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Re: (ITS#8475) Feature request: MDB low durability transactions

Nope, you're as confused as I was originally:-) LDMB doesn't know or
care when a page was written.  A page can be reused when the snapshot
which _freed_ it is known to be durable and there are no older readers.
(We could improve that by tracking page history better.  Maybe later.)

"Known to be durable" = sync datapages, write metapage, sync metapage,
note that the metapage was synced.  (We implicitly note that when
writing next txn's metapage, since we must have synced first.)  From
a data safety point of view, txns which do all that are the real txns.
Anything else is fluff, like no-sync txns if we implement them.  Their
metapages must go somewhere they *won't* be confused with durable ones.

Think of such a fluffy commit as saving an intermediate stage of a
real txn.  That's irrelevant to a later write-txn wanting to not touch
the last two durable snapshots.  It's only relevant vs. oldest reader.

So.  3rd metapage and variants - I've tried and Howard pointed out
the flaws, Howard tried and I said here we go again.  We do not need
another round, but it's just as well to have it summarized here.

(This discussion ignores MDB_NOSYNC and partly MDB_NOLOCK - if the
user enables either, it's his responsibility to compensate.)