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List: libvir-list
Subject: Re: [PATCH V3 0/3] migration: add qemu parallel migration options
From: Jiang Jiacheng <jiangjiacheng () huawei ! com>
Date: 2023-04-26 1:56:30
Message-ID: 645808e0-0a72-7753-79d1-fe003204e71c () huawei ! com
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On 2023/4/24 18:57, Jiri Denemark wrote:
> On Fri, Apr 21, 2023 at 17:39:12 +0800, Jiang Jiacheng wrote:
>>
>> Thank you for your reply and review, I'd appreciate for you to do that.
>>
>> And I'd also like to confirm that we have the following usages after the
>> modification.
>> for non-parallel migration, we can use
>>
>> --compressed
>> default to use XBZRLE for migration
>>
>> --compressed --comp-methods ...
>> use the specificed methods for migration
>>
>> --comp-methods ...
>> use the specificed methods for migration, and set the corresponding cap
>>
>> and those are forbidden since the method isn't supported with
>> non-parallel migration
>>
>> [--compressed] --comp-methods=zlib
>> [--compressed] --comp-methods=zstd
>
> Correct.
>
>> for parallel migration, we have to enable the cap using "--parallel",
>> and can use like:
>>
>> --parallel
>> default to NONE compression method for parallel migration
>>
>> --parallel --comp-methods ...
>> use the specificed methods for parallel migration
>>
>> --parallel --compressed --comp-methods ...
>> use the specificed methods for parallel migration, it's same as the
>> above
>>
>> and those are forbidden since the method isn't supported with parallel
>> migration
>>
>> --parallel [--compressed] --comp-methods=mt
>> --parallel [--compressed] --comp-methods=xbzrle
>
> Right, but see below...
>
>> In particular,
>>
>> --parallel --compressed
>>
>> is forbiddene because it leads to a result that NONE compression method
>> is chosen but compressed flag is set.
>> And I test it on libvirt-6.2.0 with qemu-6.2.0, '--parallel
>> --compressed' will set cap 'xbzrle' and 'multifd' both to true, which I
>> think is unreasonable though the migration is succeed.
>
> Hmm, that would mean xbzrle is supported even for parallel migrations
> and we should not break this functionality. I experimented a bit and
> both mt and xbzrle compression methods seem to work with parallel
> migration. Even query-migrate reports compression statistics that
> suggest it actually works since they are similar for both normal and
> --parallel migrations.
>
> On the other hand according to Juan from QEMU none of them is compatible
> with parallel migration. I will try to discuss this a bit more with him,
> but so far I think we should allow both xbzrle and mt with parallel
> migration to avoid breaking things people might be using.
>
I found that there is a patch about this in qemu, which disables multifd
capability explicitly when compression=on. Maybe xbzrle have the same
problem as mt?
ref:https://github.com/qemu/qemu/commit/6f39c90b86b9d3772779f873ed88aaa75a220aba
Thanks,
Jiang Jiacheng
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