Electrical computers and digital processing systems: memory – Storage accessing and control – Hierarchical memories
Patent
1993-04-02
1997-09-02
Chan, Eddie P.
Electrical computers and digital processing systems: memory
Storage accessing and control
Hierarchical memories
395842, 395872, 711112, 711118, 711137, G06F 1208
Patent
active
056641451
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
This invention relates to the field of data transfer between a device controller and a direct access storage device.
BACKGROUND OF THE INVENTION
Data Storage subsystems used in data processing systems commonly comprise a device controller connected to one or more storage devices on which customer data is retained. These storage devices are commonly direct access storage devices e.g. disk drives. In recent years, such storage subsystems have become increasingly sophisticated and there is an ongoing effort in the computer industry to develop subsystems which can store large amounts of data and which also provide high speed data transfer between the attached host data processing system and the devices. A number of different interfaces have been developed for use in connecting host to controller and device and controller.
One common interface is the ANSI adopted Small Computer Systems Interface standard (SCSI), details of which can be found in ANSI specifications X3.131-1986 & SCSI/2 X3T9.2/86-109. Data storage subsystems are known where controller and device are connected by means of a SCSI bus. The SCSI interface permits multi-sector operations i.e. multiple sectors of data may be transferred to and from the device in response to a single command. This has advantages over `per sector` operation (e.g. IPI) in that it avoids the need to reinstruct the device every sector.
At some location in the subsystem, it is necessary to provide one or more data buffers which are employed temporarily to hold the data being transferred between the host and the device. The location, form and number of separate areas of buffer depend on the requirements of the particular subsystem i.e. the architecture will to an extent dictate the buffering required. In terms of cost alone, it is obviously beneficial to be able to reduce the size and number of buffers.
In order to speed up the transfer of data to and from the devices, a number of performance enhancing techniques have been developed. One such is split transfer which is described in U.S. Pat. No. 4,494,157 which reduces the latency in transferring data from a rotating storage medium e.g. magnetic disk. If sectors 1 to N are requested, the split transfer will transfer the data in two bursts--first sectors N to M, followed by sectors 1 to N-1. In U.S. Pat. No. 4,494,157 the data is held in a buffer before transmission to the host. The buffer waits until it has received the second burst of data before. sending the data up to host in ascending sector order.
DISCLOSURE OF THE INVENTION
The invention provides a method of transferring data between a device controller having extended buffering and a direct access storage device having limited buffering comprising: the controller initiating data transfer by issuing a multi-sector data transfer order to the device and issuing a subsequent order to the device while the data transfer order is executing.
In a preferred method, said data transfer order specifies the data to be transferred and said subsequent order is an EXTEND order sent to the device while the data specified in the data transfer order is being transferred between device and controller, the device acting on the EXTEND order to transfer the data specified in the EXTEND order immediately upon completion of the execution of the data transfer order.
Thus in one preferred method, an EXTEND order may be employed to permit continuation of data transfer.
In a further preferred method the data transfer order is a multi-sector READ order which initiates transfer of read data from the device to a data buffer in the controller, wherein the controller transfers the read data from the controller data buffer to memory in a connected host data processing system and issues the EXTEND order to the device when there is space in the data buffer; and the device acts on the EXTEND order to send the read data specified in the EXTEND order to the controller data buffer.
Thus the EXTEND order is used to permit read ahead of data.
It is further preferred that
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Apperley Norman
Buckland Patrick Allen
Billion Richard E.
Bussan Matthew J.
Chan Eddie P.
International Business Machines - Corporation
Nguyen Hiep T.
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