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

[bentrask@comcast.net]

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.)
>