1. Get the performance object types (Get-NaPerfObject). This includes an object type 'cpdump' on some of my test systems.
2. Get the instances of the type of interest (Get-NaPerfInstance -Name cpdump). This returns no data on any of my test systems.
3. Get the counters available for the type of interest (Get-NaPerfCounter -Name cpdump). This returns numerous counters on some of my test systems.
4. Get the data (Get-NaPerfData -Name cpdump -Instances <instance names> -Counters <counter names>).
In short, because there are no instances reported for the cpdump type, no data is available. Please keep in mind that because this performance type is listed as having privilege level 'diag', it may not be available and its contents are subject to change.
I just checked on Data ONTAP 8.1, and none of the cpdump* perf objects are listed by that API, so I have to assume these were never fully implemented in earlier versions and were more recently removed. Also note that in 8.1 I see 140 counters in the wafl type, vs. 101 in 8.0.1. So perhaps that's a better place to look in the future for CP-related statistics.