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Re: [Qemu-devel] [Migration Bug? ] Occasionally, the content of VM's mem

From: Juan Quintela
Subject: Re: [Qemu-devel] [Migration Bug? ] Occasionally, the content of VM's memory is inconsistent between Source and Destination of migration
Date: Fri, 27 Mar 2015 11:36:35 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux)

Wen Congyang <address@hidden> wrote:
> On 03/27/2015 05:57 PM, Stefan Hajnoczi wrote:
>>>> You must use -drive cache=none if you want to use live migration.  It
>>>> should not directly affect memory during migration though.
>>> Otherwise, what will happen? If the user doesn't use cache=none, and
>>> tries to use live migration, qemu doesn't output any message or trigger
>>> an event to notify the user.
>> There is a risk that the destination host sees an inconsistent view of
>> the image file because the source was still accessing it towards the
>> end of migration.
> Does the flag BDRV_O_NOFLUSH cause it?

The biggest isue is reads.

Target: reads 1st block of disk
        it enters page cache of target
Source: reads and writes lots of blocks

Migration happens.

Target: rereads 1st block of disk, and gets stalled contents.

No amount of fdatasync(), syncs() whatever on source will fix this problem.

Later, Juan.

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