On 08/08/2011 04:20 AM, Dor Laor wrote:
On 08/08/2011 06:24 AM, Isaku Yamahata wrote:
This mail is on "Yabusame: Postcopy Live Migration for Qemu/KVM"
on which we'll give a talk at KVM-forum.
The purpose of this mail is to letting developers know it in advance
so that we can get better feedback on its design/implementation approach
early before our starting to implement it.
Background
==========
* What's is postcopy livemigration
It is is yet another live migration mechanism for Qemu/KVM, which
implements the migration technique known as "postcopy" or "lazy"
migration. Just after the "migrate" command is invoked, the execution
host of a VM is instantaneously switched to a destination host.
The benefit is, total migration time is shorter because it transfer
a page only once. On the other hand precopy may repeat sending same pages
again and again because they can be dirtied.
The switching time from the source to the destination is several
hunderds mili seconds so that it enables quick load balancing.
For details, please refer to the papers.
We believe this is useful for others so that we'd like to merge this
feature into the upstream qemu/kvm. The existing implementation that
we have right now is very ad-hoc because it's for academic research.
For the upstream merge, we're starting to re-design/implement it and
we'd like to get feedback early. Although many improvements/optimizations
are possible, we should implement/merge the simple/clean, but extensible
as well, one at first and then improve/optimize it later.
postcopy livemigration will be introduced as optional feature. The
existing
precopy livemigration remains as default behavior.
* related links:
project page
http://sites.google.com/site/grivonhome/quick-kvm-migration
Enabling Instantaneous Relocation of Virtual Machines with a
Lightweight VMM Extension,
(proof-of-concept, ad-hoc prototype. not a new design)
http://grivon.googlecode.com/svn/pub/docs/ccgrid2010-hirofuchi-paper.pdf
http://grivon.googlecode.com/svn/pub/docs/ccgrid2010-hirofuchi-talk.pdf
Reactive consolidation of virtual machines enabled by postcopy live
migration
(advantage for VM consolidation)
http://portal.acm.org/citation.cfm?id=1996125
http://www.emn.fr/x-info/ascola/lib/exe/fetch.php?media=internet:vtdc-postcopy.pdf
Qemu wiki
http://wiki.qemu.org/Features/PostCopyLiveMigration
Design/Implementation
=====================
The basic idea of postcopy livemigration is to use a sort of distributed
shared memory between the migration source and destination.
The migration procedure looks like
- start migration
stop the guest VM on the source and send the machine states except
guest RAM to the destination
- resume the guest VM on the destination without guest RAM contents
- Hook guest access to pages, and pull page contents from the source
This continues until all the pages are pulled to the destination
The big picture is depicted at
http://wiki.qemu.org/File:Postcopy-livemigration.png
That's terrific (nice video also)!
Orit and myself had the exact same idea too (now we can't patent it..).
Advantages:
- No down time due to memory copying.
But non-deterministic down time due to network latency while trying to
satisfy a page fault.
- Efficient, reduce needed traffic no need to re-send pages.
It's not quite that simple. Post-copy needs to introduce a protocol
capable of requesting pages.
I think in presenting something like this, it's important to collect
quite a bit of performance data. I'd suggest doing runs while running
jitterd in the guest to attempt to quantify the actual downtime
experienced too.
http://git.codemonkey.ws/cgit/jitterd.git/
There's a lot of potential in something like this, but it's not obvious
to me whether it's a net win. Should make for a very interesting
presentation :-)
- Reduce overall RAM consumption of the source and destination
as opposed from current live migration (both the source and the
destination allocate the memory until the live migration
completes). We can free copied memory once the destination guest
received it and save RAM.
- Increase parallelism for SMP guests we can have multiple
virtual CPU handle their demand paging . Less time to hold a
global lock, less thread contention.
- Virtual machines are using more and more memory resources ,
for a virtual machine with very large working set doing live
migration with reasonable down time is impossible today.
This is really just a limitation of our implementation. In theory,
pre-copy allows you to exert fine grain resource control over the guest
which you can use to encourage convergence.
Disadvantageous:
- During the live migration the guest will run slower than in
today's live migration. We need to remember that even today
guests suffer from performance penalty on the source during the
COW stage (memory copy).
- Failure of the source or destination or the network will cause
us to lose the running virtual machine. Those failures are very
rare.
In case there is shared storage we can store a copy of the
memory there , that can be recovered in case of such failure .
Overall, it looks like a better approach for the vast majority of cases.
Hope it will get merged to kvm and become the default way.
One thing I think we need to do is put together a live migration
roadmap. We've got a lot of invasive efforts underway with live
migration and I fear that without some planning and serialization, some
of this useful work with get lost.
Regards,
Anthony Liguori
There are several design points.
- who takes care of pulling page contents.
an independent daemon vs a thread in qemu
The daemon approach is preferable because an independent daemon would
easy for debug postcopy memory mechanism without qemu.
If required, it wouldn't be difficult to convert a daemon into
a thread in qemu
- connection between the source and the destination
The connection for live migration can be re-used after sending machine
state.
- transfer protocol
The existing protocol that exists today can be extended.
- hooking guest RAM access
Introduce a character device to handle page fault.
When page fault occurs, it queues page request up to user space daemon
at the destination. And the daemon pulls page contents from the source
and serves it into the character device. Then the page fault is
resovlved.
Isn't there a simpler way of using madvise verb to mark that the
destination guest RAM will need paging?
Cheers and looking forward to the presentation over the kvm forum,
Dor
* More on hooking guest RAM access
There are several candidate for the implementation. Our preference is
character device approach.
- inserting hooks into everywhere in qemu/kvm
This is impractical
- backing store for guest ram
a block device or a file can be used to back guest RAM.
Thus hook the guest ram access.
pros
- new device driver isn't needed.
cons
- future improvement would be difficult
- some KVM host feature(KSM, THP) wouldn't work
- character device
qemu mmap() the dedicated character device, and then hook page fault.
pros
- straght forward approach
- future improvement would be easy
cons
- new driver is needed
- some KVM host feature(KSM, THP) wouldn't work
They checks if a given VMA is anonymous. This can be fixed.
- swap device
When creating guest, it is set up as if all the guest RAM is swapped out
to a dedicated swap device, which may be nbd disk (or some kind of user
space block device, BUSE?).
When the VM tries to access memory, swap-in is triggered and IO to the
swap device is issued. Then the IO to swap is routed to the daemon
in user space with nbd protocol (or BUSE, AOE, iSCSI...). The daemon
pulls
pages from the migration source and services the IO request.
pros
- After the page transfer is complete, everything is same as normal case.
- no new device driver isn't needed
cons
- future improvement would be difficult
- administration: setting up nbd, swap device
Thanks in advance
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