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Re: Vacuum rate limit in KBps

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Vacuum rate limit in KBps
Date: 2012-01-19 19:05:36
Message-ID: CA+U5nML_cu_1xdka+Vi5oRHLxkRQL3Qez42c7hQg9LCoSPDDhg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Jan 15, 2012 at 9:17 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:

> I think it makes more sense to use the max read rate as the main knob,
> rather than write rate. That's because the max read rate is higher than the
> write rate, when you don't need to dirty pages. Or do you think saturating
> the I/O system with writes is so much bigger a problem than read I/O that it
> makes more sense to emphasize the writes?

Yes, the writes are more important of the two.

Too many writes at one time can overflow hardware caches, so things
tend to get much worse beyond a certain point.

Also, rate limiting writes means we rate limit WAL rate also which is
very important.

I'd like this to apply to large DDL, not just VACUUMs.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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