On Tue, 3 Mar 2015 09:47:26 -0500 Luiz Capitulino <lcapitul...@redhat.com> wrote:
> This document describes how to use memory hotplug in QEMU. > > Signed-off-by: Luiz Capitulino <lcapitul...@redhat.com> Reviewed-by: Igor Mammedov <imamm...@redhat.com> > --- > docs/memory-hotplug.txt | 76 > +++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 76 insertions(+) > create mode 100644 docs/memory-hotplug.txt > > diff --git a/docs/memory-hotplug.txt b/docs/memory-hotplug.txt > new file mode 100644 > index 0000000..f70571d > --- /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
