Fault tolerant disk array data storage subsystem

• US 5,459,857 A
• Filed: 09/27/1994
• Issued: 10/17/1995
• Est. Priority Date: 05/15/1992
• Status: Expired due to Term

• Current Assignee: Storage Technology Corp (Oracle Corporation)
• Original Assignee: Storage Technology Corp (Oracle Corporation)
• Inventors: Swiatek, Paul R., Ludlam, Henry S., Milligan, Charles A., Rudeseal, George A.
• Primary Examiner(s): Beausoliel, Jr., Robert W.
• Assistant Examiner(s): Palys, Joseph E.
• Application Number:
  US08/312,668
• Time in Patent Office: 385 Days
• Field of Search: 371/10.1, 371/11.1, 395/575, 364/268.5, 364/245
• US Class Current: 714/6.12
• CPC Class Codes: G06F 11/2064 while ensuring consistency
• CPC Class Codes: G06F 11/2071 using a plurality of controllers
• CPC Class Codes: G06F 11/2076 Synchronous techniques
• CPC Class Codes: G06F 11/2092 Techniques of failing over between control units
• CPC Class Codes: G06F 11/2094 Redundant storage or storage space G06F11/2056 takes precedence
• CPC Class Codes: G11C 29/74 using duplex memories, i.e. using dual copies

Abstract

A pair of operationally independent disk drive array data storage subsystems are used to emulate one or more physical devices shared between two control modules. The storage control units of the two data storage subsystems are interconnected by at least one data link to exchange control and data signals therebetween. The storage control units of both data storage subsystems are synchronized to maintain identical virtual device images of certain assigned virtual devices both of the data storage subsystems wherein the duplicated data records of the single virtual device are stored. The data records are therefore stored in available memory on both of the two data storage subsystems. Data is exchanged over the data link to maintain consistency of the two sets of mapping tables.

Claims

1. A disk memory system that stores data records for at least one data processor in a fault tolerant mode, comprising:

two data storage subsystems, each storing data records for said data processor, each of said data storage subsystems including;

a plurality of disk drives located in said data storage subsystem, a subset of said plurality of disk drives being configured into at least two redundancy groups, each said redundancy group consisting of at least two disk drives,

storage control means, including means, responsive to the receipt of a stream of data records from said data processor and designated by said data processor for storage in available memory space in said data storage subsystem, for selecting available memory space in one of said redundancy groups to store said received stream of data records thereon,

means, in said storage control means, for writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group,

means for maintaining data indicative of the physical location of data records stored in said plurality of redundancy groups;

means, interconnecting said storage control means with a storage control means located in the other one of said two data storage subsystems, for exchanging control and data signals therebetween; and

wherein said storage control means includes means, responsive to writing a data record to said one redundancy group, for transmitting said written data record to said storage control means connected to said exchanging means and located in said other data storage subsystem to maintain duplicate data records in both said data storage subsystems absent transmission of instructions to said data storage subsystem to create said duplicate data records by said data processor.

2. The system of claim 1 wherein each of said storage control means further comprises:

means, responsive to said data record being received from said exchanging means, for selecting available memory space in one of said redundancy groups to store said received stream of data records thereon; and

means for writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group.

3. The system of claim 1 wherein each of said storage control means further comprises:

means, responsive to said data processor requesting said data storage subsystem to read a data record from a selected redundancy group, for reading said maintained data to identify the physical storage location of said requested data record in said plurality of disk drives that comprise said selected redundancy group.

4. The system of claim 3 wherein each of said storage control means further comprises:

means, responsive to a failure of said selected redundancy group, for presenting a device not operational status to said host processor.

5. The system of claim 1 wherein said transmitting means comprises:

means for maintaining data indicative of a shared virtual device status for each of said redundancy groups, independent of said data processor;

means, responsive to said data processor transmitting a data record to said data storage subsystem for storage on a selected redundancy group, for reading said maintained shared virtual device status data for said selected redundancy group; and

means, responsive to said redundancy group being a shared virtual device, for transmitting said received data record to said other data storage subsystem absent transmission of instructions to create said duplicate data records by said data processor.

