Re: [Qemu-devel] [PATCH 03/12] block/backup: add 'never' policy to bitmap sync mode

[Max Reitz <mreitz@redhat.com>]

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On 20.06.19 03:03, John Snow wrote:
> This adds a "never" policy for bitmap synchronization. Regardless of if
> the job succeeds or fails, we never update the bitmap. This can be used
> to perform differential backups, or simply to avoid the job modifying a
> bitmap.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  qapi/block-core.json | 6 +++++-
>  block/backup.c       | 5 +++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 6d05ad8f47..0332dcaabc 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1146,10 +1146,14 @@
>  # @conditional: The bitmap is only synchronized when the operation is successul.
>  #               This is useful for Incremental semantics.
>  #
> +# @never: The bitmap is never synchronized with the operation, and is
> +#         treated solely as a manifest of blocks to copy.
> +#         This is useful for Differential semantics.
> +#

Again, this is too buzzword-y for my taste.  I don’t find it as bad
because there is not much to explain about this mode, and you do explain
it above, but still.

Like, I (me myself) read this and after the first sentence I think I’ve
understood what this is.  Then I read “for Differential semantics” and
I’m confused.  After a couple of seconds, I realize what you mean
because I’ve described in my response to patch 1.

One reason it leaves the buzzword-y taste is because “differential” is
never explained anywhere.  bitmaps.rst makes two mentions of it, but it
too just assumes I know what you mean.  Also, incremental backups are
just a certain kind of differential backups.

So you need to explain “differential” somewhere and how it differs from
“incremental” in this regard.  Why not here?

“This is useful when you wish to repeatedly perform operations in
reference to a constant synchronization point (when the bitmap was
created).”

Or something.

Max

>  # Since: 4.1
>  ##
>  { 'enum': 'BitmapSyncMode',
> -  'data': ['conditional'] }
> +  'data': ['conditional', 'never'] }
>  
>  ##
>  # @MirrorCopyMode:


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