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Fibre Channel (SAN)

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Contributor
Posts: 46
Registered: ‎06-04-2012
Accepted Solution

EHT and ED_TOV

Hi Friends,

 

I understand that EHT is the amount of time that a switch/port can hold a frame, until it can gain access to an available buffer. By default it is 220ms (FOS 7.1 above). This is mainly seen in case of a zero credit condition due to Slow Drain Device or congestion etc.

 

What about ED_TOV, this is by default 2secs and same across all switches? What condition will this trigger and how is it different to EHT?

 

Thanks in Advance!

 

Regular Contributor
Posts: 166
Registered: ‎02-05-2014

Re: EHT and ED_TOV

E_D_TOV is a fabricwide parameter which stands for Error Detect TimeOut Value. There are numerous fabricservices that depend on this parameter on which subesequent actions are executed. Wether that is to determine to discard a frame when the fabricis unable to deliver it or to determine on how to act when ISL's are formed and something unexpected happens in port-state transitions.

 

As I mentioned this is a fabric wide parameter which needs tobe the same on ALL switches in a fabric otherwise it will segment.

 

Secondly, you have to have a darn good reason to change this value. I had one customer who changed this to 5 seconds are they thought it might bea good idea to give hosts a bit more time in recovery situations. Not knowing the full extent of this parameter it has brought them nothing but trouble over many years as they were unable to break up the fabric and correct the situation.

 

In FC (and IT in general) if you don't know what it does don't touch it. Some very smart people who came up with these values had a pretty good idea how these systems work.

 

The EHT (Edge Hold Time) is only applicable on Class 3 frames that are subject to a timeout value due to credit shortages. Basically meaning the ingress port can chuck away a class 3 frame if the egress port does not have credits to send that frame.

 

 

Kind regards,
Erwin van Londen
Brocade Distinguished Architect
http://www.erwinvanlonden.net The Fibre Channel blog



Q&A -> https://hackhands.com/elo/


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Contributor
Posts: 46
Registered: ‎06-04-2012

Re: EHT and ED_TOV

Ervin a quick question,

the below timeout errors are based on EHT value of that switch?

er_tx_c3_timeout
er_rx_c3_timeout
Regular Contributor
Posts: 166
Registered: ‎02-05-2014

Re: EHT and ED_TOV

Indeed. Or, on some older kit and FOS firmware, based on an algorithm of which E_D_TOV was a partial variable. If you have newer equipment (all 8 16 and 32G kit with FOS 7.x and up observe the default values set by the factory) the algorithm no longer applies.

Kind regards,
Erwin van Londen
Brocade Distinguished Architect
http://www.erwinvanlonden.net The Fibre Channel blog



Q&A -> https://hackhands.com/elo/


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