Date: Sun, 8 Oct 1995 09:48:03 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, se@zpr.uni-koeln.de Cc: hackers@freebsd.org Subject: Re: VLB Disk Controllers Message-ID: <199510072348.JAA07022@godzilla.zeta.org.au>
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>} >Seems the NCR is faster on 512 byte transfers. >} >} Nope, the 573 us is for _8_ transfers of 512 bytes because my drive >} doesn't support multi-mode, and it's on a slower machine :-). I'm >} surprised that it is as high as 573/8 = 72 us per command. This is all >} host software (not bus related) overhead except for about 10 usec to >} write the command to the controller. >Don't think so. There are 16 transfers in the 8KB test, >and thus the per sector overhead is in fact accounted >for in the transfer rate ! That's true for IDE, but not for SCSI. The NCR would be much slower if it was forced to issue 7 more commands per 4K. That's part of its advantage. However, multi-sector transfers for IDE might give the same advantage to IDE. The reduction in the command overhead would not be 7 * 72, it would be 7 * (72 - drive_overhead_per_sector). It's likely that the drive processor is slower than an i486 and possible that the drive processor+firmware is slower than an i486+software. Thus the reduction may be negative :-). Actually :-(. >} 512 168550 >}... >} 262144 5557452 >This was measured using raw device reads ? Yes, on the inner tracks. >Ok. Using my 2GB Atlas (dd, 30 sec each): > 512 811800 > 1024 1324517 > 2048 2097152 > 4096 4194304 > 16384 6068148 > 65536 7281777 dd is not so good for this sort of test because the time for outputtinh to /dev/null is significant. Both my benchmark and dd have granularity problems - they count the time in seconds. They should use gettimeofday() instead of time(). >(BTW: I've got 1585 transfers/s in the 512 byte test. >That's 630 us per *transfer*. And the startup overhead >can't possibly be higher :) The NCR seems to be a bit faster than the Adaptec 2842. Bruce
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