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1. CloudStack should provide administrator the means to choose the type of disk controller (including sub types listed in introduction section above) for an instance. The controller to be used by VM to access virtual disk (volume) can decided for various reasons. Some of them are listed here,
   
   1. Some controllers are optimized for best performance over specific backend infrastructure like SAN. Ex: VMware Paravirtual SCSI
   2. Compatibility of some controllers with VM's virtual hardware version or guest operating system.
   3. Operating system vendor recommendation and default set of drivers distributed as part of operating system image. Ex: Windows 8.1 ISO doesn't have Lsi Logic Parallel SCSI drivers by default. Hence a virtual disk attached to this controller won't accessible during installation of OS using the ISO.
2. CloudStack should provide administrator an option which auto detects the recommended disk controller for the instance's guest operating system and applicable virtual hardware version.
3. The choice of controllers mentioned in (1) & (2) is applicable only for user instances.
4. Support backward compatibility - CloudStack should ensure legacy user instances are un-changed. Means existing user instance should not be affected by the administrator's choice mentioned in (1) & (2)
   
   1. Stopping & starting of legacy user instance should not affect the controller type in use for root & data volumes.
   2. Resetting a legacy user instance (RestoreVMCommand) of legacy instance should not affect the controller type in use for root & data volumes.
   3. Rebooting a legacy user instance
5. A modification of global configuration parameter should not affect all user instances created before the modification.
6. Attach/Detach volume operation should be supported irrespective of the type of disk controller of user instance. This needs to be fulfilled because a volume to be attached might have a specific type of disk controller in it's associated VMDK file which doesn't match the type of disk controller of user instance.
7. A mix of SCSI sub-type disk controller types are not allowed in user instance. Ex: LsiLogicSAS and VmwareParavirtual scsi controllers are not allowed in single user instance.
8. Root and data volume does not necessarily have same disk controller type. Provide means to support difference disk controllers for root and data volumes of user instance.

9. Quality risks (test guidelines). Here are some test cases to be covered.
   

   1. Deploy VM when global configuration parameter "vmware.root.disk.controller" is NOT set to osdefault.

      1. Try Stop and Start the VM and verify
      2. Try restore VM operation and then start the VM and verify
   2. Attach a fresh (not yet implemented) volume to VM deployed in test case (1)
   3. Attach an old volume (which is detached from another VM) to VM deployed in test case (1)
      
      1. Try the same case with an old volume which have used storage controller different from the VM's SCSI storage controller. Check VM's settings to see the type of SCSI controller type.
   4. Try test case (1) through (4) with global configuration parameter "vmware.root.disk.controller" is set to osdefault.
   5. Test the following cases with global configuration parameter "vmware.root.disk.controller" is set to osdefault.,
      
      1. Attach a fresh volume to legacy user instance
      2. Restore a legacy user instance and try starting it.
      3. Stop and start the user instance created in step (1)
10. Unsupported case: (What is NOT supported?)
    1. Mix of SCSI controller types in single user instance.
11. Supportability characteristics:
    • Management server log would be written with messages indicating the type of controller used while attaching a virtual disk / volume to a user instance.
    • To debug the issues specific to this feature, administrator should check following,
      • User instance's disk configuration settings using vSphere client.
      • VMDK header/meta file that describes the virtual disk's configuration
      • Log file /var/log/cloudstack/management/management-server.log in management server
      • Log file /var/log/cloud/cloud.out  in SSVM
12. Deployment requirements (fresh install vs. upgrade) if any
    • This feature is supported in fresh install as well as upgrade.
13. Interoperability and compatibility requirements:
    • Targeting VMware ESXi 4.1 or later versions.
    • http://www.vmware.com/resources/compatibility/search.php?deviceCategory=io
14. Limitations
    1. Maximum SCSI virtual disks supported per user instance is 15.
15. Target user details
    • This feature is mainly targeted for administrators

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