MAX_VM_CMD_PACKAGED_SIZE is a constant used in qemu_savevm_send_packaged
and loadvm_handle_cmd_packaged to determine whether a package is too
big to be sent or received. qemu_savevm_send_packaged is called inside
postcopy_start (migration/migration.c) to send the MigrationState
in a single blob to the destination, using the MIG_CMD_PACKAGED subcommand,
which will read it up using loadvm_handle_cmd_packaged. If the blob is
larger than MAX_VM_CMD_PACKAGED_SIZE, an error is thrown and the postcopy
migration is aborted. Both MAX_VM_CMD_PACKAGED_SIZE and MIG_CMD_PACKAGED
were introduced by commit 11cf1d984b ("MIG_CMD_PACKAGED: Send a packaged
chunk ..."). The constant has its original value of 1ul << 24 (16MB).
The current MAX_VM_CMD_PACKAGED_SIZE value is not enough to support postcopy
migration of bigger pseries guests. The blob size for a postcopy migration of
a pseries guest with the following setup:
qemu-system-ppc64 --nographic -vga none -machine pseries,accel=kvm -m 64G \
-smp 1,maxcpus=32 -device virtio-blk-pci,drive=rootdisk \
-drive file=f27.qcow2,if=none,cache=none,format=qcow2,id=rootdisk \
-netdev user,id=u1 -net nic,netdev=u1
Goes around 12MB. Bumping the RAM to 128G makes the blob sizes goes to 20MB.
With 256G the blob goes to 37MB - more than twice the current maximum size.
At this moment the pseries machine can handle guests with up to 1TB of RAM,
making this postcopy blob goes to 128MB of size approximately.
Following the discussions made in [1], there is a need to understand what
devices are aggressively consuming the blob in that manner and see if that
can be mitigated. Until then, we can set MAX_VM_CMD_PACKAGED_SIZE to the
maximum value allowed. Since the size is a 32 bit int variable, we can set
it as 1ul << 32, giving a maximum blob size of 4G that is enough to support
postcopy migration of 32TB RAM guests given the above constraints.
[1] https://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg06313.html
Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Reported-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
https://qemu.org/Hosts/Linux
https://qemu.org/Hosts/Mac
https://qemu.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu.org/qemu.git
When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
https://qemu.org/Contribute/SubmitAPatch
https://qemu.org/Contribute/TrivialPatches
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
https://qemu.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
https://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
https://qemu.org/Contribute/StartHere
-- End