using virtual synchrony for CTDB

Steven Dake sdake at
Fri Oct 6 18:30:59 GMT 2006


If latency is an issue in messages, any message that has roundtrip
response time over Ethernet medium will have higher latency then those
messages that do not have round trip responses.

If no response is required over Ethernet from the server before
proceeding to do new operations, then ptp will have less latency then
virtual synchrony.

The performance problem that vs solves is removing the round trip
response time, since every node has a copy of the data and can
immediately handle the request and may continue processing as soon as
the lock request is delivered (self-delivered) instead of waiting for a
response from a server over TCP/IP or some other PTP protocol running
over Ethernet.


On Fri, 2006-10-06 at 11:09 -0700, Tracy Camp wrote:
> Snake oil aside, 'DLM' like clustering schemes, which the CTDB proposal 
> seems like it could be grouped with, are best implemented with p-t-p 
> messages for the latency concerns already expressed.  However also using a 
> VS group communications layer to provide a generation number than can then 
> be embedded in each P-T-P message provides P-T-P w/o the overhead of VS 
> for the latecy sensitive messages.  Sort of a scheme that breaks the 
> 'control' apart from the 'data' transports.
> Tracy Camp
> On Fri, 6 Oct 2006, David Boreham wrote:
> > Steven Dake wrote:
> >
> >> I have a suggestion to use virtual synchrony for the transport mechanism
> >> of CTDB.  I think you will find using something like TCPIP unsuitable
> >> for a variety of reasons.
> >> 
> > I'm very far from being a VS expert, but when I looked into a few
> > of the open source implementations available a while back it became
> > clear (to me at least) that they have a kind of 'snake oil' property
> > in that they appear to deliver magical services but do so only by
> > using quite inefficient methods underneath the covers. For example
> > it appears that one is avoiding network round-trips but in fact to implement 
> > its
> > delivery guarantees the message middleware layer needs to propagate a token
> > around the set of participating nodes which of course involves many sends and
> > receives.
> >
> >
> >
> >
> >
> >

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