Methods and apparatus for forwarding access requests in a content addressable computer system

• US 7,428,611 B1
• Filed: 04/30/2004
• Issued: 09/23/2008
• Est. Priority Date: 04/30/2004
• Status: Active Grant

• Current Assignee: Emc IP Holding Company LLC (Dell Technologies Inc.)
• Original Assignee: EMC Corporation (Dell Technologies Inc.)
• Inventors: Kilian, Michael, Todd, Stephen J.
• Primary Examiner(s): Bataille; Pierre-Michel
• Application Number:
  US10/837,311
• Time in Patent Office: 1,607 Days
• Field of Search: 711/122, 711/108, 709/213, 709/238
• US Class Current: 711/108
• CPC Class Codes: G06F 11/2074 Asynchronous techniques
• CPC Class Codes: G06F 11/2076 Synchronous techniques
• CPC Class Codes: G06F 11/2087 with a common controller
• CPC Class Codes: H04L 67/1097 for distributed storage of data in networks, e.g. transport arrangements for network file system [NFS], storage area networks [SAN] or network attached storage [NAS]

Abstract

Embodiments of the present invention are directed to a method and apparatus for receiving an input/output (I/O) request from a host computer that specifies an operation to be performed on a content addressable storage (CAS) system and determining which operation is specified by the request before receipt of the I/O request by a content addressable storage system. In another embodiment, an I/O request from a host computer is received by a first CAS which determines if the request is to be processed by another CAS system. When it is determined that the request is to be processed by another CAS system, the first CAS system may forward the request to the other CAS system. In another embodiment, an appliance receives an I/O request from a host computer to perform an operation that accesses a unit of content. The appliance may set up a communication session between the host and a CAS system so that the unit of content may be transmitted between the host and the CAS system without passing through the appliance.

Claims

1. A method of processing an input/output (I/O) request in a computer system comprising a first content addressable storage (CAS) system and a second CAS system that each processes content units using content addresses, wherein the content address for each one of the content units is generated based, at least in part, on content of the one of the content units that is to be provided when the one of the content units is read from one of the CAS systems, the method comprising acts of:

(a) receiving the I/O request at the first CAS system, the I/O request seeking access to at least one content unit;

(b) determining whether the I/O request is to be processed by the first CAS system or the second CAS system; and

(c) when it is determined in the act (b) that the I/O request is to be processed by the second CAS system, forwarding the I/O request to the second CAS system.

2. The method of claim 1, wherein the act (b) further comprises an act of determining whether the I/O request is to be processed by the first CAS system or the second CAS system based on which of the first and second CAS systems store the at least one content unit.

3. The method of claim 1, wherein the computer system includes a CAS layer of software disposed on at least one of the first CAS system and the second CAS system, and wherein the CAS layer performs the acts of receiving the I/O request, determining whether the I/O request is to be processed by the first CAS system or the second CAS system, and forwarding the I/O request to the second CAS system when it is determined that the I/O request is to be processed by the second CAS system.

4. The method of claim 2, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a read operation to read the at least one content unit.

5. The method of claim 2, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a write operation to store the at least one content unit.

6. The method of claim 2, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a query operation to identify a set of content units that meet at least one specified criterion.

7. The method of claim 6, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

8. The method of claim 1, wherein the I/O request seeks to establish a connection between the at least one host and the at least one CAS system, and wherein the method further comprises acts of:

determining a number of previously established connections to the at least one CAS system; and

determining whether the connection sought in the I/O request would cause the number of connections to meet or exceed threshold number.

9. The method of claim 8, further comprising an act of:

when it is determined that the connection sought in the I/O request would cause the number of connections to meet or exceed the threshold number, denying the I/O request.

10. The method of claim 1, further comprising an act of:

determining whether processing of the I/O request by the first CAS system would cause a processing load of the first CAS system to meet or exceed a particular threshold.

11. The method of claim 5, wherein the first CAS system includes an execution engine for processing I/O requests, and the method further comprises acts of:

forwarding the I/O request from the CAS layer to the execution engine;

receiving, at the CAS layer, the at least one content unit from the execution engine, in response to the execution engine receiving the I/O request; and

caching, at the CAS layer, a copy of the at least one content unit.

