Okay, thanks for taking the time to discuss, and of course for all your work on MDB!

Ben

On 08/10/2016 08:50 AM, Hallvard Breien Furuseth wrote:

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.)