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Re: [Qemu-devel] Live Migration with different block devices
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] Live Migration with different block devices |
Date: |
Tue, 24 Jun 2014 20:10:26 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 |
On 06/24/2014 06:08 PM, Xiongzi Ge wrote:
> Hi Eric,
[we tend to avoid top-posting on technical lists]
>
> This really works if I set up the same configuration (/dev/vda in the
> guest) but the physical block devices are not the same after migration.The
> ABI is the same. When the guest cache stores a page like 'aaa' which is in
> the block device of Host A. If the cache in the guest is also migrated to
> Host B, but data in the block device actually are "bbb" in Host B now.
The ABI is NOT the same if the two host files do not have identical
contents; it's just that qemu cannot diagnose your bug.
> What
> will happen? Will the cache data in the guest be migrated to the other
> host? The vmstate function in the qemu seems doing something to save the
> vm state and the device state.
>
Who knows what happen? You're in undefined territory, because you
violated the premise that migration does not change disk contents behind
the guest's back.
> Should the guest re-open the block device and delete the previous cache
> data or just check the consistency of the cache and the block device?
Rather than trying to figure out what happens with undefined behavior,
how about you hot-unplug the old host device, then migrate, then
hot-plug the new host device. That way, the guest will KNOW it it is
connecting to a second disk, and not be confused by anything it cached
about the first.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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[Qemu-devel] Live Migration with different block devices, Xiongzi Ge, 2014/06/24
- Re: [Qemu-devel] Live Migration with different block devices, Brian Jackson, 2014/06/24
- Re: [Qemu-devel] Live Migration with different block devices, Eric Blake, 2014/06/24
- Re: [Qemu-devel] Live Migration with different block devices, Xiongzi Ge, 2014/06/24
- Re: [Qemu-devel] Live Migration with different block devices,
Eric Blake <=
- Re: [Qemu-devel] Live Migration with different block devices, Paolo Bonzini, 2014/06/25
- Re: [Qemu-devel] Live Migration with different block devices, Xiongzi Ge, 2014/06/25
- Re: [Qemu-devel] Live Migration with different block devices, Paolo Bonzini, 2014/06/25
- Re: [Qemu-devel] Live Migration with different block devices, Xiongzi Ge, 2014/06/25
- Re: [Qemu-devel] Live Migration with different block devices, Paolo Bonzini, 2014/06/25