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Date:	Wed, 14 May 2008 15:05:26 -0400
From:	Jeff Garzik <>
To:	Evgeniy Polyakov <>,
	Sage Weil <>, Jeff Garzik <>,,,
Subject: Re: POHMELFS high performance network filesystem. Transactions, failover,

Jamie Lokier wrote:
> Look up "one-phase commit" or even "zero-phase commit".  (The
> terminology is cheating a bit.)  As I've understood it, all commit
> protocols have a step where each node guarantees it can commit if
> asked and node failure at that point does not invalidate the guarantee
> if the node recovers (if it can't maintain the guarantee, the node
> doesn't recover in a technical sense and a higher level protocol may
> reintegrate the node).  One/zero-phase commit extends that to
> guaranteeing a certain amounts and types of data can be written before
> it knows what the data is, so write messages within that window are
> sufficient for global commits.  Guarantees can be acquired
> asynchronously in advance of need, and can have time and other limits.
> These guarantees are no different in principle from the 1-bit
> guarantee offered by the "can you commit" phase of other commit
> protocols, so they aren't as weak as they seem.

For several common Paxos usages, you can obtain consensus guarantees 
well in advance of actually needing that guarantee, making the entire 
process quite a bit more async and parallel.

Sort of a "write ahead" for consensus.


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