[SAMBA4] highestCommittedUSN support

[Luke Howard]

>> Right, but when you startup you can just search for all entries where
>> usnChanged > highestCommittedUSN, and then update the in-memory NC
>> high watermark.
>
>You may have a lower than expected value if the entries with the higher
>USN has been deleted just before the crash. When you recover you will
>not find them and so you hCUSN will not be accurate. Don't know if that
>matters anyway.

Good point, don't forget though that objects aren't actually removed from
the directory until the tombstone lifetime has lapsed (default 60 days).

So in practice this isn't a problem if your checkpoint interval is less
than the tombstone lifetime.

cheers,

-- Luke

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