Questions about the listAttachedFcdsForVM function

[akutz]

👋 Howdy,

I was reading the CSI driver code and had a few issues about the listAttachedFcdsForVM function:

• On line 359 there is a check for the device's type using a helper function for getting the type name:

  if vmDevices.TypeName(device) == "VirtualDisk" {

  However, on the very next line, 360, the actual correct way to assert the device's type is used:

  if virtualDisk, ok := device.(*vimtypes.VirtualDisk); ok

  Why check the type twice? Especially when the idiomatic approach is already present in the second attempt?

• Should this function assume that all FCDs attached to a VM must be PVCs?

  As I understand it, all PVCs --> CNS volumes --> FCDs --> VMDKs, but the reverse is not true. A VMDK is not always an FCD, an FCD is not always a CNS volume, and a CNS volume is not always a PVC. Why is CSI assuming that an FCD must have a corresponding PVC?

  Should the function be renamed to listAttachedFCDsWithPVCsForVM?

[divyenpatel]

Why check the type twice? Especially when the idiomatic approach is already present in the second attempt?

We need to correct this. It is duplicate check.

if vmDevices.TypeName(device) == "VirtualDisk" { 
   if virtualDisk, ok := device.(*vimtypes.VirtualDisk);

Should this function assume that all FCDs attached to a VM must be PVCs?

We invoke the listAttachedFcdsForVM function from getVolumesToDetachFromVM, which in turn is called during the Reconcile operation of the CnsNodeVMBatchAttachment controller.

When a volume is removed from the CnsNodeVMBatchAttachment spec, this API is used to determine which FCDs corresponding to CNS Volumes — are currently attached to the VM but no longer listed in the spec. This allows us to identify the specific disks that need to be detached from the VM.

VM might also have other disks attached that are not managed as CNS volumes. Since these non-CNS disks are unrelated to the CnsNodeVMBatchAttachment resource, we ensures they are not detached accidentally. The logic in this flow filters out such disks, ensuring that only CNS-managed volumes are detached.

[akutz]

Howdy @divyenpatel ,

Thank you for such a prompt and thorough response! Very helpful! I scheduled a zoom for later today to discuss the above. I want to better understand what happens when there is a child disk attached to a VM and the parent disk is an FCD but not a PVC...