Re: [Qemu-devel] [PATCH 04/27] add a version number to ram_list

[Juan Quintela <quintela@redhat.com>]

Michael Roth <mdroth@linux.vnet.ibm.com> wrote:
> On Tue, Jul 24, 2012 at 08:36:29PM +0200, Juan Quintela wrote:
>> From: Umesh Deshpande <udeshpan@redhat.com>
>> 
>> This will be used to detect if last_block might have become invalid
>> across different calls to ram_save_live.

>
> Was it a bug in the old code to not do this? In the unthreaded version
> it looks like last_block could still become invalid between iterations if it
> was removed in the meantime, so I'm trying to understand why it's needed
> now.

Old code assumed that we _never_ do hotplug/unplug of anything during
migration (I am still not convinced that everything works if we plug
some devices, but now at least memory works).

> And why do we re-sort when this happens? The old implementation didn't
> seem to place any guarantees on the block ordering beyond the initial
> sorting by block->idstr. After that they were sent in MRU order.

It is a "subtle" attempt to try to get the memory in order when we
suspend to disk.  So we could compare two files done while we were
stopped.  And no, I didn't liked when it was introduced.

Later, Juan.