Proposal: default virtualization storage class (#233)

Signed-off-by: Alex Kalenyuk <[email protected]>

design-proposals/default-virt-storage-class.md

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+# Overview
+Storage providers in the field may supply many ways of utilizing a back end storage solution;  
+They do so by providing different Kubernetes StorageClass parameters.  
+Some examples are `csi.storage.k8s.io/fstype` which indicates which filesystem type is created on a raw block before being passed on to a pod, or even lower level kernel [mapping options](https://access.redhat.com/articles/6978371).  
+With the growth of kubevirt adoptions, we may sometimes see a certain combination of storage class parameters is preferrable for VM workloads.
+
+This design presents a possible way of steering kubevirt users
+towards using such identified storage class.
+
+## Motivation
+- Certain Microsoft Windows versions use a peculiar I/O pattern. This particular pattern may cause [CRC errors](https://github.com/kubevirt/kubevirt/pull/9741) on certain types of storage back ends (ceph) when data is written and read across different processes or threads on a Windows VM.  
+- Storage providers like Portworx provide more than 10 storage class variations out of the box. It's possible that a specific variation is preferrable for VM workloads.
+
+## Goals
+* Provide a method for preferring a certain storage class in the cluster specifically for VM workloads
+
+## Non Goals
+* Make the preferrable storage class variation a part of CDI knowledge via [StorageProfiles](https://github.com/kubevirt/containerized-data-importer/blob/main/doc/storageprofile.md).  
+This will be a cluster admin/storage provider decision.
+
+## Definition of Users
+This feature is intended for cluster admins/storage providers who wish to advertise a specific set of parameters for provisioning VM disks.
+
+## User Stories
+* As a KubeVirt user, I want my storage to be tuned properly to run virtual machines and I want to consume this tuning automatically or with as little manual intervention as possible.
+
+## Repos
+* **containerized-data-importer**: Various controllers related to PVC population
+* **kubevirt**: storage class preference adjusted as well, if it exists
+
+# Design
+Introduce a new storage class annotation `storageclass.cdi.kubevirt.io/is-default-class`  
+which marks a storage class as the default for virtualization workloads (`contentType=kubevirt`),  
+and thus is preferred over the k8s cluster [default storage class](https://kubernetes.io/docs/tasks/administer-cluster/change-default-storage-class/) `storageclass.kubernetes.io/is-default-class` for those cases.
+
+
+## API Examples
+(tangible API examples used for discussion)
+
+## Scalability
+It is assumed scalability was one of the choosing factors when identifying a storage class to be superior for VM workloads
+
+## Update/Rollback Compatibility
+* Existing populated PVCs will not be affected, however,  
+clone sources from previous versions may have been provisioned using a different storage class variation of a provisioner.
+* Storage classes can still be explicitly requested on manifests to override virt/k8s default storage class
+
+## Functional Testing Approach
+* DV created without explicit SC  
+ virt storage class takes precedence over a default sc  
+ virt storage class is used even if there is no default sc
+* DV created with explicit SC  
+ overrides virt storage class
+* Multiple virt storage classes  
+ Follows deterministic [k8s behavior](https://github.com/kubernetes/kubernetes/pull/110559);  
+  Primary sort by creation timestamp, newest first  
+  Secondary sort by class name, ascending order
+* Clone from "standard" storage class variation to "virt" variation