12. The method of claim 11, further comprising an act of:

sending the at least one content unit from the CAS layer to the at least one host computer.

13. The method of claim 12, further comprising acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

responding to the second I/O request by reading from the cached copy of the at least one content unit at the CAS layer.

14. The method of claim 2, further comprising an act of:

caching, at the CAS layer, a copy of the at least one content unit.

15. The method of claim 14, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein the method further comprises an act of:

forwarding the I/O request to the execution engine to the store the unit of content.

16. The method of claim 15, further comprising acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

in response to the second I/O request, returning the cached copy of the at least one content unit from the CAS layer to the at least one host computer.

17. The method of claim 5, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein method further comprises an act of:

in response to the act of determining that the at least one field specifies a write request, issuing multiple write requests, from the at least one CAS layer, to the at least one CAS system, to store the at least one content unit in at least two locations, so there is at least one mirrored copy of the at least one content unit.

18. The method of claim 2, further comprising an act of:

performing, at the CAS layer, a virus scan on the at least one content unit.

19. The method of claim 5, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein the method further comprises an act of:

sending the I/O request from the CAS layer to the at least one CAS system;

receiving, at the CAS layer, the at least one content unit from the at least one CAS system;

performing, at the CAS layer, virus scanning on the at least one content unit; and

when it is determined that the at least one content unit is not infected with a virus, forwarding the at least one content unit to the at least one host.

20. The method of claim 2, further comprising an act of:

storing, at the CAS layer, information about the I/O request.

21. The method of claim 20, wherein the act of storing information about the I/O requests comprises storing information that identifies the at least one host that issued the I/O request.

22. The method of claim 20, wherein the act of storing information about the I/O requests comprises storing information that identifies at least one application program, executing on the at least one host, that issued the I/O request.

23. The method of claim 20, wherein the act of storing information about the I/O requests comprises storing information that specifies a quantity of data associated with the I/O request.

24. The method of claim 20, wherein the act of storing information about the I/O requests comprises storing, after the I/O request is processed, information that specifies a time taken by the computer system to process the I/O request.

25. The method of claim 20, wherein the act of storing information about the I/O requests comprises storing, after the I/O request is processed, information that specifies a quantity of network bandwidth consumed by the I/O request.

26. The method of claim 20, further comprising an act of:

using the information stored in the CAS layer about the I/O request as a basis for charging a customer for use of the first and second CAS systems.

27. The method of claim 5, wherein the I/O request is a first I/O request, wherein the at least one content unit is a first content unit, and wherein the method further comprises acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to store a second content unit on one of the first and second CAS systems;

forming, at the CAS layer, a third content unit that includes at least the first and second content units; and

generating, at the CAS layer, a third I/O request to store the third content unit on one of the first and second CAS systems.

28. The method of claim 27, further comprising an act of:

sending the third I/O request from the CAS layer to one of the first and second CAS systems, without sending the first and second I/O requests to either of the first and second CAS systems.

29. The method of claim 28, further comprising acts of:

receiving, at the CAS layer, a fourth I/O request from the at least one host computer to retrieve the first content unit;

generating, at the CAS layer, a fifth I/O request to retrieve the third content unit from the one of the first and second CAS systems on which the third content unit is stored;

receiving, at the CAS layer, the third content unit from the one of the first and second CAS systems;

extracting, at the CAS layer, the first content unit from the third content unit; and

sending the first content unit from the CAS layer to the at least one host computer.

30. The method of claim 2, further comprising an act of:

monitoring, at the CAS layer, information that is related to at least one quality of service parameter relating to operation of at least one of the first and second CAS systems.

31. The method of claim 30, wherein the at least one quality of service parameter relates to a processing load of at least one of the first and second CAS systems.

32. The method of claim 31, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

33. The method of claim 2, further comprising acts of:

determining which of the first and second CAS systems is allocated to store content units stored from the at least one host; and

forwarding the I/O request to the one of the first and second CAS systems allocated to store content units stored by the at least one host that issued the I/O request.

