diff options
| -rw-r--r-- | docs/memory-hotplug.txt | 76 |
1 files changed, 76 insertions, 0 deletions
diff --git a/docs/memory-hotplug.txt b/docs/memory-hotplug.txt new file mode 100644 index 0000000000..f70571df0c --- /dev/null +++ b/docs/memory-hotplug.txt @@ -0,0 +1,76 @@ +QEMU memory hotplug +=================== + +This document explains how to use the memory hotplug feature in QEMU, +which is present since v2.1.0. + +Please, note that memory hotunplug is not supported yet. This means +that you're able to add memory, but you're not able to remove it. +Also, proper guest support is required for memory hotplug to work. + +Basic RAM hotplug +----------------- + +In order to be able to hotplug memory, QEMU has to be told how many +hotpluggable memory slots to create and what is the maximum amount of +memory the guest can grow. This is done at startup time by means of +the -m command-line option, which has the following format: + + -m [size=]megs[,slots=n,maxmem=size] + +Where, + + - "megs" is the startup RAM. It is the RAM the guest will boot with + - "slots" is the number of hotpluggable memory slots + - "maxmem" is the maximum RAM size the guest can have + +For example, the following command-line: + + qemu [...] 1G,slots=3,maxmem=4G + +Creates a guest with 1GB of memory and three hotpluggable memory slots. +The hotpluggable memory slots are empty when the guest is booted, so all +memory the guest will see after boot is 1GB. The maximum memory the +guest can reach is 4GB. This means that three additional gigabytes can be +hotplugged by using any combination of the available memory slots. + +Two monitor commands are used to hotplug memory: + + - "object_add": creates a memory backend object + - "device_add": creates a front-end pc-dimm device and inserts it + into the first empty slot + +For example, the following commands add another 1GB to the guest +discussed earlier: + + (qemu) object_add memory-backend-ram,id=mem1,size=1G + (qemu) device_add pc-dimm,id=dimm1,memdev=mem1 + +Using the file backend +---------------------- + +Besides basic RAM hotplug, QEMU also supports using files as a memory +backend. This is useful for using hugetlbfs in Linux, which provides +access to bigger page sizes. + +For example, assuming that the host has 1GB hugepages available in +the /mnt/hugepages-1GB directory, a 1GB hugepage could be hotplugged +into the guest from the previous section with the following commands: + + (qemu) object_add memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1GB + (qemu) device_add pc-dimm,id=dimm1,memdev=mem1 + +It's also possible to start a guest with memory cold-plugged into the +hotpluggable memory slots. This might seem counterintuitive at first, +but this allows for a lot of flexibility when using the file backend. + +In the following command-line example, a 8GB guest is created where 6GB +comes from regular RAM, 1GB is a 1GB hugepage page and 256MB is from +2MB pages. Also, the guest has additional memory slots to hotplug more +2GB if needed: + + qemu [...] -m 6GB,slots=4,maxmem=10G \ + -object memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1G \ + -device pc-dimm,id=dimm1,memdev=mem1 \ + -object memory-backend-file,id=mem2,size=256M,mem-path=/mnt/hugepages-2MB \ + -device pc-dimm,id=dimm2,memdev=mem2 |