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HBA Transfer Rate Calcucations

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Stuart Abramson_2
Honored Contributor

HBA Transfer Rate Calcucations

Here are some Fiber Channel transfer rate ratings:

..EMC FA:........................2 Gbit/sec
..EMC Connectrix Switch Port:....2 Gbit/sec

..HP-UX FC HBA:................200 MBytes/sec

Of course they rate them in different units. Since I only care about Bytes, I have to convert 2 Gb/s to MB/s, where:
..B - Byte
..b - bit

gives me:

..2 Gb/s * 1000 Mb/Gb
..------------------------ = 250 Mbytes/sec

So, the FA is 250 MBytes/sec and the HBA is 200 MBytes/sec.

So, how can I have a "Fan-In" ratio of UNIX server HBAs to FAs of 6 to 1?

Are there "utilization" stats like an HBA is only 75% efficient and they only run 60% of the time?

do you divide by 8 to convert bits to bytes? Or do you divide by some other number (start/stop bits?)?

Is there some explanation for figureing this somewhere?
Uwe Zessin
Honored Contributor

Re: HBA Transfer Rate Calcucations

2 GigaBits/second is exactly 2.125 GigaBits/second on the cable. This includes the overhead in a fibre channel frame.

Now, any 8-bit 'data byte' is sent on the cable as a 10-bit 'transmission character'. It is for self-clocking, error detection and DC balance of the receiver.

So, 2 GigaBites/sec = 200 MegaByte/sec = user data bandwidth.
Stuart Abramson_2
Honored Contributor

Re: HBA Transfer Rate Calcucations


That's perfect for part 1 of my question.

But, how does "Fan In" work, if hte HBA and FA (on the EMC) are the same speed?

There must be some empiracal (sp?) evidence which states that a single server only drives the HBA/FA at XX% capacity...
Uwe Zessin
Honored Contributor

Re: HBA Transfer Rate Calcucations

Oh, that's a good question...

I have heard about values of 7:1 for 1 GigaBit and 14:1 for 2 GigaBit storage networks as a 'rule of thumb'.

Sounds OK as many servers are not really bandwidth-hungry - they need I/Os.