Thank you! I understand this copy-on-write behavior, but am interested if I could control it a little. What if I use records that are always much bigger than a single page, e.g. 100 kb with 4kb pages, and make sure that a record is never updated (via LMDB means) during a lifetime of an environment, - is there any scenario that the location of such a big record could be changed during a lifetime of an environment, without updating the record?

On Fri, Oct 2, 2015 at 4:38 PM, Howard Chu <> wrote:
Victor Baybekov wrote:

Docs for MDB_RESERVE say that a returned pointer to the reserved space is
valid "before the next update operation or the transaction ends." Docs
for MDB_WRITEMAP say that it "writes directly to the mmap instead of using
malloc for pages." Does combining the two options return a pointer directly to
a place in a mmap


so that this pointer could be used after a transaction ends
or after the next update?


Longer answer: maybe.

Full answer: LMDB is copy-on-write. If you update another record on the same page, in a later transaction, the contents of that page will be copied to a new page and the original page will go onto the freelist. In that case, the pointer you got must not be used again.

If you don't directly update that page and cause it to be copied, then you might get lucky and be able to use the pointer for a while. It all depends on what other modifications you do and how they affect that node or neighboring nodes.

I have a use case where I want to somewhat abuse LMDB safety for convenience.
If I could get a pointer to a place inside a mmap I could work with LMDB value
as opaque blob or as a region inside the single big mmap. This could be more
convenient than creating and opening hundreds of temporary memory mapped files
and keeping open handles to them. For example, Aeron terms could be stored
like this: a stream id per an LMDB db and a term id for a key in the db.


  -- Howard Chu
  CTO, Symas Corp. 
  Director, Highland Sun
  Chief Architect, OpenLDAP