MergeVirtualDisk function (virtdisk.h)

Article
02/22/2024

Merges a child virtual hard disk (VHD) in a differencing chain with one or more parent virtual disks in the chain.

Syntax

DWORD MergeVirtualDisk(
  [in]           HANDLE                         VirtualDiskHandle,
  [in]           MERGE_VIRTUAL_DISK_FLAG        Flags,
  [in]           PMERGE_VIRTUAL_DISK_PARAMETERS Parameters,
  [in, optional] LPOVERLAPPED                   Overlapped
);

Parameters

[in] VirtualDiskHandle

A handle to the open virtual disk, which must have been opened using the VIRTUAL_DISK_ACCESS_METAOPS flag. For information on how to open a virtual disk, see the OpenVirtualDisk function.

[in] Flags

Must be the MERGE_VIRTUAL_DISK_FLAG_NONE value of the MERGE_VIRTUAL_DISK_FLAG enumeration.

[in] Parameters

A pointer to a valid MERGE_VIRTUAL_DISK_PARAMETERS structure that contains merge parameter data.

[in, optional] Overlapped

An optional pointer to a valid OVERLAPPED structure if asynchronous operation is desired.

Return value

Status of the request.

If the function succeeds, the return value is ERROR_SUCCESS.

If the function fails, the return value is an error code. For more information, see System Error Codes.

Remarks

Note All occurrences of the term disk in this section refer to virtual disks. The term backing store refers to the physical disk storage where the VHD image file or files reside.

The MergeVirtualDisk function updates all data blocks in one or more parent disks with the data blocks from the child disk referred to by the VirtualDiskHandle parameter. This is essentially a copy operation.

Merging a disk requires that the affected disks be detached during the operation.

The caller must have READ|WRITE access to the backing store for the affected disks.

The RWDepth of the disk must be greater than the merge depth specified by the OPEN_VIRTUAL_DISK_PARAMETERS.

Merge modifies the parent disk being merged into, therefore any other differencing disks dependent on that parent will no longer be valid.

The parent disk being merged into is changed to represent the same data as was held in the child differencing disk on which the merge is performed.

Any pre-existing data in the parent disk being merged into is overwritten.

If a merge operation is interrupted, the child disk is still usable. The MergeVirtualDisk function can be rerun to finish the merge.

The depth of a merge request is the number of parent VHD image files in the differencing chain to be merged. For example, if the MergeDepth member has a value of 1, the data blocks from the specified differencing disk are moved into its parent. If the MergeDepth member has a value of 2 and the specified differencing disk's parent is also a differencing disk (meaning there is a third disk in the chain), then the data blocks from both the first and second disks are moved into the third disk (with blocks from the first disk taking precedence over blocks from the second during the final operation).

Upon completion, the affected child disks are no longer considered valid, and any future operations on them will have unsupported results. In the previous example, upon successful completion of the merge, the third disk is valid and the first and second are not. The MergeVirtualDisk function will not delete any disks that are not valid, or perform any automatic differencing relationship reconnections. This must be explicitly done by the caller.

If a merge operation is performed on a nonleaf node of a differencing disk, it is the responsibility of the caller to fix up the parent information for the child nodes of the disk that is being merged.

Requirements

Requirement	Value
Minimum supported client	Windows 7
Minimum supported server	Windows Server 2008 R2
Target Platform	Windows
Header	virtdisk.h
Library	VirtDisk.lib
DLL	VirtDisk.dll