Howard Chu wrote:
Victor Baybekov wrote:
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?
At this point in time, no, if you don't update a large record there is no
reason that it will move. That is not to say that this won't change in the
future. The documentation tells you what promises we are willing to make.
Relying on any non-documented behavior is your own responsibility.
Note that the relocation functions in LMDB are intended to accommodate blocks being moved around. The actual guts of that API haven't been implemented, but probably in 1.x we'll flesh them out. Given that support, you could at least set a callback to notify you that a block has moved. But currently, overflow pages don't move if they're not modified.
On Fri, Oct 2, 2015 at 4:38 PM, Howard Chu <hyc@symas.com
<mailto:hyc@symas.com>> wrote:
Victor Baybekov wrote:
Hi,
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
Yes.
so that this pointer could be used after a transaction ends
or after the next update?
No.
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.
Thanks!
Victor
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/