Troubleshooting DAS, Part 2: Cabling issues

Certain types of cabling issues are more likely with direct-attached storage (DAS) than with storage area network (SAN) or network-attached storage (NAS) installations. This is because DAS is usually physically close to the server or workstation it supports.

Cabling issues are common when a storage array or another external device is connected directly, although they are not as complex as with other storage architectures, especially SANs. Here are two cabling problems to watch out for in DAS installations.

Cable lengths. When it comes to cable lengths, SCSI and some other buses are quite limited. Probably the most severe limit is single-ended Ultra SCSI with a 40 MB/sec data rate. Total cable length, including cables inside devices, is only five feet. More recent SCSI implementations, such as Ultra 2, Ultra 160 and Ultra 320, use low-voltage differential (LVD) connections and can have a total cable length of up to 40 feet. (Note: These distances assume high-quality termination with the appropriate terminators.)

Two things to keep in mind: You should never exceed the recommended cable lengths. Also, the shorter the cables, the better.

Avoid sharp bends. Most cables have a minimum radius for bends. This data will either be specified in the documentation or else the manufacturer's or vendor's Web site will have it. If the cables are bent more sharply, they may perform poorly or fail to work altogether.

Pay attention to situations where the array

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or computer has been shifted by the users. (This situation is particularly common with workstations and desktops with DAS arrays.) In the process of moving things around, the cable may bend more sharply than it should, in which case, errors will probably start popping up. If a cable is bent too sharply, it may be damaged and unreliable. Just replace it.

The SCSI Trade Association provides a chart of SCSI cable lengths.

Part one of this series discusses when to replace aging disk drives in DAS installations. Part three discusses SCSI issues related to DAS.

Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in issues related to storage and storage management.
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This was first published in January 2006

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