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

On 08/08/16 03:51, bentrask@comcast.net wrote:
> The idea was that the two "floating" roots would reuse pages the way MDB
> does now. The 3rd durable root would have its pages preserved
> separately. I can see why this would cause up to a ~2X storage increase
> as the two sets diverged, but I don't see why it would need to grow
> unbounded. Apologies for this stupid question.

A transaction must not reuse data pages visible in the last snapshot
known to be durable, since that's how far back LDMB may need to revert
after abnormal termination.  Like a crash after MDB_NOMETASYNC may do.

Sync the data pages from a txn, write the metapage, eventually sync
that metapage, wait out any older read-only transactions, and *then*
you can reuse the pages the txn freed.  Not before.  So when you don't
sync, or a read-only txn won't die, LMDB degenerates to append-only.

...except if you sync the metapage and exit, next LMDB run may not
know you synced it and must assume the metapage isn't yet durable.
So it might not reuse pages visible to the _previous_ durable
metapage, until it syncs.  I'm rather losing track at this point,
but I think it may mean twice as may not-yet-usable pages as one
might expect.