Nico Williams wrote:
On Fri, Nov 25, 2016 at 09:08:37AM +0100, Florian Weimer wrote:
I think you misunderstood what I was asking (see the start of the thread).
I need to support Byzantine readers which do not follow the locking
protocol. Based on the documentation, LMDB uses locks to implement MVCC and
prevent premature page reuse.
There's a good use case for an LMDB-like COW DB such that readers need
never lock. That would require vacuuming by writing a new file and
renaming it into place, which is a trade-off.
One of the problems with LMDB's locks is that readers need write
permission to the lock file, IIRC :( but at least it's a separate file.
We developed an alternate locking protocol for Postfix. It just uses standard
fcntl() locks.
http://www.postfix.org/lmdb_table.5.html
Of course nothing comes for free - with this approach, writers don't block
readers, but readers block writers. That's the best you're going to get
without custom lock protocols like LMDB uses natively.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
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