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Re: [Qemu-devel] [PATCH 1/2] Dump: introduce a Filesystem in Userspace

From: Petr Tesarik
Subject: Re: [Qemu-devel] [PATCH 1/2] Dump: introduce a Filesystem in Userspace
Date: Tue, 10 May 2016 11:42:08 +0200

On Tue, 10 May 2016 09:48:48 +0100
"Daniel P. Berrange" <address@hidden> wrote:

> On Tue, May 10, 2016 at 07:59:41AM +0200, Petr Tesarik wrote:
> > On Mon, 9 May 2016 09:52:28 -0600
> > Eric Blake <address@hidden> wrote:
> > 
> > > On 05/07/2016 05:32 PM, Nan Li wrote:
> > > > When running the command "dump-guest-memory", we usually need a large 
> > > > space
> > > > of storage to save the dumpfile into disk. It costs not only much time 
> > > > to
> > > > save a file in some of hard disks, but also costs limited storage in 
> > > > host.
> > > > In order to reduce the saving time and make it convenient for users to 
> > > > dump
> > > > the guest memory, we introduce a Filesystem in Userspace (FUSE) to save 
> > > > the
> > > > dump file in RAM. It is selectable in the configure file, adding a 
> > > > compiling
> > > > of package "fuse-devel". It doesn't change the way of dumping guest 
> > > > memory.
> > > 
> > > Why introduce FUSE? Can we reuse NBD instead?
> > 
> > Let me answer this one, because it's me who came up with the idea,
> > although I wasn't involved in the actual implementation.
> > 
> > The idea is to get something more like Linux's /proc/kcore, but for a
> > QEMU guest. So, yes, the same idea could be implemented as a standalone
> > application which talks to QEMU using the gdb remote protocol and
> > exposes the data in a structured form through a FUSE filesystem.
> > 
> > However, the performance of such a solution cannot get even close to
> > that of exposing the data directly from QEMU. Maybe it's still the best
> > way to start the project...
> IIUC, the performance penalty will be related to the copying of guest
> RAM. All the other supplementary information you want (register state
> etc) is low volume, so should not be performance critical to copy that
> over the QMP monitor command or via libvirt monitor command passthrough.

Agreed. Even if the number of guest CPUs ever rises to the order of
thousands, the additional impact is negligible.

> So if want to have an external program provide a /proc/kcore like
> service via FUSE, the problem we need to solve here is a mechanism
> for providing efficient access to QEMU memory.

Indeed. This is the main reason for tinkering with QEMU sources at all.

> I think this can be done quite simply by having QEMU guest RAM exposed
> via tmpfs or hugetlbfs as appropriate. This approach is what is already
> used for the vhost-user network backend in an external process which
> likewise needs copy-free access to guest RAM pages.

Ha! We didn't realize this is an option. We can certainly have a look
at implementing a generic mechanism for mapping QEMU guest RAM from
another process on the host. And yes, this would address any
performance concerns nicely.

> Obviously this requires that users start QEMU in this particular setup
> for RAM, but I don't think that's a particularly onerous requirement
> as any non-trivial management application will already know how to do
> this.

Agreed. This is not an issue. Our main target would be libvirt, which
adds quite a bit of infrastructure already. ;-)

Thanks for your thoughts!

Petr T

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