34. The method of claim 2, further comprising acts of:

maintaining, at the CAS layer, an index that maps at least one content address to at least one location of a corresponding content unit on at least one of the first and second CAS systems.

35. The method of claim 2, further comprising acts of:

maintaining, at the CAS layer, an index that maps metadata associated with content unit to a content address for the content unit.

36. The method of claim 4, wherein the computer system is a first computer, wherein the first computer system is operably coupled to at least one second computer system, wherein the CAS layer of software directly manages a plurality of content units, and wherein the method further comprises acts of:

determining if the at least one content unit is directly managed by the CAS layer; and

when the at least one content unit is not directly managed by the CAS layer, forwarding the request to the at least one second computer system.

37. At least one computer readable medium encoded with instructions that, when executed on a computer system, perform a method of processing an input/output (I/O) request, wherein the computer system comprises a first content addressable storage (CAS) system and a second CAS system that each processes content units using content addresses, wherein the content address for each one of the content units is generated based, at least in part, on content of the one of the content units that is to be provided when the one of the content units is read from one of the CAS systems, and wherein the method comprises acts of:

(a) in response to receipt of the I/O request at the first CAS system, the I/O request seeking access to at least one content unit, determining whether the I/O request is to be processed by the first CAS system or the second CAS system; and

(b) when it is determined in the act (a) that the I/O request is to be processed by the second CAS system, forwarding the I/O request to the second CAS system.

38. The at least one computer readable medium of claim 37, wherein the act (a) further comprises an act of determining whether the I/O request is to be processed by the first CAS system or the second CAS system based on which of the first and second CAS systems store the at least one content unit.

39. The at least one computer readable medium of claim 37, wherein the computer system includes a CAS layer of software disposed on at least one of the first CAS system and the second CAS system, and wherein the CAS layer performs the acts of determining whether the I/O request is to be processed by the first CAS system or the second CAS system, and forwarding the I/O request to the second CAS system when it is determined that the I/O request is to be processed by the second CAS system.

40. The at least one computer readable medium of claim 38, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a read operation to read the at least one content unit.

41. The at least one computer readable medium of claim 38, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a write operation to store the at least one content unit.

42. The at least one computer readable medium of claim 38, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the I/O request specifies a query operation to identify a set of content units that meet at least one specified criterion.

43. The at least one computer readable medium of claim 42, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

44. The at least one computer readable medium of claim 37, wherein the I/O request seeks to establish a connection between the at least one host and the at least one CAS system, and wherein the method further comprises acts of:

determining a number of previously established connections to the at least one CAS system; and

determining whether the connection sought in the I/O request would cause the number of connections to meet or exceed threshold number.

45. The at least one computer readable medium of claim 44, wherein the method further comprises an act of:

when it is determined that the connection sought in the I/O request would cause the number of connections to meet or exceed the threshold number, denying the I/O request.

46. The at least one computer readable medium of claim 37, wherein the method further comprises an act of:

determining whether processing of the I/O request by the first CAS system would cause a processing load of the first CAS system to meet or exceed a particular threshold.

47. The at least one computer readable medium of claim 41, wherein the first CAS system includes an execution engine for processing I/O requests, and the method further comprises acts of:

forwarding the I/O request from the CAS layer to the execution engine;

receiving, at the CAS layer, the at least one content unit from the execution engine, in response to the execution engine receiving the I/O request; and

caching, at the CAS layer, a copy of the at least one content unit.

48. The at least one computer readable medium of claim 47, wherein the method further comprises an act of:

sending the at least one content unit from the CAS layer to the at least one host computer.

49. The at least one computer readable medium of claim 48, wherein the method further comprises acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

responding to the second I/O request by reading from the cached copy of the at least one content unit at the CAS layer.

50. The at least one computer readable medium of claim 38, wherein the method further comprises an act of:

caching, at the CAS layer, a copy of the at least one content unit.

51. The at least one computer readable medium of claim 50, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein the method further comprises an act of:

forwarding the I/O request to the execution engine to the store the unit of content.

