Re: [Xen-users] Relocation

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I'm new to this list, i checked the archives but can't find anything.

I'm not sure if anyone replied to this...  but here goes.

> I downloaded some documents (powerpoint shows, pdf files), and i'm
> fascinated by the Relocation feature of Xen. How does it work? What do i
> need for that?

To run it, you need two hosts running Xen and a guest with a network-based 
filesystem (i.e. a network block device, iSCSI, NFS root, or a SAN, etc).  
The network-based filesystem is required because after the migration the 
guest can't access the disks of the origin machine.

Live migration works by pre-copying the memory image of the virtual machine 
*whilst it's running* to the destination.  Obviously some parts of the memory 
will be updated by the running guest after the data has been copied.  A 
feature of Xen called "shadow logdirty mode" is used by the migration tool to 
record *which* parts of the memory were updated.  This data can then be 
resent to the destination host.

The pre-copying phase works in a cycle: the migration code repeatedly tries to 
copy the memory the guest modified to the destination so that it'll have an 
up-to-date copy.  The copying gets faster and faster in order to "race" with 
the virtual machine to get as much data pre-copied as possible.

When the guest is modifying data as fast as the migration code can send it, 
the guest is stopped.  The live migration code then sends the remaining data 
to the other end, and tells the destination machine to start its copy of the 
guest.  Doing this typically takes a few hundred milliseconds during which 
the guest is inactive.

> Also, is there also a way to do fail-over of a virtual machine? I know I
> can build clusters using OpenSSI and OpenMosix, but then existing TCP/IP
> connections are lost.

You can use standard mechanisms (network-based heartbeats, drbd, etc) to do 
failover, as you would for normal hosts.  More advanced techniques such as 
keeping a copy of a virtual machine running in-step on another host are being 
worked on.

Cheers,
Mark

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