Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished

Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished

[Seaway Liu(刘海伟)]

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

Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished

[Vladimir Sementsov-Ogievskiy]

Hi!

On 9/10/22 13:47, Seaway Liu(刘海伟) wrote:
> 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?
> 


As far as I understand, no, there is not.

Postcopy started actually means: target has started. So, RAM is touched by target VM process, no way to rollback.

Still, things are not so bad: when you enable dirty-bitmaps capability, but not postcopy-ram capability, RAM is migrated in precopy as usual. So, when target started, the only thing that is not yet migrated is dirty bitmap. So, in worst case (migration failure after postcopy started) you'll loose your dirty bitmap. VM is migrated and normally running on target. Unfinished bitmaps on target are automatically released (see cancel_incoming_locked()). So, in worst case you'll have to start your incremental backup chain from a new full-backup.

> 
> 
> 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


-- 
Best regards,
Vladimir

Re:Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished

[liuhaiwei9699]

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sometimes ,post-copy mode is not the best choice. For instance, Supposing migrate process will take ten minutes,but network may be interruptted In this process .
If it does happenthe , memory data of VM will be splitted into two parts, and will not be rollback.This is a bad situation


so  migrate-start-postcopy will not be setted in conservative scenario. In this case, the migration with block dirty bitmap may not be finished.




The migration of block dirty bitmap should not dependent on post-copy or pre-copy mode.


At 2022-09-10 18:58:12, "Vladimir Sementsov-Ogievskiy" <vsementsov@yandex-team.ru> wrote:
>Hi!
>
>On 9/10/22 13:47, Seaway Liu(刘海伟) wrote:
>> 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?
>> 
>
>
>As far as I understand, no, there is not.
>
>Postcopy started actually means: target has started. So, RAM is touched by target VM process, no way to rollback.
>
>Still, things are not so bad: when you enable dirty-bitmaps capability, but not postcopy-ram capability, RAM is migrated in precopy as usual. So, when target started, the only thing that is not yet migrated is dirty bitmap. So, in worst case (migration failure after postcopy started) you'll loose your dirty bitmap. VM is migrated and normally running on target. Unfinished bitmaps on target are automatically released (see cancel_incoming_locked()). So, in worst case you'll have to start your incremental backup chain from a new full-backup.
>
>> 
>> 
>> 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
>
>
>-- 
>Best regards,
>Vladimir

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Re:Re: [PATCH] bugfix:migrate with block-dirty-bitmap (disk size is big enough) can't be finished

[liuhaiwei9699]

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Hi ,Vladimir
sometimes ,post-copy mode is not the best choice. For instance, Supposing migrate process will take ten minutes,but network may be interruptted In this process .
If it does happenthe , memory data of VM will be splitted into two parts, and will not be rollback.This is a bad situation


so  migrate-start-postcopy will not be setted in conservative scenario. In this case, the migration with block dirty bitmap may not be finished.




I think  migration of block dirty bitmap should not dependent on post-copy or pre-copy mode.




Best regards
At 2022-09-10 18:18:04, "Vladimir Sementsov-Ogievskiy" <vsementsov@yandex-team.ru> wrote:
>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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