tgt: Framework for Storage Target Drivers

In order to provide block I/O services, Linux users have had to modify kernel code by hand, use binary kernel modules, or purchase specialized hardware. With the mainline kernel now having SCSI Parallel Interface (SPI), Fibre Channel (FC), iSCSI, and SCSI RDMA (SRP) initiator support, Linux target framework (tgt) aims to fill the gap in storage functionality by consolidating several target driver implementations and providing a SCSI protocol independent API that will simplify target driver creation and maintenance.

Tgt's key goal and its primary hurdle has been implementing a great portion of tgt in user space, while continuing to provide performance comparable to a target driver implemented entirely in the kernel. By pushing the SCSI state machine, I/O execution, and the management components of the framework outside of the kernel, it enjoys debugging, maintenance and mainline inclusion benefits. However, it has created new challenges. Both traditional kernel target implementations and tgt have had to transform Block Layer and SCSI Layer designs, which assume requests will be initiated from the top of the storage stack (the request queue's make_request_fn()) to an architecture that can efficiently handle asynchronous requests initiated by the the end of the stack (the low level drivers interrupt handler), but tgt also must efficiently communicate and synchronize with the user-space daemon that implements the SCSI target state machine and performs I/O.

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