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diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt new file mode 100644 index 0000000000..2d9f8c0e8c --- /dev/null +++ b/docs/nvdimm.txt @@ -0,0 +1,124 @@ +QEMU Virtual NVDIMM +=================== + +This document explains the usage of virtual NVDIMM (vNVDIMM) feature +which is available since QEMU v2.6.0. + +The current QEMU only implements the persistent memory mode of vNVDIMM +device and not the block window mode. + +Basic Usage +----------- + +The storage of a vNVDIMM device in QEMU is provided by the memory +backend (i.e. memory-backend-file and memory-backend-ram). A simple +way to create a vNVDIMM device at startup time is done via the +following command line options: + + -machine pc,nvdimm + -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE + -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE + -device nvdimm,id=nvdimm1,memdev=mem1 + +Where, + + - the "nvdimm" machine option enables vNVDIMM feature. + + - "slots=$N" should be equal to or larger than the total amount of + normal RAM devices and vNVDIMM devices, e.g. $N should be >= 2 here. + + - "maxmem=$MAX_SIZE" should be equal to or larger than the total size + of normal RAM devices and vNVDIMM devices, e.g. $MAX_SIZE should be + >= $RAM_SIZE + $NVDIMM_SIZE here. + + - "object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE" + creates a backend storage of size $NVDIMM_SIZE on a file $PATH. All + accesses to the virtual NVDIMM device go to the file $PATH. + + "share=on/off" controls the visibility of guest writes. If + "share=on", then guest writes will be applied to the backend + file. If another guest uses the same backend file with option + "share=on", then above writes will be visible to it as well. If + "share=off", then guest writes won't be applied to the backend + file and thus will be invisible to other guests. + + - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM + device whose storage is provided by above memory backend device. + +Multiple vNVDIMM devices can be created if multiple pairs of "-object" +and "-device" are provided. + +For above command line options, if the guest OS has the proper NVDIMM +driver, it should be able to detect a NVDIMM device which is in the +persistent memory mode and whose size is $NVDIMM_SIZE. + +Note: + +1. Prior to QEMU v2.8.0, if memory-backend-file is used and the actual + backend file size is not equal to the size given by "size" option, + QEMU will truncate the backend file by ftruncate(2), which will + corrupt the existing data in the backend file, especially for the + shrink case. + + QEMU v2.8.0 and later check the backend file size and the "size" + option. If they do not match, QEMU will report errors and abort in + order to avoid the data corruption. + +2. QEMU v2.6.0 only puts a basic alignment requirement on the "size" + option of memory-backend-file, e.g. 4KB alignment on x86. However, + QEMU v.2.7.0 puts an additional alignment requirement, which may + require a larger value than the basic one, e.g. 2MB on x86. This + change breaks the usage of memory-backend-file that only satisfies + the basic alignment. + + QEMU v2.8.0 and later remove the additional alignment on non-s390x + architectures, so the broken memory-backend-file can work again. + +Label +----- + +QEMU v2.7.0 and later implement the label support for vNVDIMM devices. +To enable label on vNVDIMM devices, users can simply add +"label-size=$SZ" option to "-device nvdimm", e.g. + + -device nvdimm,id=nvdimm1,memdev=mem1,label-size=128K + +Note: + +1. The minimal label size is 128KB. + +2. QEMU v2.7.0 and later store labels at the end of backend storage. + If a memory backend file, which was previously used as the backend + of a vNVDIMM device without labels, is now used for a vNVDIMM + device with label, the data in the label area at the end of file + will be inaccessible to the guest. If any useful data (e.g. the + meta-data of the file system) was stored there, the latter usage + may result guest data corruption (e.g. breakage of guest file + system). + +Hotplug +------- + +QEMU v2.8.0 and later implement the hotplug support for vNVDIMM +devices. Similarly to the RAM hotplug, the vNVDIMM hotplug is +accomplished by two monitor commands "object_add" and "device_add". + +For example, the following commands add another 4GB vNVDIMM device to +the guest: + + (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=new_nvdimm.img,size=4G + (qemu) device_add nvdimm,id=nvdimm2,memdev=mem2 + +Note: + +1. Each hotplugged vNVDIMM device consumes one memory slot. Users + should always ensure the memory option "-m ...,slots=N" specifies + enough number of slots, i.e. + N >= number of RAM devices + + number of statically plugged vNVDIMM devices + + number of hotplugged vNVDIMM devices + +2. The similar is required for the memory option "-m ...,maxmem=M", i.e. + M >= size of RAM devices + + size of statically plugged vNVDIMM devices + + size of hotplugged vNVDIMM devices |