White Paper: Tape Drive I/O Interfaces: SCSI or EIDE?

Feature

White Paper: Tape Drive I/O Interfaces: SCSI or EIDE?

Quality, performance and scalability versus short term savings

The alternatives

EIDE, as an interface, is attractive for users that mainly focus on obtaining the lowest price and, for this reason, is currently the best solution for home computing.

Compared to EIDE, SCSI devices offer better performance and higher reliability, and is, for this reason, a better solution for business applications. Microsoft and Intel (WS 98 specification) agree and advise business users to select SCSI for current and future data intensive business applications since "SCSI is a flexible I/O bus that supports good performance for access and throughput to meet a workstation's (WS) intensive data transfer needs".

The difference

EIDE uses the DMA channel of the computer, controlled by the CPU. This is cheap as the resources are already there. Ultra DMA is the state-of-the-art implementation for EIDE. Computers may have two embedded EIDE controllers, each accessing up to two peripheral devices. This limits the total number of internal peripheral devices in the system to four.

A SCSI interface needs a SCSI controller. Some computers have embedded SCSI controllers, but, in most cases, one may plug in on the system bus. Data transfers are controlled by the SCSI controller and do not put a heavy burden on the system's CPU. An 8-bit SCSI bus connects up to seven peripherals, a 16-bit SCSI bus connects up to 15 internal and external peripheral devices.

The technical advantages

In most systems, SCSI will provide a higher performance or data throughput than EIDE. Why? On EIDE, only one command is allowed at a time per IDE bus. On SCSI, multiple commands to one or more devices can be executed at a time. SCSI also supports parity checking.

SCSI is a multi-threaded I/O interface. It can process multiple I/O requests at a time and bus bandwidth can be shared among all connected devices. Not only can multiple devices process requests simultaneously, but multiple requests can also be processed simultaneously by one device. Commands are queued and are processed when a device is ready to execute. Devices are also allowed to change the order of requests to minimise the total time to service the requests. For example, one request can be processed while another is waiting for an operation to be executed. When one device has an outstanding request it disconnects from the SCSI bus until it has information to send, thereby leaving the bandwidth to other connected devices. SCSI is the only I/O connection enabling multitasking storage access under multitasking operating systems, such as Windows 98 and Windows NT.

The Ultra DMA used by EIDE remains a single-threaded I/O interface. It can execute only one I/O request at a time. Ultra DMA does not support overlapped seeks, bus bandwidth sharing or command queuing. This, of course, restricts the total data throughput as one device must wait for the previous to finish before it can start its transfers. Ultra DMA uses the system bus of the computer and is thus loading the system bus capacity.

User benefits

Most computer users want the latest and greatest. Then one should not degrade the system by building a low performance structure. One of the benefits of SCSI is that system performance is maintained when the system is expanded. The advantage of SCSI is that you can add new devices, such as extra disks, CD-R, scanners, tape drives or tape automation, on the existing interface bus. These devices can work together without creating bottlenecks or slowing down the system bus.

New applications always demand more storage capacity for installing the application itself, for working memory while processing and for storing the created files. Eventually the storage capacities of a system must be increased. When building on a SCSI system, new storage devices can easily be added without degrading the system. Adding new devices on an EIDE bus will steal bus bandwidth. The greater number of devices allowed on the bus is also a SCSI benefit, as you can have up to 15 SCSI identities on the bus vs. two + two on an EIDE bus.

Another benefit for the SCSI system user is that SCSI is the leading edge interface for peripheral devices and because of this, new products are launched with a SCSI interface first, as SCSI connects to the widest range of different peripherals available. Leading edge peripheral devices will always be available to the SCSI user, providing the benefit of getting the best performance and features. The cost sensitive user can always choose to wait for the second generation product which usually ships at a maturer price.

When the user requirements change, new devices may need to be added, but who wants to remove something old that might be used once more? Most systems have a limited number of slots for peripheral devices. It may be necessary to connect tabletop devices. If the computer has a SCSI adapter with both internal and external connectors, this is easily accomplished. Internal and external SCSI devices can be mixed on the bus and the cable can be up to three metres in single-ended systems (25 metres with differential drivers). The same flexibility is found for the choice of peripherals with an eight or 16-bit wide bus interface; both can be used on the same wide SCSI bus. For system control, SCSI multitasking storage capability makes SCSI superior in managing multiple peripherals on a system without slowing down overall performance.

Does it matter in my applications? Is SCSI really faster than EIDE?

There are many potential bottlenecks capable of affecting the overall performance of a computer system. Processor, memory, I/O bus, adapters and storage subsystems must be configured to work with each other and with the network. Most computers today are linked together, and networks also depend on the speed/performance of single computers. The I/O interface is an important part of the system, don't let it be a bottleneck, as current CPUs are so fast that the I/O usually limits performance.

A first glance the EIDE and SCSI specifications do not indicate significant performance differences, but it will show up in system benchmark testing. The degree of difference will depend on the system configuration and application. The more data-intensive the task, the greater the advantage of using SCSI. Performance testing has been executed to try to isolate the I/O influence on the performance. This shows that when using real applications such as AutoCAD, PhotoShop, Excel and FileMaker Pro, the time to open and save files is increased by 14 to 33 per cent when using EIDE compared to SCSI. In tests using a scanner, the time used by the EIDE system was 50 to 70 per cent longer for files of 13Mb to 29Mb sizes. (Source: Adaptec).

The future

It's difficult to predict what requirements for performance, speed, network configuration and storage capacity the future will bring. In general, most systems need some kind of upgrade every year. Knowing this the systems manager should prepare for this when installing or enlarging a system. New or upgraded applications and peripheral devices and growing networks will increase the workload and complexity. One of the precautions that should be made when growth is expected is to base the system on a SCSI I/O bus.

Usually, not all computers in a network will need the same annual upgrade. Some may possibly not be upgraded at all, but servers will. In addition, a server will, most likely, have a tape device attached. This is why most servers are SCSI based and why tape drives will also be SCSI devices. The same arguments will be valid for a powerful standalone WS running powerful, standalone applications. The CPU power required will increase and the I/O is most likely a bottleneck.

The cost

Most people admit that their tools are important to get the work done. Slower computers make the job slower. Why save money on tools when one uses more time working? Alternatively, why buy an expensive computer with a low performance I/O? The I/O performance issue will become increasingly significant as the computer workload grows. Saving initial investment may lead to losses in the long term. Not only when counting the accumulated numbers of hours needed for the computations, but also when looking at the system lifetime. A system that cannot grow with increased demands will reach obsoleteness early, resulting in a total system replacement, not just an upgrade. And this adds cost!

Conclusion

While EIDE and Ultra DMA provides the minimum functionality and performance required by non-demanding desktop PC users, SCSI is built for performance I/O requirements of workstations, file servers and sophisticated applications.

Obviously, all users who care about system performance should choose SCSI as their peripheral interface. But should the desktop PC user care about the performance? It depends, of course. If the applications being used are not very I/O intensive, it may function fine with an IDE hard disk and CD or DVD player.

If a tape drive is also installed, the user is probably a professional and it is then quite probable that further sophisticated applications are or will be installed. Perhaps not at outset, but it will happen. In a total cost of ownership perspective, most professional users will be better off when using SCSI I/O.

Using a professional Tandberg Data SCSI tape drive lets your system grow with your requirements and will keep you in business if your hard drive data is lost, stolen or damaged.

( 1999, Morten Ween, Corporate Product Manager, Travan. Tandberg Data ASA


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This was first published in November 1999

 

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