From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Aidan Van Dyk <aidan(at)highrise(dot)ca>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: How much do the hint bits help? |
Date: | 2010-12-22 15:52:00 |
Message-ID: | 1293033120.1193.28428.camel@ebony |
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Lists: | pgsql-hackers |
On Wed, 2010-12-22 at 10:45 -0500, Tom Lane wrote:
> Aidan Van Dyk <aidan(at)highrise(dot)ca> writes:
> > With this statement, you just moved the goal posts on the checksumming
> > ideas. In fact, you didn't just move the goal posts, you picked the
> > ball up and teleported it to another stadium.
>
> What he said. I can't imagine that anyone will be interested in any
> case other than "set the CRC immediately before writing, and check it
> upon first reading the page in". Maintaining it continuously while the
> page is in shared memory is completely insane from a cost-versus-benefit
> perspective.
If you insist on setting hint-bits, then that is probably true.
Many people experience almost no I/O these days, and there's a strong
correlation between people caring about their data and also being
willing to spend big $s on cache. We need to protect our users, however
much money they spent on cache; I would argue the more money they spent
on cache the harder we should be trying to protect them.
I'm sure it will take a little while for everybody to understand why a
full CRC implementation is both necessary and now possible. Paradigm
shifts of thought do seem like teleports, but they can be beneficial.
--
Simon Riggs http://www.2ndQuadrant.com/books/
PostgreSQL Development, 24x7 Support, Training and Services
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