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

2018-01-15 Thread Dr. David Alan Gilbert
* Matthew Schumacher (mat...@aptalaska.net) wrote:
> 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
> 3...@nongnu.org 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.

Yes, there's now a 'pause-before-switchover' which gives libvirt
a chance to quiesce the block devices.
That went in my 93fbd0314^..0331c8cabf6168 back in October.
I believe libvirt uses that; see libvirt commit 6addde2.

Dave

> Thanks,
> schu
> 
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK



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

2018-01-12 Thread Matthew Schumacher
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
3...@nongnu.org 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