[libvirt] [PATCHv4 0/3] Add support for file memorybacking

[Pavel Hrdina]

On Mon, Jan 30, 2017 at 12:06:19PM +0000, Safka, JaroslavX wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel P. Berrange [mailto:berrange at redhat.com]
> > Sent: Monday, January 30, 2017 12:35 PM
> > To: Michal Privoznik <mprivozn at redhat.com>
> > Cc: Safka, JaroslavX <jaroslavx.safka at intel.com>; libvir-list at redhat.com;
> > Mooney, Sean K <sean.k.mooney at intel.com>; Ptacek, MichalX
> > <michalx.ptacek at intel.com>
> > Subject: Re: [libvirt] [PATCHv4 0/3] Add support for file memorybacking
> > 
> > On Sat, Jan 28, 2017 at 03:03:02PM +0100, Michal Privoznik wrote:
> > > On 13.12.2016 13:12, Jaroslav Safka wrote:
> > > > Hi,
> > > > we would like to introduce 3 new elements source,access and allocation in
> > memoryBacking element.
> > > > For now it was made for numa topology.
> > > >
> > > > <memoryBacking>
> > > >      <source type="file|anonymous"/>
> > > >      <access mode="shared|private"/>
> > > >      <allocation mode="immediate|ondemand"/> </memoryBacking>
> > > >
> > > > If allocation is immediate then -mem-prealloc should be added to the qemu
> > commanline.
> > > > If source is file then
> > > > -object memory-backend-file,id=mem,size=1024M,mem-path=*lib dir
> > > > path* -numa node,memdev=mem Will be added to the qemu commandline
> > > >
> > > > If access is shared then the "share=on" parameter will be added to the
> > memory-backend-file e.g.
> > > > -object
> > > > memory-backend-file,id=mem,size=1024M,mem-
> > path=/var/lib/libvirt/qemu
> > > > ,share=on
> > > >
> > > > The access mode can be overriden by specifying token memAccess in numa
> > cell.
> > > >
> > > > The test cpu-numa-memshared was removed, because behaviour was
> > > > changed and is not needed anymore
> > >
> > > I beg to disagree. What if you don't have any <memoryBacking/>?
> > >
> > > I like these patches, but I'm not certainly sure about:
> > > a) domain XML (in the past we used to require an ACK on schema change
> > > from one of the Dans)
> > 
> > What are you unsure about ?  This XML is what I suggested in previous rounds
> > of discussion.
> > 
> > > b) the location for qemu to create its mmaped files (patch 3/3).
> > > cfg->libDir looks very suspicious.
> > 
> > Well we've got two possibilities - source=anonymous, should be using
> > /dev/shm, similar to how we do huge pages. For source=file, we need a real
> > filesystem. Something under /var/lib/libvirt is reasonable.
> > Perhaps a dedicated dir is needed ? eg /var/lib/libvirt/ram in order to provide a
> > point where the admin can mount a sufficiently large filesystem ? Or  make it
> > configurable in /etc/libvirt/qemu.conf
> > 
> 
> [Jarek] It will be ok when I do something like libDir + "/ram". Or It should be configurable on compilation time?

Oh, I see that Dan already suggested to make it configurable.  I would say
both.  Make it configurable in qemu.conf and use /var/lib/libvirt/ram as
a default path.

Pavel
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