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Re: Sync Rep: Second thoughts

From: Emmanuel Cecchet <manu(at)asterdata(dot)com>
To: Markus Wanner <markus(at)bluegap(dot)ch>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Fujii Masao<masao(dot)fujii(at)gmail(dot)com>, <aidan(at)highrise(dot)ca>, Heikki Linnakangas<heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development<pgsql-hackers(at)postgresql(dot)org>, <postgres-r-general(at)pgfoundry(dot)org>
Subject: Re: Sync Rep: Second thoughts
Date: 2008-12-24 15:39:24
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Lists: pgsql-hackers
Hi Markus,

> I didn't have much reliability issues with ensemble, appia or spread, so
> far. Although, I admit I didn't ever run any of these in production.
> Performance is certainly an issue, yes.
I may suggest another reading even though a bit dates, most of the 
results still apply:
The baseline is that if you use UDP multicast, you need a dedicated 
switch and the tuning is a nightmare. I discussed these issues with the 
developers of Spread and they have no real magic. TCP seems a more 
reliable alternative (especially predictable performance) but the TCP 
timeouts are also tricky to tune depending on the platform. We worked 
quite a bit with Nuno around Appia in the context of Sequoia and 
performance can be outstanding when properly tuned or absolutely awful 
is some default values are wrong. The chaotic behavior of GCS under 
stress quickly compromises the reliability of the replication system, 
and admission control on UDP multicast has no good solution so far.
It's just a heads up on what is awaiting you in production when the 
system is stressed. There is no good solution so far besides a good 
admission control on top of the GCS (in the application).

I am now off for the holidays.


Emmanuel Cecchet
Aster Data Systems

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