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Re: [Qemu-devel] question: I found a qemu crash about migration


From: Matthew Schumacher
Subject: Re: [Qemu-devel] question: I found a qemu crash about migration
Date: Fri, 12 Jan 2018 15:31:06 -0900
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0

Am 28.09.2017 um 19:01 hat Dr. David Alan Gilbert geschrieben:
> Hi,
>   This is a 'fun' bug;  I had a good chat to kwolf about it earlier.
> A proper fix really needs to be done together with libvirt so that we
> can sequence:
>    a) The stopping of the CPU on the source
>    b) The termination of the mirroring block job
>    c) The inactivation of the block devices on the source
>        (bdrv_inactivate_all)
>    d) The activation of the block devices on the destination
>        (bdrv_invalidate_cache_all)
>    e) The start of the CPU on the destinationOn 01/12/2018 03:21 PM,
qemu-devel-confirm+7e23769bf079599cf1f3db6b00d347e8675d87f
address@hidden wrote:
>
>
> It looks like you're hitting a race between b/c;  we've had races
> between c/d in the past and moved the bdrv_inactivate_all.
>
> During the discussion we ended up with two proposed solutions;
> both of them require one extra command and one extra migration
> capability.
>
> The block way
> -------------
>    1) Add a new migration capability pause-at-complete
>    2) Add a new migration state almost-complete
>    3) After saving devices, if pause-at-complete is set,
>       transition to almost-complete
>    4) Add a new command (migration-continue) that
>       causes the migration to inactivate the devices (c)
>       and send the final EOF to the destination.
>
> You set pause-at-complete, wait until migrate hits almost-complete;
> cleanup the mirror job, and then do migration-continue.  When it
> completes do 'cont' on the destination.
>
> The migration way
> -----------------
>    1) Stop doing (d) when the destination is started with -S
>       since it happens anyway when 'cont' is issued
>    2) Add a new migration capability ext-manage-storage
>    3) When 'ext-manage-storage' is set, we don't bother doing (c)
>    4) Add a new command 'block-inactivate' on the source
>
> You set ext-manage-storage, do the migrate and when it's finished
> clean up the block job, block-inactivate on the source, and
> then cont on the destination.
>
>
> My worry about the 'block way' is that the point at which we
> do the pause seems pretty interesting;  it probably is best
> done after the final device save but before the inactivate,
> but could be done before it.  But it probably becomes API
> and something might become dependent on where we did it.
>
> I think Kevin's worry about the 'migration way' is that
> it's a bit of a block-specific fudge; which is probably right.
>
>
> I've not really thought what happens when you have a mix of shared and
> non-shared storage.
>
> Could we do any hack that isn't libvirt-visible for existing versions?
> I guess maybe hack drive-mirror so it interlocks with the migration
> code somehow to hold off on that inactivate?
>
> This code is visible probalby from 2.9.ish with the new locking code;
> but really that b/c race has been there for ever - there's maybe
> always the chance that the last few blocks of mirroring might have
> happened too late ?
>
> Thoughts?
> What are the libvirt view on the preferred solution.
>
> Dave

Devs,

Did this issue ever get addressed?  I'm looking at the history for
mirror.c at https://github.com/qemu/qemu/commits/master/block/mirror.c
and I don't see anything that leads me to believe this was fixed.

I'm still unable to live migrate storage without risking corruption on
even a moderately loaded vm.

Thanks,
schu



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