Re: [libvirt PATCHv3 04/12] docs: add virtiofs kbase

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On Mon, Feb 03, 2020 at 04:17:31PM +0000, Daniel P. Berrangé wrote:
> On Thu, Jan 30, 2020 at 06:06:20PM +0100, Ján Tomko wrote:
> > Add a document describing the usage of virtiofs.
> > ---
> >  docs/kbase.html.in      |   3 +
> >  docs/kbase/virtiofs.rst | 152 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 155 insertions(+)
> >  create mode 100644 docs/kbase/virtiofs.rst
> > 
> > diff --git a/docs/kbase.html.in b/docs/kbase.html.in
> > index c156414c41..7d6caf3cb1 100644
> > --- a/docs/kbase.html.in
> > +++ b/docs/kbase.html.in
> > @@ -29,6 +29,9 @@
> >          <dt><a href="kbase/backing_chains.html">Backing chain management</a></dt>
> >          <dd>Explanation of how disk backing chain specification impacts libvirt's
> >            behaviour and basic troubleshooting steps of disk problems.</dd>
> > +
> > +        <dt><a href="kbase/virtiofs.html">Virtio-FS</a></dt>
> > +        <dd>Share a filesystem between the guest and the host</dd>
> >        </dl>
> >      </div>
> >  
> > diff --git a/docs/kbase/virtiofs.rst b/docs/kbase/virtiofs.rst
> > new file mode 100644
> > index 0000000000..e76ff8ca4b
> > --- /dev/null
> > +++ b/docs/kbase/virtiofs.rst
> > @@ -0,0 +1,152 @@
> > +============================
> > +Sharing files with Virtio-FS
> > +============================
> > +
> > +=== 8< delete before merging 8< ===
> > +NOTE: if you're looking at this note, this is just a proposal.
> > +See the up-to-date version on: https://libvirt.org/kbase/virtiofs.html
> > +=== 8< --------------------- 8< ===
> 
> ...this shouldn't be here still
> 
> > +
> > +.. contents::
> > +
> > +=========
> > +Virtio-FS
> > +=========
> > +
> > +Virtio-FS is a shared file system that lets virtual machines access
> > +a directory tree on the host. Unlike existing approaches, it
> > +is designed to offer local file system semantics and performance.
> > +
> > +See https://virtio-fs.gitlab.io/
> > +
> > +==========
> > +Host setup
> > +==========
> > +
> > +The host-side virtiofsd daemon, like other vhost-user backed devices,
> > +requres shared memory between the host and the guest. As of QEMU 4.2, this
> 
> requires
> 
> > +requires specifying a NUMA topology for the guest and explicitly specifying
> > +a memory backend. Multiple options are available:
> > +
> > +Either of the following:
> > +
> > +\a) Use file-backed memory
> 
> Why are you disabling list-ification here & throughout the file ?
> 
> > +
> > +Configure the directory where the files backing the memory will be stored
> > +with the ``memory_backing_dir`` option in ``/etc/libvirt/qemu.conf``
> > +
> > +::
> > +
> > +   # This directory is used for memoryBacking source if configured as file.
> > +   # NOTE: big files will be stored here
> > +   memory_backing_dir = "/dev/shm/"
> > +
> > +\b) Use hugepage-backed memory
> > +
> > +Make sure there are enough huge pages allocated for the requested guest memory.
> > +For example, for one guest with 2 GiB of RAM backed by 2 MiB hugepages:
> > +
> > +::
> > +
> > +   # virsh allocpages 2M 1024
> 
> 
> I'm not understanding from this how big the files will be for
> virtiofsd ?  How much memory should users expect to be needed
> per-guest per-virtiofsd ?

Oh, I realize on re-reading that this part of the docs is just about
the general setup of a file backed guest. So we're just talking about
guest RAM size here, virtiofsd isn't using this file backing for
anything else. IOW, ignore this comment.


Regards,
Daniel
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