MSCP SCSI controller speed

Brad Parker brad at heeltoe.com
Thu Nov 10 10:54:42 CST 2005


9000 VAX wrote:
>On 11/9/05, 9000 VAX <vax9000 at gmail.com> wrote:
>> On 11/9/05, 9000 VAX <vax9000 at gmail.com> wrote:
>> > >
>> > >         dd if=syssrc.tar of=/dev/null bs=16k count=1000
>> > 16384000 bytes transferred in 21 secs (780190 bytes/sec)
>> > >         dd if=/dev/zero of=blah.null bs=16k count=1000
>> > 16384000 bytes transferred in 35 secs (468114 bytes/sec)
>> > >
>> > > I suspect the write will be faster, and reflect the true speed of the i/
>o.
>> > It is the opposite. strange.
>>
>> OK, maybe you are right. The write operation should be faster. The OS
>> latency there is smaller. It was my controller that slowed down the
>> write operation. I need to take a look to find out why.
>
>The read operation used block mode DMA and the write operation used
>burst mode DMA (slower). It seems there is more debugging work to do.

I didn't want to get into caching, but what the heck.  It's quite
possible that once you "read" the file it will be in the block cache.
So you may get skewed results - cached blocks won't come from the
device.  The first read should be ok, however.

The simplest thing is to read/write the "raw" device (not the block device)
which will be uncached.  This, of course, runs the risk of trashing any
file system if there is one (when writing).  Please be careful.

-brad




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