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Re: (ITS#7774) LMDB assertion failure during Postfix cache cleanup
Wietse Venema writes:
> Why do you talk about map size changes when I delete a database entry??
That's how Postfix uses lmdb. It calls mdb_env_set_mapsize(). Looking
closer though, this is in src/util/slmdb.c which is by Howard and you.
Sounds like we should wait for whatever Howard has to say.
> I need a software abstraction layer between Postfix and LMDB that
> provides the following generic interface that is independent of
> LMDB internals:
It does, normally. NOLOCK and map resizing interfere, and impose
> Sofar the abstraction layer already hides the LMDB-specific MAP_FULL
> and MAP_RESIZED error conditions. If this abstraction layer needs
> additional code in order to maintain MDB cursor sanity, then please
> educate me.
ldmb.h says --
"It may be called at later times if no transactions are active in
this process. Note that the library does not check for this condition,
the caller must ensure it explicitly."
"[caller] must ensure that no readers are using old transactions
while a writer is active. The simplest approach is to use an
exclusive lock so that no readers may be active at all when a writer
That's why I talked about saving the cursor position and restoring it -
cursors are per-transaction and you need a new transaction.
Maybe you should have a single write-transaction with several cursors,
instead of several transactions. That also cures the "long-lived reader"
Howard, maybe NOLOCK should keep the reader table and just drop the
locks? More work though.