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Ultra DMA and SCSI devices

What is the difference between Ultra DMA and SCSI storage devices?

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Ultra DMA devices conform to the ATA/ATAPI protocol. The ATA Ultra 5 spec(ATA/100) http://www.t13.org calls for a burst transfer rate using DMA(Direct Memory Access) of 100MB per second. The AT spec is for interface adapters generally used with IDE (integrated drive electronics) drives, and ATAPI is the application programming interface for the AT spec, (the protocol).

Most Ultra DMA drives tend to be used in desktop PC's where very high I/O rates are not as important as on servers. DMA devices can "burst" data across the bus at rates as high as 100MB per second, but this is not a sustained throughput.

SCSI (small computer storage interconnect) devices, and the associated SCSI command set and controllers, are able to "queue" I/O through the adapter to the drive, so the CPU can move on to handle more instructions rather than doing waiting on the return of data from the drive. This is a major benefit over ATA devices, and one of the reasons you will usually find SCSI devices in high performance servers. SCSI is also the actual underlying command set used in Fibre SANs today. This allows application programmers to not have to recode for a new command set when using an optical interconnect like Fibre Channel when doing I/O. You will find that SCSI devices usually have a lower CPU utilization overhead than IDE devices. This is due to the intelligent controllers handling most of the I/O task overhead instead of the CPU in the system.

SCSI controllers also have the ability to do RAID, (redundant array of inexpensive disks)which provides for drive failure protection for your data.

Chris

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This was first published in September 2001

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