On 08/08/16 03:51, bentr...@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.
