Ask The SCSI Expert - Questions and Answers - Technology Information

Computer Technology Review,  July, 1999  

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Visit the SCSI Trade Association's web site (www.scsita.org) to ask the SCSI Expert a question.

Q: Is it possible to allow two computers to share SCSI devices? I would like two computers to be able to read and write to the same hard drive and CD-ROM drive.

I realize that this could be done by networking or by other methods, but I was curious if SCSI could accomplish this. I need availability without rebooting, though not necessarily the ability to read/write at the same time. For example, a switched solution would work if both systems recognized the shared devices without reboots, but only one had them available at any given moment.

A: Sharing a SCSI bus is fairly straightforward at the hardware level. As usual, you will need to ensure that the SCSI Ids are all unique.

Sharing at the software level is not so easy. Operating systems (including Linux) have file management software that is written with the assumption that they are the only entity updating the file system. If two file managers access the same disk at the same time, file structures get corrupted.

Even if one system only reads files, it can get confused because it might cache a structure that the other system updates. If both systems only read (as would be the case with a CD-ROM), then sharing is possible.

NFS and other networked file sharing protocols get around this problem by talking to the remote system's file manager and not directly to the disk drive. Thus, only one file manager accesses a file system at a time.

There is an early standards effort at using SCSI in a networked fashion called SCSI Secure Sockets (SSS). This very early draft standard is available at ftp.symbios.com/pub/standards/io/t10/drafts/sss/sss-r00.pdf.

Q: The SCSI terminology page defines a maximum cable length for a SCSI interface running at maximum speed: Do you have the maximum length for an interface that is running at some other speed? For example, what would be the maximum length for a Fast SCSI interface running at 4.445MB/sec?

A: The cable distance has more to do with the round trip time of the cable and the loading on the cable. Enhanced Parallel Interface (EPI) technical report reviews the issues of cable length. Thirty meters is possible with Teflon cable and very low loading from the devices. You can view the EPI technical report at http://www.symbios.com/t10.

Q: What is the difference between active and passive termination on a SCSI bus? Why would you choose one over the other? What happens to the SCSI bus when each type of termination is implemented?

A: Active termination is required for the higher speed operations and will make some systems work that will not work with passive termination. For example, on any hard disk or Jaz drive, you should always use active termination. Passive termination is used on slower equipment. If you mix the termination on the bus, it will work, but may give data errors at the higher speeds. Active termination provides higher pull up current and a better impedance match; the characteristics do not change will termpower fluctuations.

Q: Could you point me to a graph or data showing the SCSI 3.5-inch HDD maximum size estimates for the next year or two? I'm seeing 50GB now on the Seagate web site.

A: It follows Moore's Law of doubling every 18 months. In 2001, you can expect 72/100GB disks. Around 2003, you can expect 144/200GB disks. And before 2005, expect 244/400GB disks.

COPYRIGHT 1999 West World Productions, Inc.
COPYRIGHT 2000 Gale Group