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What hard drive technology to choose, SCSI or EIDE? Are SCSI hard drives better than EIDE hard drives? This is a topic of many books and magazine articles. The purpose of this report is to give a somewhat detailed comparison between SCSI (small computer system interface) hard drives and EIDE (enhanced integrated drive electronics) hard drives.

EIDE is great a solution for those on a limited budget. If money is no consideration and youre using something more modern than DOS or Windows version , SCSI should be your hard drive of choice. Unfortunately, for most of us budget is limited, and theres a real choice to be made between a SCSI based system on one hand, and an EIDE based one with more RAM, a slightly faster CPU or better screen, on the other. You need to weigh the pros and cons in your specific situation. So why purchase SCSI? A single SCSI channel supports up to seven devices. Two channels, for a total of fourteen devices, are becoming more and more common. On the EIDE side of things, four channels are defined, giving eight devices. However, common hardware and software supports only two channels so beyond four devices things become stickier.

Real high end devices are available in SCSI drives only. Fairly common examples are extremely large or fast 10,000-rpm hard disks and DAT tape drives. For that reason alone, if you need high speed out of a hard drive, SCSI is your only choice. SCSI supports many different types of devices. The ATAPI (AT Attachment Packet Interface) protocol EIDE can support a fairly wide range as well, but in practice only hard drives, CD-ROMs and tapes are really common. Magneto-optical devices etc. are possible and may eventually become corrupt. Scanners and so forth wont arrive in EIDE versions anytime soon, since it does not support external connections. Summarizing you could say that EIDE meets the needs of the majority, but not all, of the users.

EIDE has the problem that it targets the low end market. Some vendors and manufacturers tend to save a dollar too much. This has resulted in badly designed and downright buggy interfaces and chips. The worst excesses appear to be behind us. EIDE PIO (programmed input/output) mode 4 has proven relatively unreliable for a number of reasons. These problems have been addressed by the ATA- spec, and the even faster modes of Ultra-ATA ensure data integrity using CRC (cyclic redundancy checking) checksums. It will take time for all this to filter through to the marketplace though. In the mean time, mode 4 may or may not work reliably in your brand new setup. SCSI has no such reliability problems as long as your cables are decent and everything is properly terminated. To retain your busmastering ISA bus adapter when upgrading to a modern PCI system, be careful; this can result in data corruption, depending on the PCI to ISA bridge used on the motherboard.

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The scheme used to access disks of more than 504MB through the BIOS, has stabilized by now. Its still not ideal (you cannot switch between LBA (logical block addressing) and LARGE translation in some setups). Then again, the mapping used by different SCSI interfaces isnt always identical either. More worrying are the common BIOS bugs showing up at GB and 4GB capacity points. These are bugs, not EIDE limits per se, but that knowledge doesnt help you much if there are no updated flash BIOS available for your motherboard. The maturity of SCSI is a definite advantage here. Finally, there is an 8GB barrier on the horizon, but that one is a PC-specific BIOS limitation affecting SCSI and EIDE equally. Even today, SCSI is probably a safer choice; but whatever you choose, youre better off not buying the cheapest hardware available.

Even though modern EIDE interfaces (for example the PIIX used with the Intel Triton and Natoma chipsets) can handle busmastering, this is a relatively recent development. Support for SCSI has matured for years. This shows in superior features (scatter/gather), quality (drivers) and ease of use with SCSI compared to inconsistent EIDE support. Attempts to use busmastering with an ATAPI (IDE) CD-ROM still all too often fail. Even if the busmastering driver works correctly, it may still use old fashioned PIO for the CD-ROM device, because many of these simply dont support DMA, or the interface has funny restrictions with respect to ATAPI equipment.

