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Re: [Qemu-devel] Question about vNVDIMM file format

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] Question about vNVDIMM file format
Date: Mon, 16 May 2016 19:25:08 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

On Mon, May 16, 2016 at 09:53:36AM -0700, Stefan Hajnoczi wrote:
> On Mon, May 16, 2016 at 04:04:01PM +0100, Richard W.M. Jones wrote:
> > I'm playing with ext4 and DAX.
> > 
> > I'm using:
> > 
> >   -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \
> >   -device nvdimm,memdev=mem1,id=nv1
> > 
> > where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition
> > table).  I can mount this in the guest using:
> > 
> >   mount -o dax /dev/pmem0 /mnt
> > 
> > and everything appears to work.
> > 
> > I read in the mailing list that the pmem file has some internal
> > structure for storing config data, stored in the last 128 KB of the
> > file.  Is that still the case?
> AFAICT qemu.git/master does not support the ACPI _DSM for namespace
> configuration.  That means the entire /var/tmp/pmem should be visible.

That's great, thanks both for your answers.

FWIW I was able to add support to libguestfs -- at least for the
"direct" backend where we run qemu directly.  Unfortunately libvirt
does not support the vNVDIMM device yet.

I have posted the two patches needed on our mailing list.  There seems
to be some delay in our mail server, so they aren't in the archives


There are a few possible problems / questions I have:

(a) How necessary is the ACPI dependency?  We disable ACPI because it
is quite slow, adding something like 150-200ms to the boot process
(every millisecond counts for us!).  Because I previously never needed
ACPI, I never really looked into why this is, and it could be
something quite simple, so I'm going to look at this issue next.  I
understand that NVDIMMs are not regular (eg) PCI devices, so ordinary
device probing isn't going to work, and that probably answers the
question why you need to use ACPI.

(b) Could you describe what the 3 modules (nd_btt, nd_pmem, nfit) do?
Are all 3 modules necessary in the guest kernel?

(c) I've got the root filesystem (which is actually ext2, but using
the ext4.ko driver) mounted with -o dax.  What benefits / differences
should I observe?  Just general reduced memory / page cache usage?

(d) If, in future, you add the namespace metadata, what tools will be
available on the host to create a packed filesystem + metadata?
Assuming that we won't be able to export just a filesystem as I am
doing now.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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