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Re: [virt-tools-list] [PATCH] virt-manager: Add block migration support

On 10/05/2013 10:59 AM, Eiichi Tsukata wrote:
> This patch adds block migration support. There are two modes: full
> disk copy and incremental disk copy, which correspond to libvirt
> virDomainMigrate API's flags, VIR_MIGRATE_NON_SHARED_DISK and
> VIR_MIGRATE_NON_SHARED_INC. These flags are mutually exclusive,
> so GUI is implemented by ComboBox.
> Signed-off-by: Eiichi Tsukata <devel etsukata com>
> ---
>  ui/migrate.ui            | 61 ++++++++++++++++++++++++++++++++++++++++++++++--
>  virtManager/domain.py    |  7 +++++-
>  virtManager/migrate.py   | 41 +++++++++++++++++++++++++++++---
>  virtManager/uihelpers.py | 20 ++++++++++++++++
>  4 files changed, 123 insertions(+), 6 deletions(-)

At a glance the code looks fine. But I'm wary of exposing storage migration in
the UI for the following reasons:

1) The oldest storage migration code in qemu is known inefficient, not really
maintained, and usage is discouraged by qemu devs. So I'd want to
conditionally expose it depending on libvirt and qemu being new enough to do
the old style.

2) It has serious usability issues: a) you need to pre-create storage file
stubs on the destination (there are libvirt patches for this) and b) doesn't
work with tunnelled migration (which is the simplest option for end users but
isn't a blocker)

We will definitely get bug reports about the last bit if we don't at least
stick a warning in the UI about the caveats. So maybe if someone selects
storage migration we unhide a little warning box that explains things (we
could even explain 'this is the old qemu method and might suck', and when all
the libvirt bits are in place we can do a version check and not show the error

Also, I don't really understand when someone would want to use NON_SHARED_DISK
vs NON_SHARED_INC, so if a user asks I don't know what to tell them. Maybe
someone can enlighten me.

But just exposing things as-is will make my life harder while not really
adding anything for users beyond what virsh provides.

- Cole

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