Hi guys,
On a FAS2650 storage system, I am trying to compute the disks average latencies.
When querying the API with
<netapp>
<perf-object-counter-list-info>
<objectname>disk</objectname>
</perf-object-counter-list-info>
</netapp>
here is what it says about read latency :
<counter-info>
<base-counter>user_read_blocks</base-counter>
<desc>Average latency per block in microseconds for user read operations</desc>
<is-deprecated>false</is-deprecated>
<name>user_read_latency</name>
<privilege-level>admin</privilege-level>
<properties>average</properties>
<unit>microsec</unit>
</counter-info>
and here is what it says about write latency :
<counter-info>
<base-counter>user_write_blocks</base-counter>
<desc>Average latency per block in microseconds for user write operations</desc>
<is-deprecated>false</is-deprecated>
<name>user_write_latency</name>
<privilege-level>admin</privilege-level>
<properties>average</properties>
<unit>microsec</unit>
</counter-info>
Based on this information, my understanding is, if I want to compute the total average latency of a specific disk, it will go like this :
disk_total_average_latency = [ [ (user_read_latency at time t2 - user_read_latency at time t1) / (user_read_blocks at time t2 - user_read_blocks at time t1) ] + [ (user_write_latency at time t2 - user_write_latency at time t1) / (user_write_blocks at time t2 - user_write_blocks at time t1) ] ] / 1000
The division by 1000 is here to give me the total average latency in milliseconds.
Now, when I apply this formula, I get very small values, as compared to what the CLI displays.
As an example, here is the result I get when I run the above formula on one disk at 19:33:48 and 19:36:01 :
total_average_latency (19:33:48) = 0.10 ms
total_average_latency (19:36:01) = 0.14 ms
And here is what the CLI displays for the same disk :
san::*> statistics disk show -disk DDD
san : 10/24/2018 19:34:51
Busy *Total Read Write Read Write Latency
Disk Node (%) Ops Ops Ops (Bps) (Bps) (us)
------------------- ------ ----- ------ ---- ----- ----- ------- -------
DDD san-02 2 12 1 11 23552 2608128 6001
san::*> statistics disk show -disk DDD
san : 10/24/2018 19:36:21
Busy *Total Read Write Read Write Latency
Disk Node (%) Ops Ops Ops (Bps) (Bps) (us)
------------------- ------ ----- ------ ---- ----- ------ ------- -------
DDD san-02 4 22 6 15 965632 3582976 6555
So with the SDK, the order of magnitude is around 0.1 milliseconds, whereas with the CLI, the order of magnitude is around 6 milliseconds.
Here are my questions :
- Am I missing anything or doing anything wrong with my calculations ?
- Is there a known issue regarding the values displayed in the CLI ?
I hope my issue is detailed enough for you guys to help me.
Should you need any additional information, please let me know.
Thank you for your time.
Regards
