3.09 Which is better, SCSI or IDE?
Description
This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).
3.09 Which is better, SCSI or IDE?
[From: ralf@alum.wpi.edu (Ralph Valentino)]
IDE vs SCSI
Non-issues:
1) SCSI and IDE devices cost approximately the same for the same features (size, speed, access time). Shop around for good prices.
Advantages of IDE:
1) faster response time (low request overhead)
2) hard drive interface is compatible with RLL/MFM/ESDI: any driver for one (including the main system BIOS) will run the other.
3) IDE controllers are considerably cheaper ($150 and up) than SCSI host adapters.
4) Will always be the boot device when mixed with SCSI.
Advantages of SCSI:
1) Supports up to 7 devices per host adapter. This saves slots, IRQ's, DMA channels and, as you add devices, money.
2) Supports different types of devices simultaneously the same host adapter (hard drives, tape drives, CDROMs, scanners, etc).
3) SCSI devices will work in other systems as well (Mac, Sparc, and countless other workstations and mainframes). If you change platforms in the future, you will still be able to use your SCSI devices.
4) Automatically configures device type, geometry (size), speed and even manufacturer/model number(SCSI-2). No need to look up CMOS settings.
5) Busmastering DMA (available in all but a few cheap SCSI host adapters) decreases amount of CPU time required to do I/O, leaving more time to work on other tasks (in multitasking OS's only).
6) Software portability - drivers are written for the host adapter, not the specific device. That is, if you have a CDROM driver for your host adapter, you can purchase any brand or speed SCSI CDROM drive and it will work in your system.
7) Will coexist with any other type of controller (IDE/RLL/MFM/ESDI) or host adapter (other SCSI cards) without any special tricks. SCSI host adapters do not take up one of the two available hard drive controller port addresses.
8) greater bandwidth utilization (higher throughput) with multiple devices. Supports pending requests, which allows the system to overlap requests to multiple devices so that one device can be seeking while the second is returning data.
9) Ability to "share" devices between machines by connecting them to the same SCSI bus. (note: this is considerably more difficult to do than it sounds).
10) Bridges are available to hook RLL and ESDI drives to your SCSI host adapter. (note: these tend to be prohibitively expensive, though).
Warnings:
1) With otherwise equal drives, IDE will perform better in DOS due to low command overhead. SCSI, however, will perform better in multitasking OS's (OS/2, Unix, NT, etc). If you see speed comparisons (benchmarks), make sure you know what OS they were run under.
2) Most benchmarks only test one aspect of your system at a time, not the effect various aspects have on each other. For instance, an IDE drive may get faster throughput but hurt CPU performance during the transfer, so your system may actually run slower. Similar confusions arise when comparing VLB and EISA host adapters.
3) When comparing two systems, keep in mind that CPU, memory, cache, and bus speed/type will all effect disk performance. If someone gets great I/O performance with a particular controller/drive combination on his Pentium, you should not expect your 386SX-25 to get such I/O performance even with the exact same controller/drive combination.
4) Similarly sized or even priced drives may not perform equally, even if they're made by the same manufacturer. If you're going to compare two drives, make sure they have the exact same model number. (IDE drives usually have an 'A' and SCSI drives usually have an 'S' appended to their model number).
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