196Chapter K - GDT User's Manualbuild the new drive. The halting and the halt-lifting of the GDT SCSI channel is controlledby the Hot Plug function. The above mentioned halting of the SCSI channel avoids that i n-terferences caused by plugging off the drive impair the functioning of other drives still a c-tive on this SCSI channel. Moreover it prevents the possible destruction of the SCSI pro c-essors of the GDT controller or the SCSI protocol ICs of the SCSI drives. However, this i m-plies that none of the SCSI devices of the halted SCSI channel can be accessed during thetime the defective drive is being exchanged. This may affect for example: other drives ofthis disk array, other disk arrays, simple host drives (of the type disk etc.), and Not DirectAccess Devices (CD-ROMs, DAT streamers, MOs, etc.). If the GDT disk array controller has onlyone SCSI channel, or if all SCSI devices are connected to one single SCSI channel, then noSCSI device can be accessed during the time of replacement. Therefore, it is evident that itis best if the GDT controller had as many SCSI channels as possible and that all SCSI d e-vices were distributed equally to the available channels in order to avoid that the disk arrayor other SCSI devices cannot be accessed during the Hot Plug drive replac ement. This non-accessibility of the disk array during the replacement is communicated to the operatingsystem by the GDT driver program that integrated the GDT controller into the operatingsystem. If the period of non-accessibility becomes too long a system error may occur. TheHot Plug should be carried out as quick as possible.Various manufacturers offer so-called Disk Shuttle (sometimes also called Disk Shelf) subsys-tems. There are two categories of these subsystems: Non-Intelligent and Intelligent. Bothgenerally consist of the shuttle itself, a shock safe metal or plastic enclosure containing thehard disk drive, a frame which is mounted in the computer case or in an external subsy s-tem, and a sufficient cooling system.Especially the last component is very important. If the disk drives run too hot, it is verylikely that they will fail. In addition, their life time is reduced dramatically. The mountingframe has appropriate slide rails and locks that fit with the shuttle. The shuttle can be slidinto or out of the mounting. When using any of these systems you have to make sure thatthe SCSI termination (regardless of whether the drive has been terminated or not) is madein the mounting frame. In addition, always use high quality connectors for the SCSI busesand the power supply. It is very important that during the plug out the SCSI bus is disco n-nected first, and then the power supply. When plugging in again, the order is vice versa.The Non-Intelligent subsystems have no communication with the GDT controller and nointelligent backplane (i.e. with sensors).If it is intented to avoid this halting and halt-lifting of the GDT SCSI channel, an Intelligentsubsystem is needed. The GDT controllers support two different types of such subsystems:SAF-TE and DEC (TM)fault bus compliant subsystems. SAF-TE stands for SCSI AccessedFault-Tolerant Enclosure and is soon becoming part of the SCSI specification. The DEC (TM)fault bus is a special technology from Digital Equipment (TM).With these subsystems the GDT controller allows for the performance of the so-called AutoHot Plug, which reduces the whole hot plug procedure to the simple replacement of the diskshuttle which contains the desired disk drive. Furthermore these subsystems have typicallyseveral LEDs to indicate for example the disk shuttle which contains the defective diskdrive.K.3.2 ControllerThis option displays details on the GDT controller. E.g.: How much Cache RAM has the GDTcontroller. What is the current termination setting of the various SCSI channels.