6. In a disk memory system that comprises two data storage subsystems, each storing data records for said data processor, each of said data storage subsystems including a plurality of disk drives, a subset of said plurality of disk drives being configured into at least two redundancy groups, each said redundancy group consisting of at least two disk drives, wherein said two data storage subsystems are interconnected by a data link for exchanging control and data signals therebetween, a method of storing data records for at least one data processor in a fault tolerant mode, comprising the steps in one of said data storage subsystems of:

selecting, in response to the receipt of a stream of data records from said data processor, available memory space in one of said redundancy groups to store said received stream of data records thereon;

writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group;

maintaining data indicative of the physical location of data records stored in said plurality of redundancy groups;

transmitting, in response to writing a data record to said one redundancy group, said written data record to the other of said data storage subsystems via said data link to maintain duplicate data records in both said data storage subsystems absent transmission of instructions to said data storage subsystem to create said duplicate data records by said data processor.

7. The method of claim 6 further comprising the steps of:

selecting, in response to said data record being received from said data link, available memory space in one of said redundancy groups to store said received stream of data records thereon; and

writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group.

8. The method of claim 6 further comprising the steps of:

reading, in response to said data processor requesting said data storage subsystem to read a data record from a selected redundancy group, said maintained data to identify the physical storage location of said requested data record in said plurality of disk drives that comprise said selected redundancy group.

9. The method of claim 8 further comprising the step of:

presenting, in response to a failure of said selected redundancy group, a device not operational status to said host processor.

10. The method of claim 6 further comprising the steps of:

maintaining data indicative of a shared virtual device status for each of said redundancy groups independent of said data processor;

reading, in response to said data processor transmitting a data record to said data storage subsystem for storage on a selected redundancy group, said maintained shared virtual device status data for said selected redundancy group; and

transmitting, in response to said redundancy group being a shared virtual device, said received data record to said other data storage subsystem absent transmission of instructions to create said duplicate data records by said data processor.

11. A disk memory system that stores data records for at least one data processor in a fault tolerant mode, comprising:

two data storage subsystems, each storing data records for said data processor, each of said data storage subsystems including;

a plurality of disk drives located in said data storage subsystem, a subset of said plurality of disk drives being configured into at least two redundancy groups, each said redundancy group consisting of at least two disk drives,

means, responsive to the receipt of a stream of data records from said data processor and designated by said data processor for storage in available memory space in said data storage subsystem, for selecting available memory space in one of said redundancy groups to store said received stream of data records thereon,

means for writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group;

means for maintaining data indicative of the physical location of data records stored in said plurality of redundancy groups;

means, interconnecting said two data storage subsystems, for exchanging control and data signals therebetween; and

wherein each said data storage subsystem further includes;

means, connected to said exchanging means and responsive to writing a data record to one of said redundancy groups, for transmitting said written data record to the other of said two data storage subsystems connected to said exchanging means to maintain duplicate data records in both said data storage subsystems absent transmission of instructions to said data storage subsystem to create said duplicate data records by said data processor.

12. The system of claim 11 wherein each of said data storage subsystems further comprises:

means responsive to said data record being received from said exchanging means, for selecting available memory space in one of said redundancy groups to store said received stream of data records thereon; and

means for writing said received stream of data records and redundancy data associated with said received stream of data records in said selected available memory space in said one redundancy group.

13. The system of claim 11 wherein each of said data storage subsystems further comprises:

means, responsive to said data processor requesting said data storage subsystem to read a data record from a selected redundancy group, for reading said maintained data to identify the physical storage location of said requested data record in said plurality of disk drives that comprise said selected redundancy group.

14. The system of claim 13 wherein each of said data storage subsystems further comprises:

means, responsive to a failure of said selected redundancy group, for presenting a device not operational status to said host processor.

15. The system of claim 11 wherein said transmitting means comprises:

means for maintaining data indicative of a shared device status for each of said redundancy groups independent of said data processor;

means, responsive to said data processor transmitting a data record to said data storage subsystem for storage on a selected redundancy group, for reading said maintained shared device status data for said selected redundancy group; and

means, responsive to said redundancy group being a shared device, for transmitting said received data record to said other data storage subsystem independent of said data processor.

