Peter Xu <[email protected]> writes:
> COLO was broken for QEMU release 10.0/10.1 without anyone noticed.
We could arguably drop this right away. I'm not demanding we do, just
pointing out.
First, COLO is marked 'unstable' in the QAPI schema:
* MigrationCapability member x-colo:
# @unstable: Members @x-colo and @x-ignore-shared are experimental.
* MigrationParameter and MigrationParameters member x-checkpoint-delay:
# @unstable: Members @x-checkpoint-delay and
# @x-vcpu-dirty-limit-period are experimental.
* Command x-colo-lost-heartbeat:
# @unstable: This command is experimental.
There's more COLO stuff we neglected to mark, e.g. MigrationStatus
member @colo, event COLO_EXIT, commands xen-colo-do-checkpoint,
query-colo-status. We should clean that up. More on that below.
Second, it's been broken for two releases, our deprecation grace period.
In my opinion, "broken" is even stronger notice than "deprecated".
> One
> reason might be that we don't have an unit test for COLO (which we
> explicitly require now for any new migration feature). The other reason
> should be that there are just no more active COLO users, at least based on
> the latest development of QEMU.
>
> I don't remember seeing anything really active in the past few years in
> COLO development.
>
> Meanwhile, COLO migration framework maintainer (Hailiang Zhang)'s last
> email to qemu-devel is in Dec 2021 where the patch proposed an email
> change (<[email protected]>).
>
> We've discussed this for a while, see latest discussions here (our thoughts
> of deprecating COLO framework might be earlier than that, but still):
>
> https://lore.kernel.org/r/[email protected]/
> https://lore.kernel.org/r/[email protected]
>
> Let's make it partly official and put COLO into deprecation list. If
> anyone cares about COLO and is deploying it, please send an email to
> qemu-devel to discuss.
>
> Otherwise, let's try to save some energy for either maintainers or
> developers who is looking after QEMU. Let's save the work if we don't even
> know what the work is for.
>
> Cc: Lukáš Doktor <[email protected]>
> Cc: Juan Quintela <[email protected]>
> Cc: Dr. David Alan Gilbert <[email protected]>
> Cc: Zhang Chen <[email protected]>
> Cc: [email protected]
> Cc: Li Zhijian <[email protected]>
> Cc: Jason Wang <[email protected]>
> Signed-off-by: Peter Xu <[email protected]>
> ---
> docs/about/deprecated.rst | 6 ++++++
> qapi/migration.json | 5 ++---
> migration/options.c | 4 ++++
> 3 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 7abb3dab59..b499b2acb0 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -580,3 +580,9 @@ command documentation for details on the ``fdset`` usage.
>
> The ``zero-blocks`` capability was part of the block migration which
> doesn't exist anymore since it was removed in QEMU v9.1.
> +
> +COLO migration framework (since 11.0)
> +'''''''''''''''''''''''''''''''''''''
> +
> +To be removed with no replacement, as the COLO migration framework doesn't
> +seem to have any active user for a while.
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 201dedd982..3c868efe38 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -531,8 +531,7 @@
> #
> # @unstable: Members @x-colo and @x-ignore-shared are experimental.
> #
> -# @deprecated: Member @zero-blocks is deprecated as being part of
> -# block migration which was already removed.
> +# @deprecated: Member @zero-blocks and @x-colo are deprecated.
> #
> # Since: 1.2
> ##
> @@ -540,7 +539,7 @@
> 'data': ['xbzrle', 'rdma-pin-all', 'auto-converge',
> { 'name': 'zero-blocks', 'features': [ 'deprecated' ] },
> 'events', 'postcopy-ram',
> - { 'name': 'x-colo', 'features': [ 'unstable' ] },
> + { 'name': 'x-colo', 'features': [ 'unstable', 'deprecated' ] },
> 'release-ram',
> 'return-path', 'pause-before-switchover', 'multifd',
> 'dirty-bitmaps', 'postcopy-blocktime', 'late-block-activate',
Issues / doubts:
1. We delete the text why @zero-blocks is deprecated. Harmless; the
next patch drops @zero-blocks entirely. Better: swap the patches.
2. The text for @x-colo is lacking. Suggest something like "Member
@x-colo" is deprecated without replacement."
3. Does it make sense to keep x-colo @unstable?
4. Shouldn't we mark *all* the COLO interfaces the same way?
> diff --git a/migration/options.c b/migration/options.c
> index 9a5a39c886..318850ba94 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -580,6 +580,10 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps,
> Error **errp)
> warn_report("zero-blocks capability is deprecated");
> }
>
> + if (new_caps[MIGRATION_CAPABILITY_X_COLO]) {
> + warn_report("COLO migration framework is deprecated");
> + }
> +
> #ifndef CONFIG_REPLICATION
> if (new_caps[MIGRATION_CAPABILITY_X_COLO]) {
> error_setg(errp, "QEMU compiled without replication module"
