[Date Prev][Date Next] [Chronological] [Thread] [Top]

Re: LMDB: MDB_MAP_FULL doesn't allow deletions

Jeremy Bernstein wrote:

Hi Howard,

Thanks for your response. I tried using mdb_stat() previously, but I
wasn't

getting useful information. For instance, when testing a mini-DB (64K, 16
pages I believe), I had something like 1 branch page, 2 leaf pages and 0
overflow pages when I started getting MDB_MAP_FULL. My data is an 11 byte
struct with an 8 byte key FWIW, so I'm not blasting the DB with big records,
either.


It could just be that such mini DBs aren't going to be accurately statted,

but it seemed like that method wasn't going to be reliable. Did I overlook
something?
Your entire mapsize was only 64K, 16 pages? That's not going to work well. Please read the LMDB presentations to understand why not. Remember that in addition to the main data pages, there is also a 2nd DB maintaining a list of old pages, and since LMDB uses copy-on-write every single write you make is going to dirty multiple pages, and dirty pages cannot be reused until 2 transactions after they were freed. So you need enough free space in the map to store ~3 copies of your largest transaction, in addition to the static data. 


Thanks
Jeremy

Am 11.06.2013 um 19:32 schrieb Howard Chu <hyc@symas.com>:


Jeremy Bernstein wrote:

Although I didn't figure out a good way to do what I want, this is what I am
now doing:

if (MDB_MAP_FULL while putting) {
   abort txn, close the database
   reopen the database @ larger mapsize
   perform some pruning of dead records
   commit txn, close the database
   reopen the database @ old mapsize
   try to put again
}

At this point, the database is probably larger than the old mapsize. To handle
that, I make a copy  of the DB, kill the original, open a new database and
copy the records from the old DB to the  new one.

All of this is a lot more complicated and code-verbose than I want, but it
works and seems to be reliable.

Nevertheless, if there's an easier way, I'm all ears. Thanks for your
thoughts.


Use mdb_stat() before performing the _put(). If the total number of pages in use is large (whatever threshold you choose, e.g. 90%) then start pruning.

Look at the mdb_stat command's output to get an idea of what you're looking for.
--
  -- 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/