[vox] Oracle-Linux follow-up

[Jeff Newmiller]

On Tue, 6 May 2003, Eric D. Pierce wrote:

> Can someone explain what was said at the meeting about using firewire
> "hubs" as a low-end/alternative way of building a server cluster (as
> per above, instead of FibreChannel, e.g. for development/testing)?

As I understood it, it is primarily a way for more than one computer to
share access to the same peripherals, but it can also be used to add
storage by multiple devices. (The latter would need hardware or software
RAID, and/or the Logical Volume Manager.) In general, the total throughput
of a configuration that uses a hub is limited by the firewire bandwidth
and negotiation inefficiencies because each computer has to wait for the
active computer to relenquish control of the bus.  If you have something
like

   computer A         computer B
       |                  |
       |                  |
     +-+------------------+-+
     |         hub          |
     +-+------------------+-+
       |                  |
       |                  |
   peripheral A       peripheral B

then computer B cannot communicate with peripheral B until computer A
relenquishes control of the bus, so on average the throughput of each
computer-to-peripheral is halved in exchange for allowing shared access to
both peripherals.

Replacing the hub with a switch allows temporary isolation of these
communication channels, so full bandwidth is available for each
data stream, and only contention for the peripherals will slow down
access (that is, peripheral A can only communicate with one computer at a 
time).

The speaker also pointed out that a couple of Firewire cards in a linux
box could be used to implement a switch.  PCI at 66MHz can theoretically
handle 266MBps, and Firewire about 40, so four or five data streams (8 or
ten cards) should be able to get through a dedicated box if the drivers
are tuned well.  You do insert a single point failure with either a switch
or a hub, and dedicated hardware is more likely to have a better
reliability than a commodity pc, but could cost significantly more.

> Any good web sites that explain firewire networking, and/or sell
> the stuff?

Probably.

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