Hello ---- when reflect creates a synthetic back it merge the oldest  image to the full backup no matter if it is a differential or incremental  backup?  thanks

A Synthetic Full will only be created if the set contains no Differentials, and therefore only Incrementals can be merged.  The reason Differential merging isn't allowed is because if you have multiple Differentials in a set, then merging one Differential into the Full will of course alter that Full -- at which point all other Differentials and any of their child Incrementals will become useless, since those other Differentials were created relative to the Full in its original state, which no longer exists after that merge.

This is why the option is called "Create a Synthetic Full if possible".  If it's not possible, then Reflect will merge the oldest Incrementals into each other, leaving the Full intact, instead of merging the oldest Incremental(s) into the Full.

In theory the attempt could be made to merge diffs & a warning could be issued if they didn't effectively equate to another Inc... & the merge be abandoned / reverted, though I don't actually see much point to that, if you didn't want a point in time Diff, then why not use an Incrementals-forever (or until the next full) strategy?

The only circumstance where merging a Diff into a Full could be performed without collateral damage would be if there was only one Diff in the entire set.  But if you have a backup strategy that creates sets that only ever have a single Diff (plus any Incs), then there's no point making that single Diff in the first place, because a child Diff and the first direct child Inc of a Full would be exactly the same.  So as you say, I don't really see the point of allowing a Synthetic Full to be created via Diff consolidation.

+x

 

jphughan - 3 March 2021 9:36 PM

A Synthetic Full will only be created if the set contains no Differentials, and therefore only Incrementals can be merged.  The reason Differential merging isn't allowed is because if you have multiple Differentials in a set, then merging one Differential into the Full will of course alter that Full -- at which point all other Differentials and any of their child Incrementals will become useless, since those other Differentials were created relative to the Full in its original state, which no longer exists after that merge.

This is why the option is called "Create a Synthetic Full if possible".  If it's not possible, then Reflect will merge the oldest Incrementals into each other, leaving the Full intact, instead of merging the oldest Incremental(s) into the Full.

thx for the info 

The only circumstance where merging a Diff into a Full could be performed without collateral damage would be if there was only one Diff in the entire set.

I suspect such  a situation would arise quite commonly, given how typical users view using storage space. 
Not a strategy I'd recommend, just one I was observing wasn't all that unlikely for someone to think they wanted.