[v3] virtio pmem: user document

Commit Message

Pankaj Gupta Aug. 21, 2019, 12:16 p.m. UTC

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 <[e-mail]>
---
v3->v3
 - Text format fixes - Cornerlia
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

Comments

Cornelia Huck Aug. 26, 2019, 12:46 p.m. UTC | #1

On Wed, 21 Aug 2019 17:46:24 +0530
Pankaj Gupta <[e-mail]> 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 <[e-mail]>
> ---
> v3->v3
>  - Text format fixes - Cornerlia
> 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

Looks good to me now.

Reviewed-by: Cornelia Huck <[e-mail]>

Pankaj Gupta Sept. 16, 2019, 6:30 a.m. UTC | #2

Gentle ping.

Can we please merge this patch.

Thanks,
Pankaj

> 
> > 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 <[e-mail]>
> > ---
> > v3->v3
> >  - Text format fixes - Cornerlia
> > 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
> 
> Looks good to me now.
> 
> Reviewed-by: Cornelia Huck <[e-mail]>
> 
>

Patch

diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst
new file mode 100644
index 0000000000..e77881b26f
--- /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.