16. A data storage system adapted to be connected to a host computer, comprising:

a first data storage control;

a second data storage control;

a first recording means comprising a plurality of recording elements connected to said first data storage control and a second recording means comprising a plurality of different recording elements connected to said second data storage control;

each of said data storage controls being operable independent of each other to write a data record on a specified one of the plurality of recording elements to which only that said data storage control is connected;

a data link transmission path interconnecting said first and second data storage controls;

means in said first data storage control operable independent of said host computer for defining a first recording element of said first recording means as well as a first recording element of said second recording means as a copy pair of recording elements;

means unique to said first data storage control, operable independent of said second data storage control, and responsive to receipt from said host computer of a data record write request and accompanying data record for writing said data record on said first recording element of said first recording means;

means in said first data storage control for transmitting said data record write request and said accompanying data record over said data link transmission path to said second data storage control; and

means unique to said second data storage control and responsive to the receipt of said data record write request and said accompanying data record over said data link transmission path for writing said data record by said first recording element of said second recording means.

17. The data storage system of claim 16 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching means.

18. The data storage system of claim 16 wherein each of said data storage controls contains a cache memory, said data storage system further comprises:

means in each data storage control responsive to the receipt of said data record by said each data storage control for entering said received data record into the cache memory of said each data storage control;

means for returning a command complete signal to said host computer from said first data storage control when said data record is entered into said cache memory of each data storage control; and

wherein said means for writing comprises means for applying said data record in each cache memory to the recording means connected to the data storage control containing said each cache memory to control the writing of said data record by said first recording element of said recording means of each data storage control.

19. The data storage system of claim 16 further comprising:

a first path extending from said host computer to said first data storage control for the transmission of information including data record write requests and accompanying data records from said host computer to said first data storage control;

a second path extending from said host computer to said second data storage control for the transmission of information including data record write requests and accompanying data records from said host computer to said second data storage control;

said second data storage control being responsive to the receipt of said data record write requests and accompanying data records over said second path for writing said data records received over said second path on said first recording element of said second recording means.

20. The data storage system of claim 19 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching means.

21. The data storage system of claim 19 further comprising:

means in second data storage control, responsive to said receipt of said data record write requests and accompanying data records received over said second path, for transmitting said data record write requests and accompanying data records over said data link transmission path to said first data storage control; and

means in said first data storage control, operable independent of said host computer and responsive to said receipt of said data record write requests and accompanying data records over said data link transmission path, for writing said data records received by said first data storage control over said data link transmission path on said first recording element of said first recording means.

22. The data storage system of claim 21 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching means.

23. The data storage system of claim 21 wherein each of said data storage controls contains a cache memory, said data storage system further comprises:

means in each data storage control, responsive to the receipt of a data record write request and accompanying data record over said data link transmission path connected to said data storage control, for entering said received data record into the cache memory of said data storage control that received said data record write request and data record over said data link transmission path;

means for returning a command complete indication to said host computer from the data storage control that received said data record write request and data record from said host computer when said data record is entered into said cache memory of each data storage control; and

wherein said means for writing comprises means for applying said data record in each cache memory to the recording means connected to the data storage control containing said each cache memory to control the writing of said data record by said first recording element of said recording means.

24. A data storage system adapted to be connected to a host computer, comprising:

a first data storage control;

a second data storage control;

a first group of disk drives connected exclusively to said first data storage control;

a second group of disk drives connected exclusively to said second data storage control;

each of said data storage controls being operable independent of each other to write a data record by a specified one of the disk drives to which only that said data storage control is connected;

a data link transmission path interconnecting said data storage controls;

means in said first data storage control for receiving information from said host computer;

means in said first data storage control operable independent of said host computer for defining a specified one of said disk drives of said first group of disk drives and a specified one of said second group of disk drives as a copy pair of disk drives;

means in said first data storage control, responsive to receipt of a data record write request and an accompanying data record, for transmitting said received data record accompanying said data record write request over said data link transmission path to said second data storage control;

means unique to said first data storage control and further responsive to said receipt of said data record write request, for writing said accompanying data record on said one disk drive of said first group; and

means unique to said second data storage control and responsive to the reception of said data record over said data link transmission path, for writing said data record on said one disk drive of said second group.

25. The data storage system of claim 24 wherein said data link transmission path comprises a direct connection between said data storage controls that is devoid of switching means.

26. The data storage system of claim 24 wherein each of said data storage controls contains a cache memory, said data storage system further comprises:

means in each data storage control, responsive to the receipt of a data record by said each data storage control, for entering said each received data record into the cache memory of said each data storage control;

means for returning a command complete signal to said host computer from said first data storage control when a data record received by each data storage control is entered into said cache memory of said each data storage control; and

wherein said means writing comprises means for applying said data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive.

