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Re: [Xen-devel] vbd flushing during migration?

  • To: "John Byrne" <john.l.byrne@xxxxxx>
  • From: "Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>
  • Date: Mon, 31 Jul 2006 12:56:58 -0700
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 31 Jul 2006 12:57:29 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

It's slightly more than a flush that's required.  The migration
protocol needs to be extended so that execution on the target host
doesn't start until all of the outstanding (i.e. issued by the
backend) block requests have been either cancelled or acknowledged.
This should be pretty straight forward given that the backend driver
ref counts a blkif's state based on pending requests, and won't tear
down the backend directory in xenstore until all the outstanding
requests have cleared.  All that is likely required is to have the
migration code register watches on the backend vbd directories, and
wait for them to disappear before giving the all-clear to the new

We've talked about this enough to know how to fix it, but haven't had
a chance to hack it up.  (I think Julian has looked into the problem a
bit for blktap, but not yet done a general fix.) Patches would
certainly be welcome though. ;)


On 7/31/06, John Byrne <john.l.byrne@xxxxxx> wrote:


I don't see any obvious flush to disk taking place for vbd's on the
source host in XendCheckpoint.py before the domain is started on the new
host. Is there a guarantee that all written data is on disk somewhere
else or is something needed?


John Byrne

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