On 20.06.19 18:01, John Snow wrote: > > > On 6/20/19 11:00 AM, Max Reitz wrote: >> On 20.06.19 03:03, John Snow wrote: >>> We don't need or want a new sync mode for simple differences in >>> semantics. Create a new mode simply named "BITMAP" that is designed to >>> make use of the new Bitmap Sync Mode field. >>> >>> Because the only bitmap mode is 'conditional', this adds no new >>> functionality to the backup job (yet). The old incremental backup mode >>> is maintained as a syntactic sugar for sync=bitmap, mode=conditional. >>> >>> Add all of the plumbing necessary to support this new instruction. >>> >>> Signed-off-by: John Snow <js...@redhat.com> >>> --- >>> qapi/block-core.json | 30 ++++++++++++++++++++++-------- >>> include/block/block_int.h | 6 +++++- >>> block/backup.c | 35 ++++++++++++++++++++++++++++------- >>> block/mirror.c | 6 ++++-- >>> block/replication.c | 2 +- >>> blockdev.c | 8 ++++++-- >>> 6 files changed, 66 insertions(+), 21 deletions(-) >>> >>> diff --git a/qapi/block-core.json b/qapi/block-core.json >>> index caf28a71a0..6d05ad8f47 100644 >>> --- a/qapi/block-core.json >>> +++ b/qapi/block-core.json >>> @@ -1127,12 +1127,15 @@ >>> # >>> # @none: only copy data written from now on >>> # >>> -# @incremental: only copy data described by the dirty bitmap. Since: 2.4 >>> +# @incremental: only copy data described by the dirty bitmap. (since: 2.4) >> >> Why not deprecate this in the process and note that this is equal to >> sync=bitmap, bitmap-mode=conditional? >> >> (I don’t think there is a rule that forces us to actually remove >> deprecated stuff after two releases if it doesn’t hurt to keep it.) >> > > Mostly I thought it would be fine to keep as sugar. In your replies so > far I gather that "incremental" and "differential" don't mean specific > backup paradigms to you, so maybe these seem like worthless words. > > It was my general understanding that in terms of backup > paradigms/methodologies that "incremental" and "differential" mean very > specific things. > > Incremental: Each backup contains only the delta from the last > incremental backup. > Differential: Each backup contains the delta from the last FULL backup. > > You can search "incremental vs differential backup" on your search > engine of choice and find many relevant results. I took a Networking/IT > vocational degree in 2007 and these terms were taught in textbooks then. > > So I will resist quite strongly changing them, and for this reason, felt > that it was strictly a good thing to keep incremental as sugar, because > I thought that people would know what it meant.
:C OK. I’m happy as long as it’s all explained somewhere (i.e. bitmaps.rst). Personally, I’d also like a pointer to that documentation here. (Sure, people should just look there if they don’t understand something about bitmaps anyway, but I can’t see it hurting to just put a pointer here anyway.) > (More than "conditional", anyway, which is jargon I made up.) But you make it up in this series, which is great for me, because that means I get the definition (from the cover letter) without having to look it up. O:-) [...] >>> # >>> +# @bitmap-mode: Specifies the type of data the bitmap should contain after >>> +# the operation concludes. Must be present if sync is >>> "bitmap". >>> +# Must NOT be present otherwise. (Since 4.1) >> >> Do we have any rule that qemu must enforce “must not”s? :-) >> >> (No, I don’t think so. I think it’s very reasonable that you accept >> bitmap-mode=conditional for sync=incremental.) >> > > Right, I left this a secret wiggle room. If you specify the correct > bitmap sync mode for the incremental sugar, it will actually let it > slide. If you specify the wrong one, it will error out. > > However, I think this is perfectly correct advice from the API: Please > use this mode with sync=bitmap and do not use it otherwise. > > Would you like me to change it to be more technically correct and > document the little affordance I made? It’s probably better not to. Better forbid as much as we can so that we can break compatibility to users that happened to use it still “because it works”. Max
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