Async MDL Reads/sec is the frequency of reads from the file system cache that use a Memory Descriptor List (MDL) to access ...

Async MDL Reads/sec is the frequency of reads from the file system cache that use a Memory Descriptor List (MDL) to access the pages.  The MDL contains the physical address of each page in the transfer, thus permitting Direct Memory Access (DMA) of the pages.  If the accessed page(s) are not in main memory, the calling application program will not wait for the pages to fault in from disk.