Subject: Re: Throttling IO Requests in NetBSD via Congestion Control
To: Bill Studenmund <>
From: TlorD <>
List: tech-kern
Date: 08/28/2006 14:21:22
Bill Studenmund wrote:
> On Mon, Aug 21, 2006 at 04:46:59PM -0400, Kurt J. Lidl wrote:
>> On Mon, Aug 21, 2006 at 01:35:03PM -0700, Bill Studenmund wrote:
>> As for the slowdown that is seen, you might just be forcing more
>> head I/O on the platter, which will quickly dominate any other sources
>> of slowness.
> I don't think so. I can certainly see extra seek action contributing to 
> reads not catching up as much as writes are restrained (i.e. I could see 
> this contributing to a 20% slowdown in writes turning into less than a 20% 
> increase in reads). But we're seeing fewer reads overall.

Just a random thought: what if the induced sleep is interfering with the
I/O scheduler/disk buffers, thus causing more seeks?
Let me try to clarify: good reads and writes are big, sequential ones,
sent one after the other to the disk which then moves the head as little
as possible. If, by adding sleeps, writes are sent few per batch, they
can't be coalesced well and will thus be transformed into bad (small)
writes, which would skyrocket the seek number thus lowering the read
rate. I think.