Francesco Mari commented on OAK-6457:

As discussed offline with [~mduerig], one problem related to this issue is 
synthesising the generation number and the compacted flag for old segments, 
which are associated with only a full generation number. The safest way to 
achieve this is to synthesise the generation number to be equal to the full 
generation number, and to assume that the compacted flag is always on in old 
segments. Not only this is the safest approach when dealing with old segments, 
but it also works seamlessly with the new approach to full and tail compaction.

> Increment the segment version number
> ------------------------------------
>                 Key: OAK-6457
>                 URL: https://issues.apache.org/jira/browse/OAK-6457
>             Project: Jackrabbit Oak
>          Issue Type: New Feature
>          Components: segment-tar
>            Reporter: Francesco Mari
>            Assignee: Francesco Mari
>             Fix For: 1.8, 1.7.6
> OAK-3349 introduced the tail generation number in the segment header. This 
> change assigns semantics to a portion of the header that was previously not 
> used. As such, the segment version should be incremented. Proper checks 
> should also be put in place to transparently migrate segments from the old to 
> the new version number.

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