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Theoretical Max FC-AL Devices Question...



 
 
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  #1  
Old June 5th 04, 08:45 PM
R. Damian Koziel
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Default Theoretical Max FC-AL Devices Question...



This is a very simple question but one for which I'm finding
conflicting answers. What is the (theoretical) maximum number of
devices on a FC-AL?

Here's a sampling:

127:
"On a single FC-AL loop up to 127 devices can be attached, and they
are visible by simply plugging them in."
www.storus.ru/pdf/SCSI_vs_FC.pdf

126:
"In the FC-AL topology, there can be up to 126 nodes connected to a
single link, but as you might expect, the more devices that share the
loop, the lower the performance."
http://www.enterprisestorageforum.co...le.php/1586871

126:
"FC-AL allows communications between a maximum of 126 nodes, but they
share the total bandwidth of the loop."
http://www.i-tech.com/Storage%20Info...indnews251.php

127:
"The Fibre Channel loop can have any combination of hosts and devices.
The maximum number of devices is 127."
http://www.sms.com/store_gloss.htm

Oh great SAN experts, I beseech you; please divine unto me the one
true value (or is there one?) and I will forever devote my life to
spreading the good word of storage networking.

Regards,

R. Damian Koziel

  #2  
Old June 5th 04, 09:33 PM
Charles Morrall
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Default


"R. Damian Koziel" wrote in message
...


This is a very simple question but one for which I'm finding
conflicting answers. What is the (theoretical) maximum number of
devices on a FC-AL?

Here's a sampling:

127:

[snip] [127 or 126 max?]

I'd say you can have 127 devices on the loop, but at least one needs to be
an initiator (ie host adapter), to be actually useful. Having 127 target
devices (disk, tape) etc obviously does not produce much of use to anyone.
/charles


  #3  
Old June 6th 04, 02:30 PM
HV Biker
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Default

On Sat, 05 Jun 2004 19:45:55 GMT, R. Damian Koziel
wrote:

This is a very simple question but one for which I'm finding
conflicting answers. What is the (theoretical) maximum number of
devices on a FC-AL?


There is a very simple answer!

The theoretical maximum number of devices on a loop is 127.

There are a total of 128 addresses in the Arbitrated Loop Physical
Address range, as it's a single byte address.

The reason that two addresses seem to go missing in some of your
examples is because they are generally reserved for specific
functions.

The first address (effectively x00) is for connection of the loop to a
Fabric (an FL_Port). If this is not being used for a Fabric
connection, it can be used by a device (as an NL_Port). Generally
you'll find that most people quote 126 possible devices, but in
reality it's 126 + 1 FL/NL_Port, making a maximum of 127.

The last address (xFF) is a broadcast address and cannot be
reassigned.

Hope this helps.

HVB.
 




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