77 lines
2.6 KiB
ReStructuredText
77 lines
2.6 KiB
ReStructuredText
|
|
||
|
|
===========
|
||
|
|
virtio pmem
|
||
|
|
===========
|
||
|
|
|
||
|
|
This document explains the setup and usage of the virtio pmem device.
|
||
|
|
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.
|
||
|
|
|
||
|
|
How does virtio-pmem compare to the nvdimm emulation?
|
||
|
|
-----------------------------------------------------
|
||
|
|
|
||
|
|
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.
|