Re: [Qemu-devel] [PATCH 2/2] docs: add memory-hotplug.txt

[Peter Krempa <pkrempa@redhat.com>]

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On Thu, Feb 26, 2015 at 14:49:16 -0500, Luiz Capitulino wrote:
> This document describes how to use memory hotplug in QEMU.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  docs/memory-hotplug.txt | 77 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 77 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..e821449
> --- /dev/null
> +++ b/docs/memory-hotplug.txt
> @@ -0,0 +1,77 @@
> +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 hot unplug is not supported yet. This means
> +that you're able to add memory, but you're not able to remove it.
> +Also, proper 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

Limit is 256 IIRC.

> + - "maxmem" is the maximum RAM size the guest can grow

And this has to be aligned properly (see reply to 1/2).

> +
> +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 gigas 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 the front-end pc-dimm device and inserts it
> +                 into an 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

The size here is also checked for alignment AFAIK. Also a sane minimum
for linux guests is 128MiB.

> +  (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 commnands:
> +
> +  (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 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

Otherwise a very helpful text :). Shame that it wasn't available sooner
I had to figure out the details myself.

Peter

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