CompuServe Thread

Reselection is a SCSI process that can make a collection of multiple drives on a single controller work faster as an aggregate. On a SCSI drive you have a lot of smarts. In general there are so many smarts that the drive can intelligently estimate how long it will take to be ready to transfer the data required for the current transaction. If reselection is enabled it can then tell the controller (initiator in SCSIspeak) that "Gee, Boss, this is gonna take me awhile. Disconnect me from the bus and I'll wake you up next time the bus is free and I am ready to process the command." On SCSI controllers that properly handle this reselection process the initiator and target (the drive) return the bus to the "Buss Free" state. The initiator can then scan its "list" if pending commands for a command destined for a different device on that same SCSI bus. If there is one it is free to initiate that command so that you can potentially have several commands all overlapping their processing. When you have multiple drives this can lead to a major speed up in overall processing because otherwise "dead time" is "recycled" into useful purposes. (But in general EACH access will take place slower even if there are no intervening commands because there are more transactions taking place on the SCSI bus.) The upshot of all this is that if you have only one drive run with reselection disabled. (In general do this with Syquests in any case on the A3000. There is "an unfortunate interaction"…) If you have many devices and can benefit from overlapped IO then enable the reselection and reap the benefits. {@_@}