Date: Wed, 02 Dec 2009 16:00:41 -0800 From: Maxim Sobolev <sobomax@FreeBSD.org> To: Alexander Motin <mav@FreeBSD.org> Cc: FreeBSD-Current <freebsd-current@FreeBSD.org>, "Derek \(freebsd lists\)" <482254ac@razorfever.net>, Emil Mikulic <emikulic@gmail.com> Subject: Re: gmirror 'load' algorithm (Was: Re: siis/atacam/ata/gmirror 8.0-BETA3 disk performance) Message-ID: <4B16FFA9.6070002@FreeBSD.org> In-Reply-To: <4AA0075A.5010109@FreeBSD.org> References: <h7lmvl$ebq$1@FreeBSD.cs.nctu.edu.tw> <4A9E8677.1020208@FreeBSD.org> <20090903002106.GB17538@dmr.ath.cx> <4AA0075A.5010109@FreeBSD.org>
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Alexander Motin wrote: > I have played a bit with this patch on 4-disk mirror. It works better > then original algorithm, but still not perfect. > > 1. I have managed situation with 4 read streams when 3 drives were busy, > while forth one was completely idle. gmirror prefer constantly seek one > of drives on short distances, but not to use idle drive, because it's > heads were few gigabytes away from that point. > > IMHO request locality priority should be made almost equal for any > nonzero distances. As we can see with split mode, even small gaps > between requests can significantly reduce drive performance. So I think > it is not so important if data are 100MB or 500GB away from current head > position. It is perfect case when requests are completely sequential. > But everything beyond few megabytes from current position just won't fit > drive cache. > > 2. IMHO it would be much better to use averaged request queue depth as > load measure, instead of last request submit time. Request submit time > works fine only for equal requests, equal drives and serialized load, > but it is actually the case where complicated load balancing is just not > needed. The fact that some drive just got request does not mean > anything, if some another one got 50 requests one second ago and still > processes them. Can you try this one: http://sobomax.sippysoft.com/~sobomax/geom_mirror.diff It implements different logic - instead of looking for the time, it checks the outstanding requests queue length and recently served requests proximity to decide where to schedule requests. -Maxim
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