Elaborate on IO delay in server summary

Shadow Sysop

New Member
Mar 7, 2021
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I'm curious about the actual meaning of the IO delay in the Server's Summary. For example, when I clone a VM from a template, I expect high IO. All my clone templates are on it's own dedicated drive. When I clone a VM from one of these templates, the IO delay for the node hops up as high as 30-35%, but VMs on that same node but on a different disk seem relatively unaffected. Even the nodes performance in general seems about normal, which is odd because if a high IO delay is, say, on the local disk, I would probably see some performance degradation. Not the case here in this specific instance. So am I right to assume that the IO delay spikes during the clone activity is restricted to the disk that hosts all those templates? I'm only asking the community because I've never seen such high IO activity and honestly expected my PVE to be screaming bloody murder with IO delays of 30-40% . Any clarification would be greatly apperciated
 

dcsapak

Proxmox Staff Member
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Feb 1, 2016
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iodelay is basically the same as the linux kernel %iowait
from 'sar' manpage:

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

so a high iowait does not mean anything should be slow
it is just the relative percentage of 'waiting on io'/'idle cpu time'

if you cpu is fully loaded, this will generally be less, even if much io is going on

so this is always to be seen in context with cpu usage and general system load
 
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