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Re: [Qemu-devel] [PATCH v2] virtio pmem: user document
From: |
Pankaj Gupta |
Subject: |
Re: [Qemu-devel] [PATCH v2] virtio pmem: user document |
Date: |
Wed, 21 Aug 2019 07:47:10 -0400 (EDT) |
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 <address@hidden>
> > ---
> > 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
>