Re: [LMDB] What pointer is returned with combination of MDB_WRITEMAP and MDB_RESERVE?

[Howard Chu <hyc@symas.com>]

Victor Baybekov wrote:
> Hello,
>
> Working with overflow pages directly via pointers outside write transactions works great and it helps that they do not move "by design" in current versions as 
> discussed in this thread.
>
> I have two related scenarios that will give a substantial performance boost in my case.
>
> /The first one/ is updating a value in-place via pointer from aborted write transaction. If I
>
> 1) use MDB_WRITEMAP,
> 2) from **write** transaction find a record (which is small and not in an overflow page),
> 3) modify a part of it's value (for duplicates this part is not used in the compare function) directly via the MDB_VAL data pointer (e.g. interlocked_increment 
> or compare_and_swap),
> 4) and **abort** the transaction,
>
> then readers see the updated value via normal read transactions later. Since I do the direct updates from inside a write transaction all other writers should be 
> locked until I exit the transaction (abort in this case), and no pages should move since the transaction is aborted. Is this correct? Does this work "by design" 
> or "by accident" currently?

By accident. Note that LMDB 1.0 supports page checksums, and what you're doing here
will break those checksums.

> /The second one/ is about updating values in-place from read transactions. If I

Updating any value in a read transaction is ludicrous.

Basically you're abusing MDB_WRITEMAP, whose only purpose is to (potentially) optimize
normal write transactions. I've no interest in even speculating on intentional misuse
of the API.

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/