[Xen-API] Move blktap2.reset_leaf call to new SR command "epoch_begin"

[Mike McClurg <mike.mcclurg@xxxxxxxxxx>]

Hi Andrei,

(Note that I'm CC'ing the xen-api list to encourage more open
development discussions.)

I'm working on the VDI reset bug we talked about last week, where
setting on-boot=reset on a VDI, and then migrating the VM, triggers the
VDI reset after the migration is complete, because VDI reset is called
on VDI attach.

The fix for this should be to only call blktap2.reset_leaf when the VDI
attach operation is performed during a VM start or reboot. There is a
set of new SMAPIv2 calls in Xapi: begin_epoch, end_epoch, which are
called by Xapi when a VM is started and stopped (respectively), and also
during a VM reboot. These functions are not called during a VM migrate.

My proposal is that we add two new SR commands, begin_epoch and
end_epoch, and that we move the VDI reset code into begin_epoch. The two
corresponding Xapi functions would then call these SR commands.

Does this sound like a good solution to the migrate triggers VDI reset
bug? Did I miss anything here? Thanks,

Mike

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