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Re: Vacuum rate limit in KBps

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Vacuum rate limit in KBps
Date: 2012-01-20 17:35:38
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Lists: pgsql-hackers
On 01/20/2012 10:37 AM, Robert Haas wrote:
> On Thu, Jan 19, 2012 at 11:29 PM, Greg Smith<greg(at)2ndquadrant(dot)com>  wrote:
>> vacuum_cost_page_hit = 0.1
>> vacuum_cost_page_miss = 1.0
>> vacuum_cost_page_dirty = 2.0
>> Now add in the new setting, which is explicitly said to be the read value:
>> vacuum_cost_read_limit = 8000 # maximum page miss read rate in
>> kilobytes/second
> That may be a little better, but I still don't think it's worth
> breaking backward compatibility for.  I mean, suppose I don't care
> about read rate, but I want to limit my dirty data rate to 1MB/s.
> What parameters should I set?

vacuum_cost_page_dirty = 8.0

The resulting maximum rates will then be:

hit = 80MB/s
miss = 8MB/s
dirty = 1MB/s

The question you should ask yourself next is "how do I limit my dirty data rate to 1MB/s in 9.1?"  Working that out by hand is a good exercise, to show just how much less complicated this proposal is over the current state of things.  Show me how it's possible to do that in way we can expect new DBAs to follow, then the idea of keeping strong backwards compatibility here would have some weight.  I see sticking too closely to the current scheme as being more bug-level compatibility; it's fundamentally broken, by being too difficult to use, to most people in its current form.

Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support

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