enabling DMA on hard drives

[Peter L. Peres]

>    enabling it (and thus increasing the Mb/sec of the drive).  If not
>    enabled, the difference in performance could be substantial.

Unfortunately unless one is using high end SCSI drives the throughput 
will remain very low, limited by the c**p controller and r/w head 
throughput (normal ide/eide/ata/sata will not go beyond 10-12MBps and 
that is very fast by the average standards of off the shelf hdds in my 
experience). hdparm will tell you if you can gain from DMA. hdparm -Tt 
will determine the cache and the real disk r/w speeds. The latter is 
usually 10 times less then the cache r/w speed and is the real 
bottleneck. Enabling DMA should not (and does not as far as I could 
test) improve this. The initialisation code in the kernels puts the hdds 
into a 'best match' mode with the available capabilities. And if one 
does use SCSI then it's the SCSI controller's DMA that must be turned 
on.

Peter
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