Date: Wed, 06 Mar 2002 13:14:48 -0700 From: Ian <freebsd@damnhippie.dyndns.org> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: <freebsd-hackers@freebsd.org> Subject: Re: A weird disk behaviour Message-ID: <B8ABC6C8.ACA5%freebsd@damnhippie.dyndns.org> In-Reply-To: <Pine.SOL.4.21.0203061247260.6633-100000@onyx>
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>> >> When I instrumented code in various places to try to track down why there >> was such a huge difference when fragsize != blocksize I found that the >> killer was repeated read-modify-write cycles, especially on filesystem >> metadata. Creating a file and writing a few bytes to it could result in >> dozens of blocks read then written, and some of the blocks got re-read >> several times in the process. It was always a mystery to me why the same >> sectors would get read over and over again (isn't that what buffer and >> filesystem caches are for?) But I know for certain the physical reads were >> happening because the instrumentation for that was in a custom raid driver >> of our own. > > Could you tell me where is your custom raid driver? I mean, is it part of > the operating system or inside the disk controller? > It was custom hardware and software for an embedded system. We had a motherboard with 18 adaptec AIC 78xx chips on it, and our own software raid layer (modified freebsd kernel) that did special-purpose stuff (striping and error recovery using custom parity-generating hardware also on the mobo). I mentioned the raid driver only as a way of indicating that I was sure I was instrumenting real physical IO at the very lowest talk-to-the-drive layer, and I was seeing read-modify-write cycles at that layer that indicated the same sectors were being read over and over again (filesystem metadata sectors) during the higher-level operation of create-file, write 1 byte to file, close file. -- Ian To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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