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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>
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2.7 KiB
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76 lines
2.7 KiB
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========================
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QEMU virtio pmem
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========================
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This document explains the setup and usage of the virtio pmem device
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which is available since QEMU v4.1.0.
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The virtio pmem device is a paravirtualized persistent memory device
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on regular (i.e non-NVDIMM) storage.
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Usecase
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--------
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Virtio pmem allows to bypass the guest page cache and directly use
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host page cache. This reduces guest memory footprint as the host can
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make efficient memory reclaim decisions under memory pressure.
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o How does virtio-pmem compare to the nvdimm emulation supported by QEMU?
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NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not
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persist the guest writes as there are no defined semantics in the device
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specification. The virtio pmem device provides guest write persistence
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on non-NVDIMM host storage.
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virtio pmem usage
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-----------------
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A virtio pmem device backed by a memory-backend-file can be created on
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the QEMU command line as in the following example:
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-object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G
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-device virtio-pmem-pci,memdev=mem1,id=nv1
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where:
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- "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>"
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creates a backend file with the specified size.
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- "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem
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pci device whose storage is provided by above memory backend device.
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Multiple virtio pmem devices can be created if multiple pairs of "-object"
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and "-device" are provided.
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Hotplug
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-------
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Virtio pmem devices can be hotplugged via the QEMU monitor. First, the
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memory backing has to be added via 'object_add'; afterwards, the virtio
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pmem device can be added via 'device_add'.
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For example, the following commands add another 4GB virtio pmem device to
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the guest:
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(qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G
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(qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2
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Guest Data Persistence
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----------------------
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Guest data persistence on non-NVDIMM requires guest userspace applications
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to perform fsync/msync. This is different from a real nvdimm backend where
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no additional fsync/msync is required. This is to persist guest writes in
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host backing file which otherwise remains in host page cache and there is
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risk of losing the data in case of power failure.
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With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides
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a hint to application to perform fsync for write persistence.
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Limitations
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------------
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- Real nvdimm device backend is not supported.
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- virtio pmem hotunplug is not supported.
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- ACPI NVDIMM features like regions/namespaces are not supported.
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- ndctl command is not supported.
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