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Are long Consistency Points normal?

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ontap-9
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perf
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Applies to

  • ONTAP 9
  • Clustered ONTAP 8.x
  • Data ONTAP 7-mode

Answer

  • If no performance impact, then the occasional long Consistency Point (CP) are normal and expected.
  • Although more frequent long CPs events may be an indication of other issues and should be investigated. 

Additional Information

ONTAP can do several background tasks in CP. If latency is low and the system has idle cycles, rather than waste them, ONTAP will devote idle cycles to background work. A few examples are deswizzling, various wafl scans, or deletions.

CPs happen asynchronously of the write path, as writes are acknowledged when written to NVRAM. Latency is unaffected as writes can be buffered until CP completion. Latency will be observed during this time as being stead.

If latency goes up during this time, above expected levels, then investigation is needed. The long CP is a symptom of something else that is stuck or not responding efficiently such as a hardware or other ONTAP issue, or workload issue. The secondary issue must be rectified and considered the primary issue, not the long CP.

"wafl.cp.toolong" message seen in ONTAP logs when disk is about to fail or being failed

"wafl.cp.toolong" message seen in ONTAP Logs during high disk workload

"wafl.cp.toolong" message seen in ONTAP logs for archive, backup, or disaster recovery storage system

Consistency Point (CP) process

Examples of messages logged:

  • ONTAP 9:
Mon Dec 23 00:20:36 EST [FilerA: wafl_exempt08: wafl.cp.toolong:error]: Aggregate fas_01_DATA_AGGR experienced a long CP...
This message occurs when a WAFL(R) consistency point (CP) takes longer than 30 seconds. A CP lasting more than 30 seconds might cause client latency and potentially a client outage.
  • Clustered ONTAP 8.x, Data ONTAP 7-mode:
Mon Feb 22 16:14:08 CLT [FilerA: wafl_CP_proc: wafl.cp.toolong.warning:warning]: params: ...
Mon Feb 22 16:14:08 CLT [FilerA: wafl_CP_proc: wafl.cp.slovol.warning:warning]: params: ...