Date: Tue, 4 Apr 1995 00:18:47 +1000 From: Bruce Evans <bde@zeta.org.au> To: rgrimes@gndrsh.aac.dev.com, taob@gate.sinica.edu.tw Cc: freebsd-current@FreeBSD.org Subject: Re: New installation notes Message-ID: <199504031418.AAA26234@godzilla.zeta.org.au>
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>> But I were to do >> something like two 'dd if=blah of=/dev/null bs=65536' on files from >> each drive simultaneously, I should still be able to hit close to the >> maximum throughput, no? Probably not. On my system (DX2/66 VLB BT445C), one 7200rpm drive seems to hit a limit at 5.4MB/sec and reading another 3600rpm drive at the same time reduces the total throughput. >One needs to be very carefull when doing benchmarks to understand >what it is you are really measureing. And to attempt to make the >test measure what you really want it to measure. I image a carefully >written C program could do the ``repeat 1000 dd'' above and obtain >close to SCSI bus bandwidth if that is what you wanted to measure. >(I also suspect in this case the real bottleneck would be the disk >controller itself). Yes :-(. For the BT445C, repeatedly reading the same 512-byte block is almost exactly as slow as reading sequential 512-byte blocks. The speed depends on the drive too. I get about 180K/sec for a slow drive and 300K/sec for a fast drive. An Ultrastor 34F controller is 40% slower. Bruce
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