On Tue, May 10, 2016 at 08:19:38AM +0200, Petr Tesarik wrote: > On Mon, 9 May 2016 17:32:50 +0100 > "Daniel P. Berrange" <berra...@redhat.com> wrote: > > > On Mon, May 09, 2016 at 06:20:22PM +0200, Petr Tesarik wrote: > > > On Mon, 9 May 2016 17:13:07 +0100 > > > "Daniel P. Berrange" <berra...@redhat.com> wrote: > > > > > > > On Mon, May 09, 2016 at 09:52:28AM -0600, Eric Blake 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? > > > > > > > > The commit message talks of letting QEMU dump to RAM avoiding disk I/O. > > > > IOW, it seems like it could just dump to any tmpfs directory. > > > > > > > > I'm not really seeing a compelling reason why QEMU needs to mount a fuse > > > > filesystem itself - whatever app is using QEMU could handle mounting of > > > > fs without QEMU's involvement at all. > > > > > > The ultimate goal is to export internal QEMU state (memory content, > > > register values) as an ELF file, so you could simply reuse any existing > > > tools that can work with ELF dump files (gdb, crash, makedumpfile, > > > readelf, etc.) instead of re-inventing the wheel for each of those > > > tools. > > > > > > This cannot be really done from outside of QEMU without too much > > > overhead (how would you access guest memory from outside QEMU?). > > > > Maybe I'm missing something, but IIUC the 'dump-guest-memory' monitor > > command in QEMU already dumps in ELF format which can be used by standard > > ELF tools. If you don't want that dump to hit disk, then you could mount > > a tmpfs and then tell QEMU to write to that. > > It's not the same kind of beast: > > 1. You need double the amount of RAM in the host. Oh, yes, some > folks like to create VMs with a RAM size of a few hundred GBs of > RAM, and then it may not be negligible... > > 2. The memory must still be copied. This is made a bit worse by the > fact that tmpfs does not pre-allocate enough RAM, so even copying > a few GBs takes several seconds. > > 3. Most importantly, if the file is created on the fly, it's a live > memory source, i.e. repeated reads will reflect changes in the > running guest. > > Some use cases are substantially slower with the dump-then-use > approach. For example, makedumpfile can estimate the resulting dump > size based on data from the running kernel. It reads only a tiny > portion of system RAM to do the analysis, but since only makedumpfile > knows the exact addresses, you would still need a full dump for that. > > With the FUSE approach, guest pages are served on demand when the > application requests them.
AFAICT, what you describe here is not what this patch set is actually doing. This patch isn't modifying the guest-dump-memory monitor command at all - it is just mounting a fuse filesystem and saying you use the guest-dump-memory as normal to write to that filesystem. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