27. The data storage system of claim 24 further comprising:

a first path extending from said host computer to said first data storage control for the reception of information including data records from said host computer;

a second path extending from said host computer to said second data storage control for the transmission of information including data records to said second data storage control;

means in said second data storage control for receiving information over said second path from said host computer;

means in said second data storage control, responsive to the receipt of a data record write request and an accompanying data record, for writing said accompanying data record on said second group of disk drives;

means in said second data storage control, operable independent of said host computer, for defining a specified one of said disk drives of said first group of disk drives and a specified one of said second group of disk drives as a copy pair of disk drives;

means in said second data storage control, responsive to a subsequent receipt of a data record write request and accompanying data record directed to said specified one disk drive of said second group, for transmitting said received data record over said data link transmission path to said first data storage control;

means unique to said second data storage control and responsive to said subsequent receipt of said data record write request and accompanying data record for writing said received data record on said specified one disk drive of said second group; and

means unique to said first data storage control and responsive to the receipt of said data record over said data link transmission path for writing said data record on said specified one disk drive of said first group of disk drives.

28. The data storage system of claim 27 wherein each of said data storage controls contains a cache memory, further comprising:

means in each data storage control, responsive to the receipt of a data record by said each data storage control, for entering said received data record into the cache memory of said each data storage control;

means for returning a command complete signal to said host computer from said data storage control that receives a data record from said host computer when said data record is entered into said cache memory of each data storage control; and

wherein said means for writing comprises means for applying a data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive.

29. A data storage system adapted to be connected over channel interface paths to a first and a second host computer for the writing of multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers, said data storage system comprising:

a first and a second data storage control connected via separate ones of said channel interface paths to each of said host computers;

a first group of disk drives connected to said first data storage control and a second group of disk drives connected to said second data storage control;

a direct data link transmission path devoid of switching means interconnecting said data storage controls;

means in each of said data storage controls for receiving a data record from either of said host computers over said channel interface paths;

means unique to each data storage control independent of the other data storage control,]and responsive to the reception of a data record over one of said channel interface paths for writing said data record on a disk drive of the group to which said each data storage control receiving said data record is connected;

means in said each data storage control, responsive to the receipt of said data record over one of said channel interface paths for transmitting said data record over said data link transmission path to the other one of said data storage controls; and

means unique to said other data storage control and responsive to the reception of said data record over said data link transmission path for writing said data record on a disk drive of the group to which said other data storage control is connected.

30. The data storage system of claim 29 wherein each of said data storage controls contains a cache memory, further comprising:

means in each data storage control responsive to the receipt of said data record over a channel interface path for entering said received data record into the cache memory of each of said data storage controls;

means for returning a command complete signal to said host computer from the data storage control that receives a data record over a channel interface path when said received data record is entered into said cache memory of each of said data storage controls; and

wherein said means for writing comprises means for applying said data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive of each data storage control.

31. A data storage system adapted to be connected over channel interface paths to a first and a second host computer, apparatus for the writing of multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers comprising:

a first and a second data storage control being operable for receiving a data record from either of said host computers over one of said channel interface paths;

said first data storage control being operable independent of said second data storage control in response to the receipt of a data record over one of said channel interface paths for writing said data record on a disk drive of a group of drives to which only said first data storage control is connected;

means in said first data storage control operable in response to the receipt of said data record over one of said channel interface paths for transmitting said data record over a data link transmission path to said second data storage control;

means in said second data storage control operable in response to the receipt of said data record over one of said channel interface paths for transmitting said data record over a data link transmission path to said first data storage control; and

means in said second data storage control operable in response to the receipt of said data record over said data link transmission path for writing said data record on a disk drive of a group to which only said second data storage control is connected.

32. The apparatus of claim 31 further comprising:

means in each data storage control operable in response to the receipt of said data record over a channel interface path for entering said received data record into a cache memory of each of said each data storage control;

means for returning a command complete signal to said host computer from the data storage control that receives a data record over a channel interface path when said received data record is entered into said cache memory of each of said data storage controls; and

means for applying said data record in each cache memory to a disk drive of a group connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive of a group connected to said each data storage control.

