Thanks a lot! My proof-of-concept code works OK.
I do not understand all subtle details of mmap reliability, could you please help with these two:
If I write data to a pointer to an opaque blob as discussed above, and my process crashes before mdb_env_sync, but OS doesn't crash - will that data be secure in the mmap file?
Also, am I correct that mdb_env_sync synchronizes all dirty pages in the mmap file as seen by a file system, regardless how they were modified - either via LMDB API or via a direct pointer writes?
As for "you could at least set a callback to notify you that a block has moved" - if that is implemented, it would be nice to have a notification *before* a block is moved (with old and new address, so that right after the callback it is OK to use the new address), otherwise this non-intended but convenient use of LMDB won't work anymore.
Best regards, Victor
On Sat, Oct 3, 2015 at 1:27 AM, Howard Chu hyc@symas.com wrote:
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 reservedspace 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 onthe 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 workwith 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/