pmem.rst > @@ -0,0 +1,75 @@ > + > +======================== > +QEMU virtio pmem > +======================== > + > + This document explains the setup and usage of virtio pmem device s/virtio pmem device/the virtio pmem device/ > + which is available since QEMU v4.1.0. > + > + The virtio pmem is a paravirtualized persistent memory device on s/The virtio pmem/The virtio pmem device/ > + regular(i.e non-NVDIMM) storage. missing blank before '(' > + > +Usecase > +-------- > + > + Allows to bypass the guest page cache and directly use host page cache. "Virtio pmem allows to..." ? > + 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 missing blank before '(' > + 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 of size on a mem-path. "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'." Please lose the '"' (copy/paste leftover, I presume? :) > + > +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 application to s/application/applications/ ? > + 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. Only some nits from my side, otherwise looks good to me.
Hi Cornelia, > > 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> > > --- > > v1->v2 > > - Fixes on text format and 'Guest Data persistence' > > section - Cornelia > > > > docs/virtio-pmem.rst | 75 ++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 75 insertions(+) > > create mode 100644 docs/virtio-pmem.rst > > > > diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst > > new file mode 100644 > > index 0000000000..0346e61674 > > --- /dev/null > > +++ b/docs/virtio-pmem.rst > > @@ -0,0 +1,75 @@ > > + > > +======================== > > +QEMU virtio pmem > > +======================== > > + > > + This document explains the setup and usage of virtio pmem device > > s/virtio pmem device/the virtio pmem device/ done > > > + which is available since QEMU v4.1.0. > > + > > + The virtio pmem is a paravirtualized persistent memory device on > > s/The virtio pmem/The virtio pmem device/ o.k > > > + regular(i.e non-NVDIMM) storage. > > missing blank before '(' sure > > > + > > +Usecase > > +-------- > > + > > + Allows to bypass the guest page cache and directly use host page cache. > > "Virtio pmem allows to..." ? done. > > > + 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 > > missing blank before '(' done. > > > + 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 of size on a mem-path. > > "a backend file with the specified size" ? done. > > > + > > + - "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'." > > Please lose the '"' (copy/paste leftover, I presume? :) Done :) > > > + > > +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 application > > to > > s/application/applications/ ? done. > > > + 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. > > Only some nits from my side, otherwise looks good to me. Thank you for the review. Will post a v3 with the changes. Best regards, Pankaj >
diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst new file mode 100644 index 0000000000..0346e61674 --- /dev/null +++ b/docs/virtio-pmem.rst @@ -0,0 +1,75 @@ + +======================== +QEMU virtio pmem +======================== + + This document explains the setup and usage of virtio pmem device + which is available since QEMU v4.1.0. + + The virtio pmem is a paravirtualized persistent memory device on + regular(i.e non-NVDIMM) storage. + +Usecase +-------- + + 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 of size on a mem-path. + + - "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 application 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.
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> --- v1->v2 - Fixes on text format and 'Guest Data persistence' section - Cornelia docs/virtio-pmem.rst | 75 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 docs/virtio-pmem.rst