Re: [Xen-devel] [PATCH RFC] Live migration for VMs with QEMU backed local storage

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jun 23, 2017 at 03:42:20AM -0400, Bruno Alvisio wrote:
> This patch is the first attempt on adding live migration of instances with 
> local
> storage to Xen. This patch just handles very restricted case of fully
> virtualized HVMs. The code uses the "drive-mirror" capability provided by 
> QEMU.
> A new "-l" option is introduced to "xl migrate" command. If provided, the 
> local
> disk should be mirrored during the migration process. If the option is set,
> during the VM creation a qemu NBD server is started on the destination. After
> the instance is suspended on the source, the QMP "disk-mirror" command is 
> issued
> to mirror the disk to destination. Once the mirroring job is complete, the
> migration process continues as before. Finally, the NBD server is stopped 
> after
> the instance is successfully resumed on the destination node.
> 
> A major problem with this patch is that the mirroring of the disk is performed
> only after the memory stream is completed and the VM is suspended on the 
> source;
> thus the instance is frozen for a long period of time. The reason this happens
> is that the QEMU process (needed for the disk mirroring) is started on the
> destination node only after the memory copying is completed. One possibility I
> was considering to solve this issue (if it is decided that this capability
> should be used): Could a "helper" QEMU process be started on the destination
> node at the beginning of the migration sequence with the sole purpose of
> handling the disk mirroring and kill it at the end of the migration sequence? 
> 

In theory we could, but I am very cautious about this. I _think_ we can
change the timing QEMU is started. It can be started earlier, but take
precaution that it shouldn't resume the guest.

In any case, start with the simple setup first.

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