mirror of https://gitee.com/openkylin/qemu.git
94 lines
3.3 KiB
Plaintext
94 lines
3.3 KiB
Plaintext
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
|