IDE versus SCSI: Which disk interface is best?





The first PC/AT computers introduced by IBM in the early eighties

used low-capacity, ST506 disk drives. The disk controller used in

these early machines, a Western Digital WD1003, was quite

unsophisticated by today's standards. The WD1003 controlled up to

two ST506 drives, and used the PC's processor to move the data to

and from a buffer in the controller.



Demand for higher disk storage capacities soon reached the limits

of ST506 drives. A new disk interface which offered a solution to

this limitation was ESDI (Enhanced Small Disk Interface). Because

of the large degree of similarity between the older ST506 and new

ESDI interfaces, it was easy for controller manufacturers to build

ESDI controllers which could emulate the original WD1003 ST506

controller register set and protocol. This avoided the necessity of

writing a new BIOS as well as new disk drivers for every PC

operating system. As a result, ESDI was initially the interface of

choice for high-capacity drives used in PCs.



Unfortunately, ESDI also had limitations. Like the ST506, it

transferred data serially, one bit at a time. This placed practical

limitations on the data transfer rate, and because of the fixed

rotational speed, also the maximum number of bits that could be

squeezed onto each track on the disk media. In the late eighties,

drive manufacturers introduced the first fully intelligent SCSI

disk drives by including an embedded disk controller in the drive

electronics.



In addition to offering faster, parallel data transfer, SCSI drives

offered significant architectural advantages over the previous

ST506 and ESDI drives. Since each SCSI drive had its own embedded

disk controller, it was now possible for the computer to issue

simultaneous commands to every drive in the system. Each drive

could then fully overlap its seek, rotational positioning, and read

or write operations with the other SCSI drives. The data could be

fully buffered on the drive and transferred at high speed over the

shared SCSI bus when a time slot was available.



The primary problem with SCSI drives was initially the lack of

software support. Unlike ESDI, SCSI looked nothing like the old

ST506 interface, and thus SCSI controllers which emulated the

WD1003 ST506 disk controller were much more difficult to build. In

the late eighties, DPT built such a controller - the PM2001. The

PM2001 required no special software drivers since it emulated the

old WD1003 ST506 controller register set. The drive companies also

introduced an equivalent product by embedding a WD1003 register set

compatible controller on the disk drive. These drives were called

Integrated Drive Electronics, or IDE, drives.



In addition to requiring no special software, IDE drives offered

the advantage of eliminating the need for any external controller

or host bus adapter. Only a simple paddle card with a couple of ICs

to implement the drive selection logic was required. Since the cost

to build an IDE drive was identical to the cost of a SCSI drive,

many low-end desktop PCs offered special connectors for one or two

IDE drives, thereby freeing up a bus slot and reducing cost.

However, because of a limitation of the original WD1003 controller

register set, no more than two IDE drives are supported by the

standard BIOS.



Each advantage of IDE, however, came with a cost. As PCs advanced

beyond single-user applications, the ability to overlap the

operation of multiple drives became instrumental in obtaining

higher I/O rates from the disk subsystem. IDE drives, which were

tied to the old WD1003 ST506 controller protocol, could only

single-task I/O operations. Once a command had been issued to an

IDE drive, the operating system had to wait until the command had

been completed before issuing another command to any drive.



In addition, WD1003 protocol relied on the processor to move the

data one word at a time between a buffer on the controller and the

computer's RAM. This limited the speed of the data transfer, as

well as precluding hardware scatter-gather support which can be

implemented by bus-mastering SCSI controllers.



Although IDE drives do not require a host adapter card, more and

more PCs require tape backup, CD-ROM or optical drive support, and

thus must still be configured with a SCSI host bus adapter.



In short, in the late eighties, IDE drives initially offered a high

degree of software compatibility at a low cost. However, the

incremental cost of supporting SCSI drives has dropped to zero in

many cases since the SCSI host bus adapter can be shared with other

devices like tape and optical drives. In addition, all major

operating systems today include software support for a large range

of SCSI devices. The disadvantages of IDE, although not as apparent

under DOS, severely limit the performance of multiuser operating

systems like Novell Netware, Unix, or Windows NT, which realize

large benefits from multitasked, overlapped I/O, bus mastering,

scatter/gather memory access, and I/O command queuing, all offered

by SCSI drives with intelligent host bus adapters.





Why are the limitations of IDE not apparent under DOS?

 

  1. IDE does not support overlapped, multitasked I/O. Neither

     does DOS. 



  2. IDE does not support I/O command queuing. Neither does DOS. 



  3. IDE does not support bus mastering, thus data transfer must

     be done via slower PIO (processor I/O). However, unless the

     record size is quite large, reducing data transfer time

     through bus mastering produces little additional performance

     under single-user operating systems like DOS. 



  4. Since IDE supports only PIO data transfer, scatter/gather

     memory access is not possible. However, simple operating

     systems like DOS do not use virtual memory addressing which

     causes memory fragmentation, and thus DOS does not benefit

     from scatter-gather memory access. 



  5. No more than two IDE drives can be supported in a PC system

     without a special, non-standard BIOS. However, most DOS users

     do not require more than two drives. 



  6. Because of limitations in the original WD1003 ST506 controller

     register set addressing, IDE imposes drive capacity

     limitations of 528MB per drive without a special, non-standard

     BIOS. However, few DOS systems require drive capacity greater

     than 528MB. 



  7. Unlike SCSI, IDE does not support tape or optical drives.

     However, most DOS systems today are still not configured with

     tape or optical drives.





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