What is IO delay?

[masgo]

I am new to proxmox and want to understand it better. A node shows me the "IO delay" value. What is IO delay? How is it calculated? Knowing this would make it easy to answer questions like: what IO delay is acceptable? Is 5% a high value or is 50% acceptable? etc.

I tried searching the documentation and found nothing on what it is. Google and forum searches give me hundreds of pages where people discuss how to reduce it, or what is causing it, but nothing on what this value actually means.

Related question: having a node with mixed storage, e.g., SSDs, HDDs or even CEPH or other networked storages. How does the storage type ifluences the IO delay value?

 

[dcsapak]

io delay is simple the 'iowait' metric of the linux kernel, which values are ok is very dependent on your config and situation,

there are many pages which describe what it is, e.g. the man page of iostat(1) says the following:

%iowait

Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request.

 

[masgo]

Geat, thanks! I know %iowait and iostat.

Would be nice if proxmox called it this way or included some mouse-over hint to it or at least something in the manual when I click the "help" button.

As a suggestion: a different metric for IO might be better. %iowait can be highly missleading, especially when comparing different CPUs and especially when multiple process are running on the cpu. Maybe something comparable to the device-based output of iostat? In my experience looking at tps and read/write speeds usually helped me find a problem much faster than %iowait.