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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 1/2] Dump: introduce a Filesystem in Userspace
Date: Tue, 10 May 2016 10:56:42 +0100
User-agent: Mutt/1.6.0 (2016-04-01)

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...

If you want no overhead and are willing to pause the guest, use QEMU's
gdb stub (directly, no extra FUSE file system layer).  If you cannot
pause the guest then take a copy of memory with dump-guest-memory to

There might be a middle-ground where you can copy-on-write pages and let
the guest continue to run, but this is probably not worth the

I find it hard to see where adding more code or using FUSE would make
things better?


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