33. A data storage system containing a plurality of data storage controls and adapted to be connected over channel interface paths to a first and a second host computer for the writing of multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers comprising:

each data storage control being operable independent of all other data storage controls in response to the receipt of a data record over one of said channel interfaces paths from either of said host computers for writing said data record on a disk drive of a group of drives to which only said each data storage control receiving said data record is connected;

said each data storage control being operable in response to the receipt of said data record over one of said channel interface paths for transmitting said data record over a data link transmission path to another one of said data storage controls; and

means in said other data storage control being operable in response to the receipt of said data record over said data link transmission path for writing said data record on a disk drive of a group of drives to which only said other one data storage control is connected.

34. A data storage system adapted to be connected over first and second channel interface paths, respectively, to a first and a second host computer, respectively, for writing a copy of data records received by said data storage system over any of said channel interface paths from either of said host computers comprising:

a first data storage control for receiving a data record write request and an accompanying data record over said first channel interface path from said first host computer;

a second data storage control for receiving a data record write request and an accompanying data record over said second path channel interface from said second host computer;

means in said second data storage control operable in response to the receipt of each data record write request from said second host computer and directed to a specified disk drive connected to said second data storage control for writing said data record accompanying each data record write request on said specified disk drive to which said second data storage control is connected;

means in said first data storage control operable independent of said first host computer for defining a specified one of said disk drives of said first group of disk drives and a specified one of said second group of disk drives as a copy pair of disk drives;

means in said first data storage control operable in response to receipt of a data record write request and an accompanying data record not directed to said specified drive of said first data storage control for writing the data record accompanying said request on a disk drive of a group to which said first data storage control is connected;

means in said first data storage control operable in response to a subsequent receipt of a data record write request and an accompanying data record directed to said specified disk drive for transmitting said data record over a data link transmission path to said second data storage control; and

means in said second data storage control operable in response to the receipt of said data record over said data link transmission path for writing said data record on a disk drive to which said second data storage control is connected.

35. In a data storage system, adapted to be connected over channel interface paths to a first and a second host computer, apparatus for writing multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers, comprising:

a first and a second data storage control for receiving a data record from either of said host computers over one of said channel interface paths;

each said data storage control being operable independent of the other data storage control in response to the receipt of a data record over one of said channel interface paths for writing said data record on a disk drive of a group of disk drives to which only said each data storage control receiving said data record is connected;

means unique to each said data storage control, operable in response to the receipt of said data record over one of said channel interface paths, for transmitting said data record over a data link transmission path to the other one of said data storage controls; and

means unique to said other data storage control, responsive to the receipt of said data record over said data link transmission path, for writing said data record on a disk drive of a group to which only said other one data storage control is connected.

36. The apparatus of claim 35 further comprising:

means in each data storage control, responsive to the receipt of said data record over one of said channel interface paths for entering said received data record into a cache memory of each of said each data storage control;

means for returning a command complete signal to said host computer from the data storage control that receives a data record over a channel interface path when said received data record is entered into said cache memory of each of said data storage controls; and

wherein said means for writing comprises means for applying said data record in each cache memory to a disk drive of a group connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive of a group connected to said each data storage control.

37. In a data storage system adapted to be connected over first and second channel interface paths, respectively, to a first and a second host computer, respectively, apparatus for the writing of a copy of data records received by said data storage system over any of said channel interface paths from either of said host computers, comprising:

means in a first data storage control for receiving a data record write request and an accompanying data record over said first channel interface path from said first host computer;

means in a second data storage control for receiving a data record write request and an accompanying data record over said second channel interface path from said second host computer;

means in said second data storage control, responsive to the receipt of each data record write request from said second host computer and directed to a specified disk drive connected to said second data storage control for writing said data record accompanying each data record write request on said specified disk drive to which said second data storage control is connected;

means in said first data storage control operable independent of said host computer for defining the provision of shared service upon a subsequent receipt of a data record write request and an accompanying data record directed to a specified phantom drive of a group of disk drives connected to said first data storage control;

means in said first data storage control, responsive to a subsequent receipt of a data record write request and an accompanying data record not directed to said phantom drive of said first data storage control for writing the data record accompanying each data record write request on a disk drive of a group to which said first data storage control is connected;

means in said first data storage control, responsive to a subsequent receipt of a data record write request and an accompanying data record directed to said specified phantom disk drive for transmitting said data record over a data link transmission path to said second data storage control; and

means in said second data storage control, responsive to the receipt of said data record over said data link transmission path, for writing said data record on a disk drive to which said second data storage control is connected.

