I have updated my GKE nodes to version v1.18.17-gke.1900 on 18th of May 2021. Nevertheless the recent COS version is not included in that GKE version. Where do I find which COS version is in which GKE version? (In the docs I only see which version is available but not which version will be installed)
For example following COS Release Notes say Kubernetes version v1.18.13 but this is not the case and I didn't find any other mapping because as mentioned above I got 1.18.17.
https://cloud.google.com/container-optimized-os/docs/release-notes/m85#cos-85-13310-1209-29
In the GKE release notes, you can find the list of current GKE minor versions available for each release channel as well as the COS version. Unfortunately, the current list does not include 1.18.x.
GKE generally uses the current COS LTS releases available when at the time the initial GKE major/minor version is released. GKE 1.18 initially GA'd in April 2020 at which time COS 85 was the current. So all GKE 1.18 versions will almost always be based on COS 85. The version of COS 85 available will usually be patch version available around the time of each GKE 1.18 release.
If you upgraded in May, then cos-85-13310-1209-29 would have been the COS version as it came out mid-April.