94 lines
3.3 KiB
Plaintext
94 lines
3.3 KiB
Plaintext
QEMU memory hotplug
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===================
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This document explains how to use the memory hotplug feature in QEMU,
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which is present since v2.1.0.
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Guest support is required for memory hotplug to work.
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Basic RAM hotplug
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-----------------
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In order to be able to hotplug memory, QEMU has to be told how many
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hotpluggable memory slots to create and what is the maximum amount of
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memory the guest can grow. This is done at startup time by means of
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the -m command-line option, which has the following format:
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-m [size=]megs[,slots=n,maxmem=size]
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Where,
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- "megs" is the startup RAM. It is the RAM the guest will boot with
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- "slots" is the number of hotpluggable memory slots
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- "maxmem" is the maximum RAM size the guest can have
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For example, the following command-line:
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qemu [...] -m 1G,slots=3,maxmem=4G
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Creates a guest with 1GB of memory and three hotpluggable memory slots.
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The hotpluggable memory slots are empty when the guest is booted, so all
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memory the guest will see after boot is 1GB. The maximum memory the
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guest can reach is 4GB. This means that three additional gigabytes can be
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hotplugged by using any combination of the available memory slots.
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Two monitor commands are used to hotplug memory:
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- "object_add": creates a memory backend object
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- "device_add": creates a front-end pc-dimm device and inserts it
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into the first empty slot
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For example, the following commands add another 1GB to the guest
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discussed earlier:
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(qemu) object_add memory-backend-ram,id=mem1,size=1G
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(qemu) device_add pc-dimm,id=dimm1,memdev=mem1
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Using the file backend
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----------------------
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Besides basic RAM hotplug, QEMU also supports using files as a memory
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backend. This is useful for using hugetlbfs in Linux, which provides
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access to bigger page sizes.
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For example, assuming that the host has 1GB hugepages available in
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the /mnt/hugepages-1GB directory, a 1GB hugepage could be hotplugged
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into the guest from the previous section with the following commands:
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(qemu) object_add memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1GB
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(qemu) device_add pc-dimm,id=dimm1,memdev=mem1
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It's also possible to start a guest with memory cold-plugged into the
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hotpluggable memory slots. This might seem counterintuitive at first,
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but this allows for a lot of flexibility when using the file backend.
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In the following command-line example, an 8GB guest is created where 6GB
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comes from regular RAM, 1GB is a 1GB hugepage page and 256MB is from
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2MB pages. Also, the guest has additional memory slots to hotplug more
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2GB if needed:
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qemu [...] -m 6GB,slots=4,maxmem=10G \
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-object memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1G \
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-device pc-dimm,id=dimm1,memdev=mem1 \
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-object memory-backend-file,id=mem2,size=256M,mem-path=/mnt/hugepages-2MB \
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-device pc-dimm,id=dimm2,memdev=mem2
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RAM hot-unplug
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---------------
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In order to be able to hot unplug pc-dimm device, QEMU has to be told the ids
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of pc-dimm device and memory backend object. The ids were assigned when you hot
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plugged memory.
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Two monitor commands are used to hot unplug memory:
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- "device_del": deletes a front-end pc-dimm device
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- "object_del": deletes a memory backend object
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For example, assuming that the pc-dimm device with id "dimm1" exists, and its memory
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backend is "mem1", the following commands tries to remove it.
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(qemu) device_del dimm1
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(qemu) object_del mem1
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