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Troubleshooting DAS, Part 3: SCSI issues

SCSI is still a fertile source of potential problems in a direct-attached storage (DAS) system. If your DAS system will not recognize the storage devices or the connection to storage is unreliable, there are five things to check on the SCSI bus.

SCSI is the most common method of directly connecting a Windows server to an external storage device, such as a RAID array. While this method doesn't involve the network complexities that arise with network-attached storage (NAS) or storage area networks (SANs), SCSI is still a fertile source of potential problems in a direct-attached storage (DAS) system.

If the DAS system will not recognize the storage devices or the connection to storage is unreliable, there are five things to check on the SCSI bus.

1. Is the controller properly seated and the connectors tight? When making a SCSI installation, one of the most common sources of trouble is having poor electrical connections. While this problem may be common, it is about the easiest to fix. Simply re-seat the controller card (cleaning the contacts if necessary) and make sure all the connections are tight and secure.

2. Is the system terminated correctly? A SCSI bus can have only two terminators, one at each end. Also, make sure you have the right kind of terminators: Recent versions of SCSI require active (not passive) terminators.

3. Are the device IDs correct? Each device on the SCSI bus, including the controller, needs its own ID. Conflicting ID numbers will confuse the bus and result in unreliable operation. Some devices, such as the controller, may have fixed device numbers, such as 7; you will have to adjust the other device numbers to avoid conflicts.

4. Is everything at the same SCSI level? Over the years there have been a dozen kinds of parallel SCSI interfaces with different speeds, connectors and bus configurations. Some of them are more or less compatible, but others aren't compatible at all. Make sure you are connecting only compatible devices, and remember that mixing faster and slower devices on the same bus, even if they are compatible, will drag everything down to the speed of the slowest device.

5. Are you mixing fast and slow devices on the same controller/bus? Although a SCSI can support a wide variety of devices, from storage to scanners, it is best to separate storage devices on their own bus. Mixing fast devices, such as storage, with slow devices, such as scanners, can result in operation that is less than reliable. Part one of this series discussed when to replace a hard drive in DAS installations. Part two discussed cabling problems inherent 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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