There are many reasons why SCSI hard disk drives can show low performance scores compared with similar ATA drives. To make an accurate comparison, drives must have similar features. The drives must have the same rotational speed, same capacity point, and the roughly the same date of manufacturer. The operating system, SCSI and ATA hard drive controllers all must be optimized for maximum hard drive performance.
A hard drive in a pre-configured system may already be optimized to its limits. When adding any new device or application, there is opportunity to impact performance. All computer systems represent a balance between performance penalties and desirable benefits and features.
Three areas outside of the hard drive directly impact performance--hard drive cache, drive controller settings, and system board capabilities. The SCSI controller can (and often does) disable hard drive cache memory - for the sake of data integrity. There is a performance penalty with the disk drive's cache disabled. It's faster to get data from cache. With the cache disabled, a potential bottle neck is created by the read/write heads, the media, and the drive's read/write channel.
SCSI utilities, such as EZ-SCSI from Adaptec, SCSI VIEW from BUS LOGIC, or a SCSI Mode page editor - PTI SCSI TOOL BOX, or Extreme Protocol Solutions EXTREME SCSI from have the ability to enable a hard drive's read and write cache.
You are responsible to confirm the operating system's default and optional features. Is DMA (direct memory access) enabled by the operating system? (Microsoft operating system users can view hard drive disk drive properties from the My Computer icon). Be aware that even if you did not actively change a function, parameters can be altered by default action of a software application.
DMA and Bus Mastering
At the system board level DMA may be disabled (or not supported) by the system board. Read the system board documentation to find out. ATA hard drives now support DMA and UDMA (also called Ultra ATA). The operating system, system board BIOS, CPU, all must support DMA or UDMA (Ultra DMA) transfer. Does the system board support Bus Mastering? This related feature can impact drive performance. Bus Mastering allows the CPU to offload commands to the system RAM, thus freeing up CPU cycles.
SCSI Controller
The default settings of the SCSI controller are best for most workloads. You should confirm all default settings for the SCSI controller with the device manufacturer. The hard drive will perform to the constraints set by the SCSI controller. If yours is an embedded (on the system board) SCSI controller, read the system board documentation. Embedded SCSI controllers are typically manufactured to the specifications of a specific system-board manufacturer and may contain a different feature set from their SCSI host adapter counterparts.
Feature vs. Benefit
Even if the SCSI drive is optimized for its environment - the ATA drive may still be faster. In recent years the ATA interface is catching up to SCSI speeds (now at Ultra ATA/133). A notable SCSI benefit allows up to fifteen dissimilar devices to independently use the same channel. This is useful in multi-drive environments (RAID.) System designers specify SCSI for multi-tasking, multi-user environments. This is where SCSI realizes it biggest performance gains over ATA drives.
There will always be contention for the best and the fastest hard-drive interface. Your choice depends on your intended workload - file server or personal computer? Every interface has unique benefits and penalties. If high data throughput in a single user environment is the only requirement, then ATA may be your best choice. In a multi-tasking, multi-user environment, with demands for the highest level of signal integrity and fail-safe characteristics, then SCSI may be the better solution.
