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Cisco Bug: CSCus64439 - "PFC feature operational" not updated causes storage latency and aborts.

Last Modified

Mar 20, 2018

Products (1)

  • Cisco Unified Computing System

Known Affected Releases

2.0(1q)A 2.1(1a)A 2.2(1d)A

Description (partial)

Symptom:
Servers experiencing poor storage performance and seeing an increasing number of aborts in the UCS Mezz logs for Cisco VIC 1240/1280/1340/1380 and within OS logs.

Conditions:
- IOM reset (including FI reboots which in turn reboots the IOM)
- IOM re-seated
- Cable between IOM and FI pulled/replaced.
- UCS system is upgraded from code prior to Known Fixed Releases, and infrastructure components are completed before adapters/blades

In some circumstances, the VIC does not properly honor Pause Registers sent by the switch/IOM (PGS and PFC updates).  This leads to FCoE frames being dropped on the IOM. As this behavior is a race condition, it can occur on some blades, but not others. Typically it will be observed for an entire chassis; however, if there are multiple chassis in the system others can still be affected if their Pause configuration is correct due to the amount of traffic being flooded onto the network.

Identifying issues:

In adaptor syslog search for "feature operational" and review the when you see the following sequence:

150209-16:12:35.817073 mcp.uif_dcbx Port 0-0: PFC feature operational
150209-17:31:11.454181 mcp.uif_dcbx Port 0-0: PFC feature inoperative
150209-17:31:20.512303 mcp.uif_dcbx Port 0-0: PGS feature operational
150209-17:31:24.482730 mcp.uif_dcbx Port 0-0: PFC feature operational

In some case, but not all, the second "PFC feature operational" message is ignored and PFC does not actually become operational.

IOM show-tech-support-iom-nxos.out may also show the following interrupt being triggered while the issue is occurring:

: |3 |000001ad | wo_cr[3] | frames rcvd without credit for pausable classes. Pause is missing.
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