Date: Thu, 03 Dec 2009 08:37:08 +0100 From: Miroslav Lachman <000.fbsd@quip.cz> To: Maxim Sobolev <sobomax@FreeBSD.org> Cc: "Derek \(freebsd lists\)" <482254ac@razorfever.net>, Alexander Motin <mav@FreeBSD.org>, FreeBSD-Current <freebsd-current@FreeBSD.org>, Emil Mikulic <emikulic@gmail.com> Subject: Re: gmirror 'load' algorithm (Was: Re: siis/atacam/ata/gmirror 8.0-BETA3 disk performance) Message-ID: <4B176AA4.60204@quip.cz> In-Reply-To: <4B16FFA9.6070002@FreeBSD.org> References: <h7lmvl$ebq$1@FreeBSD.cs.nctu.edu.tw> <4A9E8677.1020208@FreeBSD.org> <20090903002106.GB17538@dmr.ath.cx> <4AA0075A.5010109@FreeBSD.org> <4B16FFA9.6070002@FreeBSD.org>
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Maxim Sobolev wrote: > 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. Have you any numbers from benchmarks for different type of load? (I will try it when I found some free time) Maybe both algorithms can be implemented, one as 'load-offset' and one as 'load-queue', so users can use which one is better for their workload. Miroslav Lachman
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