From: | Greg Smith <greg(at)2ndquadrant(dot)com> |
---|---|
To: | Fujii Masao <masao(dot)fujii(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: write ahead logging in standby (streaming replication) |
Date: | 2009-11-13 02:15:05 |
Message-ID: | 4AFCC129.1010908@2ndquadrant.com |
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Lists: | pgsql-hackers |
Fujii Masao wrote:
> Personally, I think that semi-synchronous replication is sufficient for HA.
>
Whether or not you think it's sufficient for what you have in mind,
"synchronous replication" requires a return ACK from the secondary
before you say things are committed on the primary. If you don't do
that, it's not true sync replication anymore; it's asynchronous
replication. Plenty of people decide that a local commit combined with
a promise to synchronize as soon as possible to the slave is good enough
for their apps, which as you say is getting referred to as
"semi-synchronous replication" nowadays. That's an awful name though,
because it's not true--that's asynchronous replication, just aiming for
minimal lag. It's OK to say that's what you want, but you can't say
it's really a synchronous commit anymore if you do things that way.
--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com
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