Re: POHMELFS high performance network filesystem. Transactions, failover,performance.

[Jeff Garzik]

Evgeniy Polyakov wrote:
> Hi Sage.
>
> On Wed, May 14, 2008 at 06:35:19AM -0700, Sage Weil (sage@xxxxxxxxxxxx) wrote:
> > > > What is your opinion of the Paxos algorithm?
> > > It is slow. But it does solve failure cases.
> > For writes, Paxos is actually more or less optimal (in the non-failure 
> > cases, at least).  Reads are trickier, but there are ways to keep that 
> > fast as well.  FWIW, Ceph extends basic Paxos with a leasing mechanism to 
> > keep reads fast, consistent, and distributed.  It's only used for cluster 
> > state, though, not file data.
>
> Well, it depends... If we are talking about single node perfromance,
> then any protocol, which requries to wait for authorization (or any
> approach, which waits for acknowledge just after data was sent) is slow.

Quite true, but IMO single-node performance is largely an academic 
exercise today.  What production system is run without backups or 
replication?


> If we are talking about agregate parallel perfromance, then its basic
> protocol with 2 messages is (probably) optimal, but still I'm not
> convinced, that 2 messages case is a good choise, I want one :)

I think part of Paxos' attraction is that it is provably correct for the 
chosen goal, which historically has not been true for hand-rolled 
consensus algorithms often found these days.

There are a bunch of variants (fast paxos, byzantine paxos, fast 
byzantine paxos, etc., etc.) based on Classical Paxos which make 
improvements in the performance/latency areas.  There is even a Paxos 
Commit which appears to be more efficient than the standard transaction 
two-phase commit used by several existing clustered databases.

	Jeff



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