Re: [Xen-devel] Xen and migration

[Jacob Gorm Hansen <jg@xxxxxx>]

On Thu, Oct 02, 2003 at 10:25:38PM +0100, Ian Pratt wrote:

> We'd probably modify the guest OS to cause it to evict the entire
> buffer cache before asking for a suspend, to minimize the state.

I spent quite a lot of time dealing with the issues of suspending and resuming
guest images in NomadBIOS (which is based on L4 and L4Linux), I did not have
the problem of having to change the guest pagetables because L4 provides
recursive address spaces, but other resources such as task numbers had to be
remapped instead. I ended up needing quite a bit of cooperation from the guest
and its applications (through a special signal handler, everything is
transparent to user programs), but in the end it got to work quite well. A good
thing about L4 is the entire address space of a guest can be unmapped, and
resulting page faults be propagated to the host environment, which allows for
the use of 'precopy' migration, effectively running the guest on the
originating host while copying pages across, until the working set is
sufficiently small that an outside client will not even notice that the system
is suspended, migrated, and resumed on the new host (typical downtime is less
than 1/10th of a second for a loaded system).

About the buffer cache I am not sure it matters, perhaps better to use good
checksums to eliminate duplicate page transfers of identical pages, as the disk
image will have to be copied as well, and - in the case of a network filesystem
- the cached pages may have to be refetched from the file server if evicted.

> 
> Volunteers welcome ;-)

I am currently awaiting a response to my Ph.D. application, I was planning to
continue work on NomadBIOS in a Grid setting, but perhaps we can find ways to
cooperate, I am impressed by the thoroughness of the Xen-paper. If I get the
funding I will start my work around November or December.

If you have the time, please have a look at http://www.nomadbios.dk.

> Cheers,
> Ian

best,
Jacob



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