on 6/2/2000 8:08 AM, Greg Morin at [EMAIL PROTECTED] wrote:

> I'm afraid I don't understand SCSI buses as well as I thought then
> ;-) Why does this create a bottleneck for internal HD->VXA writes?

Any I/O bus, be it SCSI, network, or whatever, has a limit to the amount of
data that can be passing through it at any given moment. This is a bit of a
simplification, but if you're trying to read data at full speed from your
HD, you cannot simultaneously write at full speed to a tape drive on the
same bus, because the bus is saturated with the read. If the tape drive is
on another bus, you can read at full speed on one bus and write at full
speed on the other bus at the same time.

> The internal drive is Fast SCSI-2 (and benchmarks in the 8 MB/s
> read/write range with HDT 4)... so it seems like the drive itself
> should not be slowing things down. Also, ignoring the internal drive
> tests, in normal network backup use the VXA still performs more
> slowly, anywhere from 25%-50% slower than the DDS-2 (identical
> backups, I'm running backups to each device nightly).

That could still be caused by the fact that the VXA is having to share time
on the bus with the system disk.

As a comparison, our Retrospect server is a beige G3/300 desktop. The
internal HD is on the ATA bus, an external DDS-3 tape is on the built-in
SCSI bus, and an external VXA drive is by itself on an Iomega fast SCSI PCI
card. Backing up the internal HD, I get about 160Mb/min to the VXA and
80Mb/min to the DDS-3.

Jon L. Gardner '89, Computer Systems Manager <mailto:[EMAIL PROTECTED]>
Texas A&M University Dept. of Food Services <http://food.tamu.edu/>
Tel 979.458.1839 Fax 979.845.2157 ICQ 34792860
PGP public key available at <http://food.tamu.edu/pgp/jon.html>

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