What's the difference between "disk.device.write.requests.rate" and "disk.device.iops" ?

asked 2019-10-09 00:54:18 -0500

corey gravatar image

I would like to get the IOPS value from each instance, so I searched some info about Gnocchi metrics, and I found two metrics: disk.device.read/write.requests.rate and disk.device.iops.

According to this https://docs.openstack.org/ceilometer/latest/admin/telemetry-measurements.html (page), disk.device.read/write.requests.rate means "The number of read/write requests" per second, and disk.device.iops means "Average disk iops per device".

My question is: what is the main difference between these two metrics? I can't find out which is the most suitable choice for me.

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answered 2019-10-09 01:11:34 -0500

updated 2019-10-14 00:52:42 -0500

I don't find the term "rate" on that page, but I understand IOPS as the IOs per second, both read and write, whereas read requests metrics cover read requests, and write request metrics cover write requests only.

Regarding source of measurement information, first see https://docs.openstack.org/ceilometer.... The first method is services providing information to Ceilometer via the message queue. After all, services should know everything about their objects; e.g. Nova should know all details about its instances. This method is named notification.

Since the services don't contribute all the samples that Ceilometer would like to have, it also has its own pollingagents that get information from sources like hypervisors. An instance's disk I/O is most likely sampled by a pollster running on the compute host. Such a pollster knows how to use libvirt or whatever other hypervisor API is suitable to gather the required data. See also the lists on https://docs.openstack.org/ceilometer....

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So (disk.device.read.requests + disk.device.write.requests) == disk.device.iops ?

corey gravatar imagecorey ( 2019-10-09 01:14:35 -0500 )edit

That's what I believe, yes. I have not confirmed it.

Bernd Bausch gravatar imageBernd Bausch ( 2019-10-09 03:36:54 -0500 )edit

´╝áBernd Bausch, would you mind if I ask one more question? How does Nova get these informations? I want to know how it works? I know I should check the source code, but I have no idea where to start.

corey gravatar imagecorey ( 2019-10-13 22:56:26 -0500 )edit

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Asked: 2019-10-09 00:54:18 -0500

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Last updated: Oct 14 '19