This situation will improve in time, not least due to standardization efforts on the hardware level. Once busmastering works, it drives the CPU usage of EIDE devices to low enough levels that its not much of a consideration for single user systems. Busmastering is important in multitasking environments, since it greatly reduces the number of CPU cycles consumed by data transfers. Do not expect it to raise the transfer rates themselves. For single user machines, EIDE busmastering implementations do a decent job. Servers, on the other hand, exist to pump large amounts of data around; for these, you wouldnt consider anything but the best busmastering capable interfaces.

With SCSI you can put all your devices to work simultaneously, provided youre using an operating system supporting this feature (DOS and Win do not). The devices will interfere slightly with each other due to the limited total bandwidth of the SCSI bus (10-40MB/s for todays hardware; actually, the command overhead may be more of a problem). With EIDE, only one of the two devices on each channel (cable) can be active at a given time. Between different channels though there is no such restriction. Modern interfaces and operating systems like OS/ or Linux allow access to as many devices as youve got channels -- usually two. This can be put to good use with some careful planning.

Exceptions to this rule are some badly designed interface chips which prevent this or even corrupt data if you try (CMD640!). Probably for that reason, this feature has some support in Win5 only for the PIIX (used on Intel Triton and Natoma based mainboards). In all other cases Win5 will access no more than one EIDE device at any given time, which really impacts performance if multiple tasks are performing I/O. This is especially bad with ATAPI tapes and CD-ROMs, since commands on these devices can take a pretty long time to complete.

EIDE will certainly incorporate some form of command overlap in the future but its unlikely to be as powerful as SCSI. Access to multiple devices is important in multitasking environments, and positively vital in multi-user and server type applications. SCSI is, and will remain, more flexible here. EIDE suffers doubly because of the loveless support offered by Win5. The intelligence of the SCSI controller and protocol allows some nifty optimizations. One of the most important is tagged command queuing; a device can absorb a number of commands and execute them in a different order than they were issued. The operating system will do the same to some extent if you need to access a hard disk on, say, cylinder 0, 1000, and 500, it doesnt take many CPU cycles to figure out that youd better swap the last two. A device can always do a better job of it because it has complete information on its own internal structure and current state. Many devices arent all that intelligent about it in practice, though. The effects are difficult to quantify but usually small. This, too, is a feature that really comes into its own in multitasking situations.

The first thing to note is that no single device except a solid-state drive can achieve 16MB/s sustained throughput, let alone 40MB/s. The reason that SCSI bandwidth should be exceptionally high is that it needs to be shared by all devices on the bus. Multiple devices may very well generate a controlled data flow of tens of megabytes per second. On the other hand, the 16MB/s bandwidth of the EIDE bus is shared by just two devices, making the per device bandwidth higher than even UW-SCSI. Even so, the most recent development in EIDE is Ultra-ATA, which adds a MB/s DMA mode (DMA/) to its repertoire. This is more of marketing than practical value at present. Another example ATAPI devices (tape, CD-ROM) may or may not support advanced PIO modes, may or may not support DMA, and there are three different operating modes which can make a real difference in CPU usage (microprocessor DRQ, interrupt DRQ, accelerated DRQ). The half-hearted EIDE support offered by Win5 has already been remarked upon. Microsoft has caused a tiny uproar on the ATA mailing list by declining to commit to further development not in favor of SCSI though they want to focus their efforts on the next generation interface, which will be a fast serial one.

In conclusion, theres very little end user consciousness about important performance options in EIDE hardware and software, which of course doesnt tend to encourage rapid improvement. In the mean time, the choice for most of us is still pretty much limited to either SCSI or EIDE. SCSI would be directed more toward high speed data transactions while EIDE would be more suitable for the home user where high data transfer is not really an issue.

Work Cited

Gary Field, Peter M. Ridge. The Book of SCSI I/O for the New Millennium.

New York No Starch Press, 000.

Michael E. Powers. Clean Up Your Hard Drive, v1..

New York Michael E. Powers, 00.

Operating Systems NIIT school book

http//www.epinions.com/cmd-review-5AD-CCFE88B-87F5-prod

http//www.aiyamicro.com/scsi_101.htm

http//www.paralan.com/artech1.html

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