1. Technical Field
The present invention relates generally to communication protocols between a host computer and an input/output (I/O) Adapter. More specifically, the present invention is directed to a system and method for enabling user space middleware or applications to pass block mode storage requests directly to a physical I/O adapter without run-time involvement from the local Operating System (OS), or, in a virtual system, the local hypervisor.
2. Description of Related Art
Operating systems, according to the present state of the art, do not permit user space middleware or applications, such as a database, to directly access persistent storage that is identified through the Operating System's Raw Mode Storage I/O interface or the Operating System's Logical Volume Storage I/O interface. As a result, the user space middleware must invoke an Operating System (OS) call and incur several task switches every time an I/O operation is performed. The first task switch is caused when the middleware or application transfers a storage request to the OS. A second task switch occurs when the OS passes control back to the user space middleware or application, after the OS completes processing the middleware or application storage request and passes the storage request to the storage adapter.
A third task switch occurs when the storage adapter completes the associated block mode I/O storage operations and interrupts the processing being performed by an application so that the OS may process the storage adapter's completion. The final task switch occurs when the OS finishes processing the storage adapter's completion and gives control back to the middleware or application that transferred the storage request to the OS. In addition to these task switches the storage adapter typically has a single request queue to process work from the operating system.
The four task switches described above may be considered wasted processor cycles, because all work on the thread being switched is stopped until the task switch is complete. On some servers, the number of storage operations performed by a user space middleware or application program may be quite large. Modern, high-end servers may have millions of these operations per second, resulting in several million task switches per second.