You asked..

>I'm always puzzled by these numbers. How come copying files between my HDs
>never seem to exceed 2.5MB/Sec (burst speed)? My SCSI card is rated as
>20MB/Sec Maximum, but copying speed seem to be around 1MB average. What
>keeps it from reaching the maxium potential speed?

There are a lot more factors involved than just the SCSI bus speed. You have to
take into account that the disk itself is not capable of unlimited speed. The
platters pass under the heads only so fast, and the data will not come off any
faster. Furthermore, many modern disks use a zone recording method where the
sectors/track varies from the inside to the ouside of the platters. This is
simply because the outer tracks have a greater circumference and can hold more,
so the data transfer is higher when reading the outer tracks.

Other factors may include the performance of the SCSI controller. Some cheap
controllers use PIO to transfer the data, and will not get as good performance
as a DMA bus master controller.

Calculating the theoretical transfer rate from the disk can be calculated once
you know the sector density, and the rotational rate of the disk.

Russ Poffenberger               Engineering Specialist
Schlumberger Technologies ATE   DOMAIN: [log in to unmask]
1601 Technology Drive           CIS:    72401,276
San Jose, Ca. 95110             Voice: (408)437-5254  FAX: (408)437-5246