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QEMU memory hotplug
===================
This document explains how to use the memory hotplug feature in QEMU,
which is present since v2.1.0.
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 [...] -m 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, an 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
RAM hot-unplug
---------------
In order to be able to hot unplug pc-dimm device, QEMU has to be told the ids
of pc-dimm device and memory backend object. The ids were assigned when you hot
plugged memory.
Two monitor commands are used to hot unplug memory:
- "device_del": deletes a front-end pc-dimm device
- "object_del": deletes a memory backend object
For example, assuming that the pc-dimm device with id "dimm1" exists, and its memory
backend is "mem1", the following commands tries to remove it.
(qemu) device_del dimm1
(qemu) object_del mem1
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