Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining()

[Balamuruhan S <bala24@linux.vnet.ibm.com>]

On 2018-04-04 13:36, Peter Xu wrote:
> On Wed, Apr 04, 2018 at 11:55:14AM +0530, Balamuruhan S wrote:
> 
> [...]
> 
>> > too. So still I'll put aside the "which one is better" question.
>> >
>> > For your use case, you can have a look on either of below way to
>> > have a converged migration:
>> >
>> > - auto-converge: that's a migration capability that throttles CPU
>> >   usage of guests
>> 
>> I used auto-converge option before hand and still it doesn't help
>> for migration to complete
> 
> Have you digged about why?  AFAIK auto-convergence will at last absort
> merely the whole vcpu resource (99% of them maximum).  Maybe you are
> not with the best throttle values?  Or do you think that could be a
> auto-convergence bug too?

I am not sure, I will work on it to find why.

> 
>> 
>> >
>> > - postcopy: that'll let you start the destination VM even without
>> >   transferring all the RAMs before hand
>> 
>> I am seeing issue in postcopy migration between POWER8(16M) -> 
>> POWER9(1G)
>> where the hugepage size is different. I am trying to enable it but 
>> host
>> start
>> address have to be aligned with 1G page size in 
>> ram_block_discard_range(),
>> which I am debugging further to fix it.
> 
> I thought the huge page size needs to be matched on both side
> currently for postcopy but I'm not sure.

you are right! it should be matched, but we need to support
POWER8(16M) -> POWER9(1G)

> CC Dave (though I think Dave's still on PTO).