Re: [PATCH v2 1/1] io: make zerocopy fallback accounting more accurate

[Markus Armbruster <armbru@redhat.com>]

Tejus GK <tejus.gk@nutanix.com> writes:

> Currently, the dirty-sync-missed-zero-copy stat is incremented only when
> an entire batch of IO operations fails to use zerocopy and falls back to
> a normal copy. As long as at least one IO in the batch is successfully
> zero-copied, the whole batch is treated as a success. This hides
> individual IO fallbacks and makes the migration stat less accurate than
> it could be.
>
> Make the stat more accurate by reporting at a finer granularity, i.e, by
> incrementing for every individual IO fallback that occurs.
>
> Suggested-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>

[...]

> diff --git a/qapi/migration.json b/qapi/migration.json
> index f925e5541b..94977b8810 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -58,8 +58,7 @@
>  #     (since 7.0).
>  #
>  # @dirty-sync-missed-zero-copy: Number of times dirty RAM
> -#     synchronization could not avoid copying dirty pages.  This is
> -#     between 0 and @dirty-sync-count * @multifd-channels.
> +#     synchronization could not avoid copying dirty pages.
>  #     (since 7.1)
>  #
>  # Since: 0.14

Dropping the documented upper limit might make the value harder to
interpret.  Thus my question: how is this value to be used?