First Class Disk (FCD) Backup

First Class Disk (FCD), also known as Improved Virtual Disk, provides storage lifecycle management on virtual disks, independent of virtual machines. An FCD may be created natively using the FCD interfaces or an existing virtual disk may be promoted to an FCD.

First Class Disk is a named virtual disk not associated with a VM. The vSphere API for handling FCD is called VSLM, virtual storage lifecycle management.

In vSphere 6.5, VDDK supported the backup of FCD in attached mode, but not in detached mode. To back up FCD, programs had to attach the FCD to a dummy VM (such as one without a guest OS) and then back up the VM.

In vSphere 6.7, VDDK supports the backup of detached FCD, in any transport mode. Detached FCD is neither related to nor attached to a VM. It is identified by a combination of its UUID and the datastore ID where it resides. An FCD must be attached to a VM for regular I/O, but not for snapshot based backup.

First Class Disks are identified by UUID. This UUID is globally unique and the primary identifier for the FCD. The UUID remains valid even if its FCD is relocated or snapshotted. FCD operations are performed using the VStorageObjectManagerBase managed object, extended for either ESXi hosts or vCenter Server. See the vSphere API Reference for details.

FCD Workflows and Operations

Create and Delete: FCDs can be created using the createDisk_Task method. An existing VMDK can be promoted to FCD using the registerDisk method. Once a disk has been registered as an FCD it cannot be unregistered, however registering a disk as an FCD has no impact other than enabling the VSLM APIs to manage them.

Snapshot: FCD snapshots use the same underlying mechanisms as regular snapshots and have the same restrictions as regular snapshots. FCD snapshots are independent of VM snapshots. Each FCD snapshot receives a separate snapshot ID. Snapshots are managed using the VStorageObjectManager specific createSnapshot_Task and deleteSnapshot_Task methods. You can revert an FCD to a given snapshot using the RevertVStorageObject_Task method. Reverting an FCD removes all snapshots after the selected snapshot. To access data at the time of the snapshot without discarding subsequent snapshots, you can use the VStorageObjectManagerCreateDiskFromSnapshot method or VADP restore.

Backup and Restore: FCDs that are not attached to VMs can be backed up using the VADP interfaces. For backup applications, the best practice is to back up VMDKs and FCDs attached to the VMs, and also back up FCDs that are not attached to a VM.

Discovering FCDs for Backup

FCDs can be listed on a per-datastore basis. The recommended practice is to iterate across the list of datastores from vCenter, get the FCDs for each datastore, then perform a backup of each FCD. VADP can back up both attached and detached FCDs.

Use the Datacenter managed object's datastore property to list all datastores in a datacenter. For each datastore, use the listVStorageObject method to list the FCDs. For each FCD, use the VStorageObject.getConfig.getId API to retrieve the FCD's ID. The FCD ID is used in connParams-> below. The FCD snapshot ID is used in the connParams->vStorageObjSpec.ssid to specify the snapshot to back up from.

New Connect Parameters and Functions

VDDK 6.7 has a new union in the VixDiskLibConnectParams structure, and new function calls to allocate and free the structure, VixDiskLib_AllocateConnectParams() and VixDiskLib_FreeConnectParams(). The new typedef is a union of:

  • vmxSpec, the traditional specification string for a VM.
  • VixDiskLibVStorageObjectSpec specifying the FCD's UUID, datastore MoRef, and snapshot UUID.

VixDiskLib_AllocateConnectParams() is the recommended way to allocate an instance of VixDiskLibConnectParams. Programs can allocate VixDiskLibConnectParams as before, although then they cannot use new features in 6.7 such as detached FCD.

