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Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big eno


From: 刘海伟
Subject: Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished
Date: Sat, 10 Sep 2022 10:47:02 +0000

hi,i have a question
if failed in migration using post-copy mode,is there some way to restore the 
memory data back to soucre VM?




发自我的小米
在 Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>,2022年9月10日 下午6:18写道:

On 9/10/22 09:35, liuhaiwei wrote:
> From: liuhaiwei <liuhaiwei@inspur.com>
>
> bug description as  https://gitlab.com/qemu-project/qemu/-/issues/1203
> Usually,we use the precopy or postcopy mode to migrate block dirty bitmap.
> but if block-dirty-bitmap size more than threshold size,we cannot entry the 
> migration_completion in migration_iteration_run function
> To solve this problem, we can setting  the pending size to a fake 
> value(threshold-1 or 0) to tell  migration_iteration_run function to entry 
> the migration_completion,if pending size > threshold size
>


Actually, bitmaps migrate in postcopy. So, you should start postcopy for it to 
work (qmp command migrate-start-postcopy). This command simply set the boolean 
variable, so that in migration_iteration_run() we'll move to postcopy when 
needed. So, you can start this command immediately after migrate command, or 
even before it, but after setting the "dirty-bitmaps" capability.

Fake pending is a wrong thing to do, it means that you will make downtime to be 
larger than expected.

--
Best regards,
Vladimir

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