38. A method of operating a data storage system adapted to be connected to a host computer, said data storage system comprising a first data storage control, a second data storage control, a data link transmission path interconnecting said first and second data storage controls, a first plurality of recording elements connected to said first data storage control and a second plurality of different recording elements connected to said second data storage control, said method comprising the steps of:

operating each of said data storage controls to write a data record on a specified one of the plurality of recording elements to which each said data storage control is connected;

defining, independent of said host computer, in said first data storage control a first recording element of said first plurality of recording elements as well as a first recording element of said second plurality of recording elements as a copy pair of recording elements;

operating said first data storage control in response to the reception from said host computer of a data record write request and accompanying data record to write said data record on said first recording element of said first plurality of recording elements;

transmitting said data record write request and said accompanying data record from said first data storage control over said data link transmission path to said second data storage control; and

operating said second data storage control in response to the receipt of said data record write request and said accompanying data record over said data link transmission path to write said data record on said first recording element of said second plurality of recording elements.

39. The method of claim 38 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching.

40. The method of claim 38 wherein each of said data storage controls contains a cache memory, said method further comprises the steps of:

operating each data storage control in response to the receipt of said data record by said each data storage control to enter said received data record into the cache memory of said each data storage control;

returning a command complete signal to said host computer from said first data storage control when said data record is entered into said cache memory of each data storage control; and

applying said data record in each cache memory to the recording elements connected to the data storage control containing said each cache memory to control the writing of said data record by said first recording element of said plurality of recording elements of each data storage control.

41. The method of claim 38 wherein said data storage system comprises a first path extending from said host computer to said first data storage control for the reception of information including data record write requests and accompanying data records from said host computer, a second path extending from said host computer to said second data storage control for the transmission of information including data record write requests and accompanying data records from said host computer to said second data storage control, said method further comprising the step of:

operating said second data storage control in response to the receipt of said data record write requests and accompanying data records over said second path to write said data records received over said second path on said first recording element of said second plurality of recording elements.

42. The method of claim 41 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching.

43. The method of claim 41 further comprising the steps of:

transmitting from second data storage control, in response to said receipt of said data record write requests and accompanying data records received over said second path, said data record write requests and accompanying data records over said data link transmission path to said first data storage control; and

operating said first data storage control in response to said receipt of said data record write requests and accompanying data records over said data link transmission path to write said data records received by said first data storage control over said data link transmission path on said first recording element of said first recording element.

44. The method of claim 43 wherein said data link transmission path comprises a direct connection between said first and second data storage controls that is devoid of switching.

45. The method of claim 44 wherein each of said data storage controls contains a cache memory, said method further comprises the steps of:

operating each data storage control in response to the receipt of a data record write request and accompanying data record over said path connected to said data storage control to enter said received data record into the cache memory of said data storage control that received said data record write request and data record over said path;

returning a command complete to said host computer from the data storage control that received said data record write request and data record from said host computer when said data record is entered into said cache memory of each data storage control; and

applying said data record in each cache memory to the recording elements connected to the data storage control containing said each cache memory to control the writing of said data record by said first recording element of said recording elements.

46. A method of operating a data storage system adapted to be connected to a host computer, said data storage system comprising a first data storage control, a second data storage control, a data link transmission path interconnecting said data storage controls, a first group of disk drives connected to said first data storage control and a second group of disk drives connected to said second data storage control, said method comprising the steps of:

operating each of said data storage controls to write a data record on a specified one of the disk drives to which said each data storage control is connected;

receiving in said first data storage control information from said host computer;

operating said first data storage control in response to receipt of a data record write request and an accompanying data record to write said received data record on said first group of disk drives;

defining, independent of said host computer, a specified one of said disk drives of said first group of disk drives and a specified one of said second group of disk drives as a copy pair of disk drives;

transmitting, from said first data storage control, in response to receipt of said data record write request and an accompanying data record, said received data record accompanying said data record write request over said data link transmission path to said second data storage control;

operating said first data storage control in response to said receipt of said data record write request, to write said accompanying data record on said one disk drive of said first group; and

operating said second data storage control in response to the reception of said data record over said data link transmission path, to write said record data on said one disk drive of said second group.

