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Windows Server can actually support up to eight buses per adapter, 128 target IDs per bus and 254 LUNs per target ID. However, these numbers only work if the hardware supports the hierarchical addressing feature in SCSI. If it doesn't, the system is limited to eight LUNs.
To use more than eight LUNs, or to even find out if you can use more than eight LUNs, you need some information and possibly some support from your hardware vendor.
Sometimes you get lucky. If a storage device reports the hierarchical addressing bit (HiSupport bit) set as part of its standard inquiry data, and if the vendor's drivers support them, then the Large LUN feature is automatically enabled and you don't have to do anything. However, Microsoft recommends contacting the vendor to find out if the bit is set and if the drivers will support large LUNs.
If you're not lucky, you have to make changes to the registry, which only works if the storage devices support the SCSI REPORT LUNS commands. You will also need specific information about the devices' hardware IDs and registry entries. If the hardware can be made to support Large LUNs, the vendor should be able to provide you with the necessary information.
There are a couple of problems you may encounter if you manually enable Large LUN support. You could end up with too many disks or not enough LUNs.
If the host bus adapter's (HBA's) device driver enables Large LUN support in a proprietary fashion and you have to manually enable the feature, you may end up with duplicate disks. This happens when both the Windows Large LUNs feature and the HBA Large LUNs feature are enabled.
The other problem occurs when LUN 0 isn't present. In that case, Windows only reports eight LUNs no matter how many are present because it can't send the REPORT LUNS command.
In both of these cases, you need to consult with your hardware vendor about the parameters to use and possible fixes.
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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 storage and other computer issues.
This was first published in March 2005