neese@adaptx1.UUCP (04/28/91)
>/* ---------- "Separate controllers to enhance per" ---------- */ >I'm not real familiar with SCSI drives so I have a question about their >performance (actually it's on the controller). When you run MFM or RLL, >the controller can only read or write one device at a time whether it is >a hard drive or floppy. One database vendor suggested putting database >files or separate hard drives to increase performance. I protested that >it wouldn't make any difference unless the drives were on separate >controllers. Does this apply to SCSI controllers as well or does it >access drives simultaneously (or almost so)? SCSI has provisions for overlapping commands to each device on the SCSI bus. This is done via a disconnect. It works like; If a command is sent to a device on the bus and the device must do a seek or does not have the data in it's local buffer/cache, the device will disconnect from the SCSI bus and allow the bus to be used by another device. Intelligent adapters do this transparently to the host system. Dumb adapters have to do it in software. There are benefits to putting 2 devices on 2 different adapters though. For instance, if you had 2 disks and put one each adapter, you could disable the disconnect (if the adapter/software will allow it) and potentially increase the overall throughput of the disk subsystem. Disconnects/reconnects consume quite a bit of time on the SCSI bus (upwards to a milli-second). It also depends on the bandwidth balance of the host bus and the SCSI bus. If the data transactions are small, there would be a definate gain using 2 or more adapters in the host bus. But if the data transactions are large, there may not be a very significant gain. Several variables are involved in all of this. It is very dependent on the application you will be using. Roy Neese Adaptec Senior SCSI Applications Engineer UUCP @ neese@adaptex uunet!cs.utexas.edu!utacfd!merch!adaptex!neese