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Fibre Channel (SAN)

Posts: 0

Buffer Credits on 48K Director



We're getting a lot of TimeBB_CreditZero (thousands within a few minutes)


The 48K itself is only about 50% utilised but all ports regardless of being in use or not are online... if i shut off the ports that are not in use would that free up buffer credits?






Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: Buffer Credits on 48K Director



The answer is no.


On the other hand, what is the current config of the isl?

What is the distance between the switchs?

Do you see discards?

What is the average frame size through the isl?


Kind rgds

Posts: 0

Re: Buffer Credits on 48K Director

The tim_txcrd_z counter increments at 2.5us (microsecond) ticks, meaning it can grow at most ~400,000 times per second !


If you see rates at roughly 10% of that (and you'd need to closely monitor every few minutes), you may have issues (on long distance it may be exacerbated). For ISLs as "felipon" suggested check the classics: distance, speed, average frame size (remember: a BB_Credit represents a _full_ size frame) and possible link quality issues (attenuation) etc.


Another classic cause for BB_Credit starvation are slow drain devices, which are basically slow in responding and may hence generate back-pressure.


On a side note: I seem to recall that Brocade used to recommend as best practice that all unsused ports should be disabled (persistently maybe even). But I'm not sure if that still holds true these days.



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