Sage Weil wrote:
Hi Jeff,
Do you still plan to replace bdb (and it's replication) with a something
based on paxos? I'm considering replacing the Ceph monitors (which
currently implement paxos, but in a very ceph-specific way) with cld if it
can meet the basic requirements.
What I'd kind of like to see is a clean implementation of a paxos
library--one that leaves out message transport and storage--to build a
replicated write-ahead log. And then a separate library for handling the
database/namespace served up by cld (be it regular files, bdb, whatever)
that leaves replication up to paxos. It looks like Google ended up doing
something similar with Chubby (see
http://labs.google.com/papers/paxos_made_live.html).
Does this sound like the direction you guys are heading in?
You mean something like http://linux.yyz.us/misc/paxreg.c ? :)
Yes, a straight PAXOS implementation is definitely in the plans, for
similar reasons as Google describes: CLD just doesn't need full db4
replicated transactions, when a PAXOS replicated, write-ahead logging
database would work just fine.
That said, the urgency of this task is lowered, because current db4
(after much trial, tribulation and debugging) did eventually start
supporting PAXOS. Some of the Google hints paper hint at this, noting
that Sleepycat/Oracle "eventually" fixed all the problems -- but by then
it was too late, Google had moved on and done their own db.
Most importantly, from the view of a CLD client (libcldc user), CLD will
provide the necessary guarantees today. When CLD switches to native
PAXOS, the CLD client API will not change at all. So, the switchover
should be transparent from the client's point of view.
Jeff
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