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

[Igor Mammedov]

On Mon, 2 Mar 2015 15:52:48 +0100
Peter Krempa <address@hidden> wrote:

> 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 <address@hidden>
> > ---
> >  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.
I've avoided mentioning minimum size limits as it's pure guest OS limitation
that has nothing to do with hardware (QEMU).
For example windows works just fine with 2M sizes.

> 
> > +  (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