[Stgt-devel] User-mode iSER

Tom Tucker tom
Tue Aug 1 20:42:44 CEST 2006

What do people think about something like this...

The target architecture is implemented to the extent possible entirely
in user-mode. The architecture intends to support multiple Target Device
Types, SCSI Transport Types and Network Transport Types. The enclosed
figure below illustrates the components of the architecture.

At the top of the figure are the different Target Device Type Drivers.
These "drivers" are implemented in user-mode as libraries and plug into
the Target Interface Layer. The Target Device Type drivers each support
a particular class of device. For example, the SCSI Disk Driver supports
SCSI disks, the Block Device driver supports generic block devices
(e.g. /dev/md0, etc..) and the File Device Driver supports files as
target devices. In most cases, the Target Device Type Drivers call
existing system call interfaces to communicate with the actual target
device, e.g. open, close, read, write, ioctl, etc... High performance
implementations may use private kernel interfaces to improve

The Target Interface Layer implements a generic target device
independent API called the Target Device API, and a SCSI transport
independent API called the SCSI Transport API. This Target Interface
Layer implements a target/SCSI transport switch that allows any Target
Device Type to be associated with any SCSI Transport Type. 

The SCSI Transport Class Drivers implement support for the various SCSI
transport types: SRP Transport implements the SCSI RDMA Protocol
transport, FCP Transport implements the Fiber Channel transport type,
and the iSCSI Transport implements the iSCSI transport type. These
drivers sit between the Target Interface Layer and the Network Interface

The Network Interface Layer implements a SCSI transport independent API
called the Transport Class API and a network transport independent API
called the Transport API. The Network Interface Layer allows a SCSI
Transport Class driver to support multiple network transports. For
example, the iSCSI Transport driver will support TCP, IB, and iWARP as
network transports. The details of a particular SCSI Transport Class's
device enumeration, login and management are implemented in the SCSI
Transport Class driver (e.g. iSCSI Transport). The details of a
particular network transport's connection management paradigm are
implemented in the Transport Provider driver (e.g. RDMA driver).

The Transport Provider Drivers implement the Transport Provider API and
provide core network I/O services to the Network Interface Layer. The
Transport API is a transport independent interface for creating
endpoints, service points, accepting incoming connection requests and
performing I/O on an endpoint.

The Management Agent interfaces with the Target Interface Layer and
performs management functions such as creating targets, devices, loading
and storing persistent configurations and other management related

The various API referred to above are basically simplified versions of
the existing scsi_transport_template, scsi_host, scsi_host_template
interfaces, etc... from the current kernel implementation. The
interfaces between the various components, however, can be reduced to
function calls since everything resides user mode. 

I think the only tough issue here is with copy avoidance for the network
user/kernel interface and target device user/kernel interfaces.
Initially, these could be prototyped without regard to this issue and
see what kind of performance we could get. The RDMA network transports
already provide copy avoidance, however, TCP/FC would require some


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