1QEMU memory hotplug 2=================== 3 4This document explains how to use the memory hotplug feature in QEMU, 5which is present since v2.1.0. 6 7Guest support is required for memory hotplug to work. 8 9Basic RAM hotplug 10----------------- 11 12In order to be able to hotplug memory, QEMU has to be told how many 13hotpluggable memory slots to create and what is the maximum amount of 14memory the guest can grow. This is done at startup time by means of 15the -m command-line option, which has the following format: 16 17 -m [size=]megs[,slots=n,maxmem=size] 18 19Where, 20 21 - "megs" is the startup RAM. It is the RAM the guest will boot with 22 - "slots" is the number of hotpluggable memory slots 23 - "maxmem" is the maximum RAM size the guest can have 24 25For example, the following command-line: 26 27 qemu [...] -m 1G,slots=3,maxmem=4G 28 29Creates a guest with 1GB of memory and three hotpluggable memory slots. 30The hotpluggable memory slots are empty when the guest is booted, so all 31memory the guest will see after boot is 1GB. The maximum memory the 32guest can reach is 4GB. This means that three additional gigabytes can be 33hotplugged by using any combination of the available memory slots. 34 35Two monitor commands are used to hotplug memory: 36 37 - "object_add": creates a memory backend object 38 - "device_add": creates a front-end pc-dimm device and inserts it 39 into the first empty slot 40 41For example, the following commands add another 1GB to the guest 42discussed earlier: 43 44 (qemu) object_add memory-backend-ram,id=mem1,size=1G 45 (qemu) device_add pc-dimm,id=dimm1,memdev=mem1 46 47Using the file backend 48---------------------- 49 50Besides basic RAM hotplug, QEMU also supports using files as a memory 51backend. This is useful for using hugetlbfs in Linux, which provides 52access to bigger page sizes. 53 54For example, assuming that the host has 1GB hugepages available in 55the /mnt/hugepages-1GB directory, a 1GB hugepage could be hotplugged 56into the guest from the previous section with the following commands: 57 58 (qemu) object_add memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1GB 59 (qemu) device_add pc-dimm,id=dimm1,memdev=mem1 60 61It's also possible to start a guest with memory cold-plugged into the 62hotpluggable memory slots. This might seem counterintuitive at first, 63but this allows for a lot of flexibility when using the file backend. 64 65In the following command-line example, an 8GB guest is created where 6GB 66comes from regular RAM, 1GB is a 1GB hugepage page and 256MB is from 672MB pages. Also, the guest has additional memory slots to hotplug more 682GB if needed: 69 70 qemu [...] -m 6GB,slots=4,maxmem=10G \ 71 -object memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1G \ 72 -device pc-dimm,id=dimm1,memdev=mem1 \ 73 -object memory-backend-file,id=mem2,size=256M,mem-path=/mnt/hugepages-2MB \ 74 -device pc-dimm,id=dimm2,memdev=mem2 75 76 77RAM hot-unplug 78--------------- 79 80In order to be able to hot unplug pc-dimm device, QEMU has to be told the ids 81of pc-dimm device and memory backend object. The ids were assigned when you hot 82plugged memory. 83 84Two monitor commands are used to hot unplug memory: 85 86 - "device_del": deletes a front-end pc-dimm device 87 - "object_del": deletes a memory backend object 88 89For example, assuming that the pc-dimm device with id "dimm1" exists, and its memory 90backend is "mem1", the following commands tries to remove it. 91 92 (qemu) device_del dimm1 93 (qemu) object_del mem1 94