From: wgstuken@immd4.informatik.uni-erlangen.de (Wolfgang Stukenbrock) Subject: Re: 2 SCSI cards in the same Linux box? Date: Thu, 1 Jul 1993 08:19:55 GMT
oyang@cs.monash.edu.au (Kai Shing O'Yang) writes:
>My proposed setup is to have a Adaptec 1742 EISA SCSI card connected to 2
>FAST-SCSI II drives while having the 1542b connected to slower devices
>like CD-ROM and tape. Would I have any advantage on this config than
>hooking all the devices with the 1742?
Normaly a good idea, but in a PC environment ...
A good FAST-SCSI controller is able to transfer up to 8 MB/sec average
over the SCSI bus. This will allow you to put two disk with each up to
5 MB/sec transferrate from disk-cache to media on that bus without the
bus beeing the bottleneck. The "normal"-SCSI controller may transfer
up to 3 MB/sec (depending of the connection devices) avarage. Better
SCSI-disk will allow up to 5 MB/sec with a transferrate of to the media
equal to that. So on "normal"-SCSI one disk will saturate the SCSI-bus.
Theese values are theoritical values. "real" values are a little bit
smaller, because of the seek times of the drive. Practice has shown, that
on BSD filesystems these values are reached up to 2/3. This meens three
disk on FAST-SCSI without overloading the bus, and 2 disk on "normal"-SCSI
with a little overload.
Multible SCSI-busses makes only sence if your machine needs more data-
transfers on average as one SCSI-Bus can handle. And you need multiple
disks on that bus with equal distributed access to it. This meens either
some striping SW or multiple filesystems active at the same time.
Next, for two SCSI-contorllers the transfer rate of the EISA bus is slow.
You will need 100% of memroy bandwith and CPU time just for transfering
the data. If you have a file server, that may be good, but otherwise ..
>I tend to believe that slower devices will hold the SCSI bus for longer
>times and will affect performance on the faster hard disks (if the hard
>disks are on the same scsi bus as the tape, cdrom, etc).
That's right. If you have a very slow tape (for example) and transfer
data to it all the time, you will loose some bandwith. But how often does
this happen? What other programms will run at the same time requiring
the full bandwith to the disks?
Thinks are a little bit different, if you have some stupid SCSI-devices
that are not able to release the bus if they need a little stop during
transfer. (e.g. Microtec still builds such a rubish and some old very old
tapes may have this problem. There may be also some old "PC"-SCSI-disk
that will not work as they should.) In this case the SCSI bus will hang
over a long time, just stopping the hole system. A second SCSI-bus for
theese stupid devices will help.
> ...
If you are out of SCSI Id's, you need a second SCSI-controller.
You should use intelligent SCSI-controller with a large write cache. If theese
work as they should, this will leed to full SCSI-bus speed. I don't know if
Adaptec will do this.
But a FAST-SCSI controller doesn't make any sense in the PC at home!