[SAMBA4] highestCommittedUSN support

[Luke Howard]

>we could do, but it would mean re-writing the rootdse record on each
>add/modify, in addition to the re-writing of the @BASEINFO record. I'm
>concerned about the overhead of that. That's why I did
>highestCommittedUSN as a generated attribute, much like the currentTime
>field, except that it comes from the @BASEINFO record which ldb_tdb is
>already maintaining.

You could just store it in memory and checkpoint it at regular intervals,
it's easy to recover at startup -- this is effectively what OpenLDAP
syncrepl does.

>Is there something else that maintaining it separately would gain us?

None, although it's possible if you want because the USN namespace is
NC-local as far as replication is concerned.

-- Luke

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