What does the "tim_txcrd_z" mean from the "portstatshow" command output on Brocade switch?

• Brocade SAN switch
• Slow Drain

• The "tim_txcrd_z "（Time TX Credit Zero (2.5Us ticks)）parameter in the "portstatshow" command output means the following:
  • The number of times that the port was unable to transmit frames because the transmit BB (buffer-to-buffer)credit was zero.
  • The purpose of this statistic is to detect congestion or a device affected by latency.
  • This parameter is sampled at intervals of 2.5 microseconds, and the counter is incremented if the condition is true. Each sample represents 2.5 microseconds of time with zero Tx BB Credit.
  • An increment of this counter means that the frames could not be sent to the attached device for 2.5 microseconds, indicating degraded performance.
• To identify a slow drain device:
• For a single link;

Take the counter "stat_ftx" and the "tim_txcrd_z" counters and put them in the following calculation.
tim_txcrd_z / frames tx = YYY * 100 = ZZZ %

• For a trunk link:

Take the SUM of all members of the trunk of stat_ftx and the single tim_txcrd_z counters and put them in the following calculation:
tim_txcrd_z / (SUM of all members frames tx) = YYY * 100 = ZZZ %

Note: A trunk is showing different counters.The tim_txcrd_z counter is on each trunk member the same, this displays the logical tim_txcrd_z (all members together).The stat_ftx counter is on each trunk member different and displays the counter on a per-member basis.

According to the Brocade performance guidelines, a ratio of approaching 10% or greater should be considered as a red flag and above 20% is becoming a real issue, and further investigation is needed.