Except for connection parameters, VDDK code remains the same. All transport modes may be used. Here is sample C++ code to allocate and initialize the connect parameters structure:

auto connParams = VixDiskLib_AllocateConnectParams();
connParams-> = "XXXXXX" // FCD UUID
connParams->vStorageObjSpec.datastoreMoRef = "datastore-N" // datastore
connParams->vStorageObjSpec.ssid = "XXXXXX" // FCD snapshot UUID
VixDiskLib_FreeConnectParams(connParams); // free connect params near end

For a vmxSpec, here is a sample of legacy C++ code, and a revision:

// old:
VixDiskLibConnectParams connParams;
connParams.vmxSpec = "moref=vm-XX"; // vm moref
// new:
auto connParams = VixDiskLib_AllocateConnectParams;
connParams->specType = VIXDISKLIB_SPEC_VMX;
connParams->vmxSpec = "moref=vm-XX"; // vm moref
VixDiskLib_FreeConnectParams(connParams); // free connect params near end

Subsequently, VixDiskLib_Open ignores its disk path parameter because path location is determined by VixDiskLibVStorageObjectSpec. The snapshot UUID is specified in VixDiskLibVStorageObjectSpec, so the disk path is invisible to the customer anyway. Consequently programs can pass NULL as the disk path parameter to VixDiskLib_Open. Programs can also pass an FCD's UUID as the path parameter:

VixDiskLib_Open(const VixDiskLibConnection connection,
                const char *path,
                uint32 flags,
                VixDiskLibHandle *diskHandle);

However for the VixMntapi open disks call, passing a null diskNames parameter may cause a crash. Programs should pass the FCD's UUID only, and specify numberOfDisks = 1.

VixMntapi_OpenDisks(VixDiskLibConnection connection,
                    const char *diskNames[],
                    size_t numberOfDisks,
                    uint32 openFlags,
                    VixDiskSetHandle *handle);

FCD Managed Objects and Tasks

The following managed objects and tasks are available in vSphere for manipulating FCD storage. For details, see the vSphere Web Services API Reference on under the vSphere Management SDK.

  • HostVStorageObjectManager manages FCD when connected to an ESXi host.
  • VcenterVStorageObjectManager manages FCD when connected to a vCenter Server.
  • VirtualMachine.AttachDisk_Task() attaches FCD to a VM.
  • VirtualMachine.DetachDisk_Task() detaches FCD from the VM.

Behavior of FCD With Changed Block Tracking

The behavior of FCD with changed block tracking (CBT) differs from that of a regular VMDK disk.

  1. Attaching an FCD with CBT disabled (the default) to a VM with CBT enabled causes CBT on the FCD to become enabled. However, detaching that FCD from the VM does not disable CBT.
  2. Attaching an FCD with CBT enabled to a VM with CBT disabled throws an error, unless the FCD is attached as "independent nonpersistent" disk.

    Use ReconfigVM_Task and connect the disk with the diskMode set to independent_nonpersistent in the backing info. (VirtualDiskSeSparseBackingInfo, VirtualDiskFlatVer2BackingInfo, VirtualDiskRawDiskMappingVer1BackingInfo and VirtualDiskSparseVer2BackingInfo support this mode.)

Behavior of FCD with vMotion

An FCD can be attached to more than one VM.

When your program calls VixDiskLib_PrepareForAccess, VDDK disables the vSphere API method RelocateVM_Task for all attached VMs, one after another. If an error occurs while disabling, VMs whose relocate method was disabled remain disabled, whereas subsequent VMs can still be relocated. Programs should call VixDiskLib_EndAccess to re-enable the relocate method, regardless of VixDiskLib_PrepareForAccess return status. Otherwise vMotion or Storage vMotion could fail for the remaining disabled VMs.

Code Sample for Connect Parameters

In the development kit, see lines in the vixDiskLibSample.cpp program where connection parameters are allocated with the VixDiskLib_AllocateConnectParams() call.

In vSphere 6.7 and later, the Web Services SDK contains FCD code samples in Java for creating, attaching, detaching, and deleting FCD.

In the vSphere 6.7 U2 release, Container Native Storage (CNS) supports FCD, and support for more solutions will be added in the future.