The SetData operation will discard all existing buffer data.
The operation will overwrite (with a write-only operation) every location within the locked surface. This is a valid option when using dynamic textures, dynamic vertex buffers, and dynamic index buffers. You may not use this option to update a portion of a surface.
For vertex and index buffers, the SetData operation will discard the entire buffer. A pointer to a new memory area is returned so that the direct memory access (DMA) and rendering from the previous area do not stall.
For textures, the SetData operation will overwrite (with a write-only operation) every location within the region being locked.
Portions of existing data in the buffer may be overwritten during this operation.
The SetData operation will not overwrite existing data in the vertex and index buffers. Specifying this option allows the driver to return immediately from a SetData operation and continue rendering.