🚀 go-pugleaf

pclink@cheops.qld.tne.oz.au (Rick) writes:

>I was talking to a sales rep about adding a CDROM (CDR-84) to my existing SCSI
>system (TMC885 + ST4767N), and he said that this will degrade the system
>performance.  He said most users opt for a second SCSI card into which
>they plug the CDROM.  Is this correct?  I thought that each device on a
>SCSI bus worked independently of each other, and that data rate was
>determined by the addressed device.  Am I wrong?

You aren't wrong, but the sales rep said a little bit truth too.

The situtation is the following:

Your disk has an burst transfer rate of up to 4 Mbytes/sec (with sync transfer).
Modern hard disks have Media transfer rates of up to 3 Mbytes/sec, so you
could use up to 75 % of the available bandwidth with one disk.

Your CDROM has an burst transfer rate of up to 1.5 Mbyte/sec. I've never
seen an CDROM out there, which uses Sync transfer!
The CDROM drive has a Media transfer rate of 176 kBytes/sec. So it needs
11 % of the bandwidth. Also an transfer of a CD data block needs
appx. 1.5 msec. This calculation is only true, when the CD drive buffers
the data blocks, before transferring to SCSI, but I hope they do it.

So at least if you have the sync transfers enabled, you have enough bandwidth
to use both a disk and a CD on your SCSI bus. Also its' rather seldom
that both drives would transfer continuously, so you could even add more drives.

If you have one disk, and one CD, there is nothing against mixing them.

But if you have a large heavily loaded fileserver with many disks, you don't
want to mix both CD-Roms and harddisks on the same SCSI Path. Probably you
could mix them instead with tapedrives, like Exabyte or DAT.

Sincerely,
Klaus

--
Klaus Steinberger               Beschleunigerlabor der TU und LMU Muenchen
Phone: (+49 89)3209 4287        Hochschulgelaende
FAX:   (+49 89)3209 4280        D-8046 Garching, Germany
Internet: Klaus.Steinberger@Physik.Uni-Muenchen.DE