virtio-pmem.rst (2662B)
1 2 =========== 3 virtio pmem 4 =========== 5 6 This document explains the setup and usage of the virtio pmem device. 7 The virtio pmem device is a paravirtualized persistent memory device 8 on regular (i.e non-NVDIMM) storage. 9 10 Usecase 11 ------- 12 13 Virtio pmem allows to bypass the guest page cache and directly use 14 host page cache. This reduces guest memory footprint as the host can 15 make efficient memory reclaim decisions under memory pressure. 16 17 How does virtio-pmem compare to the nvdimm emulation? 18 ----------------------------------------------------- 19 20 NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not 21 persist the guest writes as there are no defined semantics in the device 22 specification. The virtio pmem device provides guest write persistence 23 on non-NVDIMM host storage. 24 25 virtio pmem usage 26 ----------------- 27 28 A virtio pmem device backed by a memory-backend-file can be created on 29 the QEMU command line as in the following example:: 30 31 -object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G 32 -device virtio-pmem-pci,memdev=mem1,id=nv1 33 34 where: 35 36 - "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>" 37 creates a backend file with the specified size. 38 39 - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem 40 pci device whose storage is provided by above memory backend device. 41 42 Multiple virtio pmem devices can be created if multiple pairs of "-object" 43 and "-device" are provided. 44 45 Hotplug 46 ------- 47 48 Virtio pmem devices can be hotplugged via the QEMU monitor. First, the 49 memory backing has to be added via 'object_add'; afterwards, the virtio 50 pmem device can be added via 'device_add'. 51 52 For example, the following commands add another 4GB virtio pmem device to 53 the guest:: 54 55 (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G 56 (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2 57 58 Guest Data Persistence 59 ---------------------- 60 61 Guest data persistence on non-NVDIMM requires guest userspace applications 62 to perform fsync/msync. This is different from a real nvdimm backend where 63 no additional fsync/msync is required. This is to persist guest writes in 64 host backing file which otherwise remains in host page cache and there is 65 risk of losing the data in case of power failure. 66 67 With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides 68 a hint to application to perform fsync for write persistence. 69 70 Limitations 71 ----------- 72 73 - Real nvdimm device backend is not supported. 74 - virtio pmem hotunplug is not supported. 75 - ACPI NVDIMM features like regions/namespaces are not supported. 76 - ndctl command is not supported.