Re: [aclug-L] Info Request

[John Goerzen <jgoerzen@xxxxxxxxxxxx>]

Bob Deep <bobd@xxxxxxxxxxxx> writes:

> How about USB?  Does it offer any advantages aside from not burning an
> interrupt per device?  What is the effective bandwith available?  Does
> Linux support it yet?

USB offers 1.5 MB (that's megaBYTE) per second (they claim 12 Mb
[megaBIT], which confuses a lot of people because SCSI times are
measured in MB).  Therefore, SCSI is still a FAR better solution for
disks, disk-like devices (Zip, ...), tapes, scanners, and other
high-bandwidth options, despite what Apple would like you to believe.

With proper OS support, SCSI is hot-pluggable to a certain extent.  If 
you can ensure that all devices on the chain are idle, you can rmmod
your SCSI card's module, twiddle with the chain as much as you want,
and then modprobe it back.  This does not work if any filesystems are
mounted from any SCSI devices on that chain.  USB is touting their
hot-plugging capability.  Really what they should be touting is "OS
support for hot-plugging" -- SCSI is capable of the same, it's just
that *doze doesn't support it.

Note that even the most ancient SCSI was 5 MB/sec, and most modern SCSI 
is 40 MB/s at minimum with 80 MB/s becoming popular, to USB is
significantly slower than SCSI.  I only mention this because Apple is
replacing SCSI ports with USB ports on the iMac, which is a totally
stupid thing to do IMHO.

That said, USB is significantly faster than serial ports, which max
out at about 12 KB/s (max speed is 115.2 Kbps, for 8 bits + 2 control
bits, we get 11.52 KB/s).  USB is also faster than traditional
parallel ports.  Its advantage over both of these is chaining,
hot-plug capability, and speed.  However, this does not make it
revolutionary or anything like that -- modern SCSI is more than 25
times faster and lets you hook up to 15 devices on a single chain.
FC-AL (Fibre Channel Arbitrated Loop), the successor to SCSI, is over
250 times faster than USB and requires only two copper wires in a
cable, and supports 127 devices per chain, with a maximum chain length 
of several miles.  FC-AL is, however, rather expensive at the moment.

AFAIK, there is no current release-quality USB support in Linux but it 
is being worked on.

John

-- 
John Goerzen   Linux, Unix consulting & programming   jgoerzen@xxxxxxxxxxxx |
Developer, Debian GNU/Linux (Free powerful OS upgrade)       www.debian.org |
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