How many SCSI controller ?

[Philip Chan_1]

Hi,

I'm trying to clarify my understanding with the "SCSI" technology.

Let say server "A" has a SCSI-2 controller with 80Mb/sec I/O bandwidth which has only one disk attached to it with max transfer rate at 40Mb/sec. At the same time I would like to add a new tape drive with 30Mb/sec max I/O rate to the system.

My question is, from performance perspective, can I attach the tape drive to the existing SCSI-2 controller? or should I get a second SCSI-2 card for the tape drive to avoid overloading the first SCSI card?

Frankly speaking, I don't buy the idea of using ONE controller for EVERY I/O eager devices as long as the total throughput don't exceed the controller limit.

Any comments are welcome.

Thanks in advance.

~Philip

[Edward Sedgemore]

Im not sure how you can have SCSI-2 controllers with 40Mb of bandwidth! They must be Ultra-SCSI controllers. Discs are so fast nowadays that for maximum throughput 1 disk per controller is ideal. The old maximum with SCSI-2 (5 MB/s) and FWD SCSI (15 MB/s) controllers and the disks that usually came with them was 4 disks per controller (as not all 4 disks were in use constantly so as to flood the controller).

If you add a tape drive then only if youre going to use it during the day when the disk is being used do you need to worry about them interfereing with each other. If its only used for backups at night then it will slow the backup down a little when its actually backing up the disk on the same controller, but not by too much.
Normal practicse is to put tape drives, especially DLT ones on their own SCSI controller for maximum throughput and minimal backup/restore times, but you can have anywhere from 2-4 disks per controller on the new Ultra-SCSI controllers.

[Philip Chan_1]

Excuse me, I mean ... ultra2-scsi.

Perhaps I should ask my question in another way, before a scsi controller used up its bandwidth, should a second one be added?

I knew the normal practise is to let the DLT drive to run on its own controller, but why doing this when there are spare bandwidth?

[Edward Sedgemore]

Perhaps I should ask my question in another way, before a scsi controller used up its bandwidth, should a second one be added?

If you demand high performance from your server then yes.

I knew the normal practise is to let the DLT drive to run on its own controller, but why doing this when there are spare bandwidth?

If you have spare bandwidth on a controller then put the DLT on it, no problem. However, DLT's are slowed down by ANY other devices on the same controller, even if theyre only being used a little.