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Re: Vacuum rate limit in KBps

From: Jim Nasby <jim(at)nasby(dot)net>
To: Greg Smith <greg(at)2ndquadrant(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: 2012-01-18 02:00:16
Message-ID: 5C4C2A81-9334-4069-A8BD-EA49C464B1B9@nasby.net (view raw or flat)
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On Jan 15, 2012, at 8:13 PM, Greg Smith wrote:
> On 01/15/2012 04:17 PM, Heikki Linnakangas 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?
> 
> I haven't had the I/O rate logging available for long enough to have a good feel for which is more important to emphasize.  I'm agnostic on this.  I'd have no problem accepting the argument that exposing the larger of the two rates--which is the read one--makes for a cleaner UI.  Or that it is the one more like other knobs setting precedents here.

Could we expose both?

On our systems writes are extremely cheap... we don't do a ton of them (relatively speaking), so they tend to just fit into BBU cache. Reads on the other hard are a lot more expensive, at least if they end up actually hitting disk. So we actually set page_dirty and page_hit the same.
--
Jim C. Nasby, Database Architect                   jim(at)nasby(dot)net
512.569.9461 (cell)                         http://jim.nasby.net



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