47. The method of claim 46 wherein said data link transmission path comprises a direct connection between said data storage controls that is devoid of switching.

48. The method of claim 46 wherein each of said data storage controls contains a cache memory, further comprising the steps of:

operating each data storage control in response to the receipt of a data record by said each data storage control to enter said each received data record into the cache memory of said each data storage control;

returning a command complete signal to said host computer from said first data storage control when a data record received by each data storage control is entered into said cache memory of said each data storage control; and

applying said data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive.

49. The method of claim 46 wherein said data storage system further comprises a first path extending from said host computer to said first data storage control for the reception of information including data records from said host computer, a second path extending from said host computer to said second data storage control for the transmission of information including data records to said second data storage control, said method further comprising the steps of:

receiving in said second data storage control a data record write request over said second path from said host computer;

operating said second data storage control in response to the receipt of each received data record write request and an accompanying data record to write said accompanying data record on said second group of disk drives;

operating said second data storage control to transmit said received data record over said data link transmission path to said first data storage control,

operating said second data storage control in response to a receipt of said data record write request and accompanying data record to write said received data record on said specified one disk drive of said second group, and

operating said first data storage control in response to the receipt of said data record over said data link transmission path to write said data record on a disk drive of said first group of disk drives.

50. The method of claim 46 wherein each of said data storage controls contains a cache memory, further comprising the steps of:

operating each data storage control in response to the receipt of a data record by said each data storage control to enter said received data record into the cache memory of said each data storage control;

returning a command complete signal to said host computer from said data storage control that receives a data record from said host computer when said data record is entered into said cache memory of each data storage control; and

applying a data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive.

51. A method of operating a data storage system adapted to be connected over channel interface paths to a first and a second host computer for the writing of multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers, said data storage system comprising a first and a second data storage control connected via separate ones of said channel interface paths to each of said host computers, a first group of disk drives connected to said first data storage control and a second group of disk drives connected to said second data storage control, a direct data link transmission path devoid of switching apparatus interconnecting said data storage controls, said method comprising the steps of:

receiving in each of said data storage controls a data record from either of said host computers over said channel interface;

operating each data storage control independent of the other data storage control and in response to the reception of a data record over one of said direct data link transmission paths to write said data record on a disk drive of the group to which said each data storage control receiving said data record is connected;

operating said each data storage control in response to the receipt of said data record over one of said channel interface paths to transmit said data record over said data link transmission path to the other one of said data storage controls independent of said host computer;

operating said other data storage control independent of the other data storage control and in response to the reception of said data record over said data link transmission path to write said data record on a disk drive of the group to which said other data storage control is connected.

52. The method of claim 51 wherein each of said data storage controls contains a cache memory, said method further comprises the steps of:

operating each data storage control in response to the receipt of said data record over a channel interface path to enter said received data record into the cache memory of each of said data storage controls;

returning a command complete signal to said host computer from the data storage control that receives a data record over a channel interface path when said received data record is entered into said cache memory of each of said data storage controls; and

applying said data record in each cache memory to a disk drive connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive of each data storage control.

53. A method of operating a data storage system adapted to be connected over channel interface paths to a first and a second host computer for the writing of multiple copies of data records received by said data storage system over any of said channel interface paths from either of said host computers, said method comprising the steps of:

operating a first and a second data storage control for receiving a data record from either of said host computers over one of said channel interface paths;

operating each data storage control in response to the receipt of a data record over one of said channel interface paths for writing said data record on a disk drive of a group of drives to which only said each data storage control receiving said data record is connected;

operating said each data storage control in response to the receipt of said data record over one of said channel interface paths for transmitting said record over a data link transmission path to the other one of said data storage controls; and

operating said other data storage control in response to the receipt of said data record over said data link transmission path for writing said data record on a disk drive of a group to which only said other one data storage control is connected.

54. The method of claim 53 wherein said method further comprises the steps of:

operating each data storage control in response to the receipt of said data record over a channel interface path for entering said received data record into a cache memory of each of said each data storage control;

returning a command complete signal to said host computer from the data storage control that receives a data record over a channel interface path when said received data record is entered into said cache memory of each of said data storage controls; and

wherein said step of writing comprises the step of applying said data record in each cache memory to a disk drive of a group connected to the data storage control containing said each cache memory to control the writing of said data record by said disk drive of a group connected to said each data storage control.