From: Pankaj Gupta <pagupta@redhat.com>
To: qemu-devel@nongnu.org
Cc: pagupta@redhat.com, david@redhat.com, cohuck@redhat.com,
lcapitulino@redhat.com, mst@redhat.com, stefanha@redhat.com,
riel@surriel.com, nilal@redhat.com
Subject: [Qemu-devel] [PATCH v2] virtio pmem: user document
Date: Wed, 21 Aug 2019 13:06:30 +0530 [thread overview]
Message-ID: <20190821073630.2561-1-pagupta@redhat.com> (raw)
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
+ 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.
--
2.21.0
next reply other threads:[~2019-08-21 8:03 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-21 7:36 Pankaj Gupta [this message]
2019-08-21 11:24 ` [Qemu-devel] [PATCH v2] virtio pmem: user document Cornelia Huck
2019-08-21 11:47 ` Pankaj Gupta
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