Re: [Qemu-devel] [PATCH 05/10] block: Inactivate BDS when migration completes

[Eric Blake <eblake@redhat.com>]

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On 12/22/2015 09:46 AM, Kevin Wolf wrote:
> So far, live migration with shared storage meant that the image is in a
> not-really-ready don't-touch-me state on the destination while the
> source is still actively using it, but after completing the migration,
> the image was fully opened on both sides. This is bad.
> 
> This patch adds a block driver callback to inactivate images on the
> source before completing the migration. Inactivation means that it goes
> to a state as if it was just live migrated to the qemu instance on the
> source (i.e. BDRV_O_INCOMING is set). You're then supposed to continue
> either on the source or on the destination, which takes ownership of the
> image.
> 
> A typical migration looks like this now with respect to disk images:
> 
> 1. Destination qemu is started, the image is opened with
>    BDRV_O_INCOMING. The image is fully opened on the source.
> 
> 2. Migration is about to complete. The source flushes the image and
>    inactivates it. Now both sides have the image opened with
>    BDRV_O_INCOMING and are expecting the other side to still modify it.

The name BDRV_O_INCOMING now doesn't quite match semantics on the
source, but I don't have any better suggestions.  BDRV_O_LIMITED_USE?
BDRV_O_HANDOFF?  At any rate, I fully agree with your logic of locking
things down on the source to mark that the destination is about to take
over write access to the file.

> 
> 3. One side (the destination on success) continues and calls
>    bdrv_invalidate_all() in order to take ownership of the image again.
>    This removes BDRV_O_INCOMING on the resuming side; the flag remains
>    set on the other side.
> 
> This ensures that the same image isn't written to by both instances
> (unless both are resumed, but then you get what you deserve). This is
> important because .bdrv_close for non-BDRV_O_INCOMING images could write
> to the image file, which is definitely forbidden while another host is
> using the image.

And indeed, this is a prereq to your patch that modifies the file on
close to clear the new 'open-for-writing' flag :)

> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c                   | 34 ++++++++++++++++++++++++++++++++++
>  include/block/block.h     |  1 +
>  include/block/block_int.h |  1 +
>  migration/migration.c     |  7 +++++++
>  qmp.c                     | 12 ++++++++++++
>  5 files changed, 55 insertions(+)
> 

> @@ -1536,6 +1540,9 @@ static void migration_completion(MigrationState *s, int current_active_state,
>          if (!ret) {
>              ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
>              if (ret >= 0) {
> +                ret = bdrv_inactivate_all();
> +            }
> +            if (ret >= 0) {
>                  qemu_file_set_rate_limit(s->file, INT64_MAX);

Isn't the point of the rate limit change to allow any pending operations
to flush without artificial slow limits?  Will inactivating the device
be too slow if rate limits are still slow?

But offhand, I don't have any strong proof that a different order is
required, so yours makes sense to me.

You may want a second opinion, but I'm okay if you add:
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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