52. The at least one computer readable medium of claim 51, wherein the method further comprises acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

in response to the second I/O request, returning the cached copy of the at least one content unit from the CAS layer to the at least one host computer.

53. The at least one computer readable medium of claim 41, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein method further comprises an act of:

in response to the act of determining that the at least one field specifies a write request, issuing multiple write requests, from the at least one CAS layer, to the at least one CAS system, to store the at least one content unit in at least two locations, so there is at least one mirrored copy of the at least one content unit.

54. The at least one computer readable medium of claim 38, wherein the method further comprises an act of:

performing, at the CAS layer, a virus scan on the at least one content unit.

55. The at least one computer readable medium of claim 41, wherein the first CAS system includes an execution engine for processing I/O requests, and wherein the method further comprises an act of:

sending the I/O request from the CAS layer to the at least one CAS system;

receiving, at the CAS layer, the at least one content unit from the at least one CAS system;

performing, at the CAS layer, virus scanning on the at least one content unit; and

when it is determined that the at least one content unit is not infected with a virus, forwarding the at least one content unit to the at least one host.

56. The at least one computer readable medium of claim 38, wherein the method further comprises an act of:

storing, at the CAS layer, information about the I/O request.

57. The at least one computer readable medium of claim 56, wherein the act of storing information about the I/O requests comprises storing information that identifies the at least one host that issued the I/O request.

58. The at least one computer readable medium of claim 56, wherein the act of storing information about the I/O requests comprises storing information that identifies at least one application program, executing on the at least one host, that issued the I/O request.

59. The at least one computer readable medium of claim 56, wherein the act of storing information about the I/O requests comprises storing information that specifies a quantity of data associated with the I/O request.

60. The at least one computer readable medium of claim 56, wherein the act of storing information about the I/O requests comprises storing, after the I/O request is processed, information that specifies a time taken by the computer system to process the I/O request.

61. The at least one computer readable medium of claim 56, wherein the act of storing information about the I/O requests comprises storing, after the I/O request is processed, information that specifies a quantity of network bandwidth consumed by the I/O request.

62. The at least one computer readable medium of claim 56, wherein the method further comprises an act of:

using the information stored in the CAS layer about the I/O request as a basis for charging a customer for use of the first and second CAS systems.

63. The at least one computer readable medium of claim 41, wherein the I/O request is a first I/O request, wherein the at least one content unit is a first content unit, and wherein the method further comprises acts of:

receiving, at the CAS layer, a second I/O request from the at least one host computer to store a second content unit on one of the first and second CAS systems;

forming, at the CAS layer, a third content unit that includes at least the first and second content units; and

generating, at the CAS layer, a third I/O request to store the third content unit on one of the first and second CAS systems.

64. The at least one computer readable medium of claim 63, wherein the method further comprises an act of:

sending the third I/O request from the CAS layer to one of the first and second CAS systems, without sending the first and second I/O requests to either of the first and second CAS systems.

65. The at least one computer readable medium of claim 64, wherein the method further comprises acts of:

receiving, at the CAS layer, a fourth I/O request from the at least one host computer to retrieve the first content unit;

generating, at the CAS layer, a fifth I/O request to retrieve the third content unit from the one of the first and second CAS systems on which the third content unit is stored;

receiving, at the CAS layer, the third content unit from the one of the first and second CAS systems;

extracting, at the CAS layer, the first content unit from the third content unit; and

sending the first content unit from the CAS layer to the at least one host computer.

66. The at least one computer readable medium of claim 38, wherein the method further comprises an act of:

monitoring, at the CAS layer, information that is related to at least one quality of service parameter relating to operation of at least one of the first and second CAS systems.

67. The at least one computer readable medium of claim 66, wherein the at least one quality of service parameter relates to a processing load of at least one of the first and second CAS systems.

68. The at least one computer readable medium of claim 67, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

69. The at least one computer readable medium of claim 38, wherein the method further comprises acts of:

determining which of the first and second CAS systems is allocated to store content units stored from the at least one host; and

forwarding the I/O request to the one of the first and second CAS systems allocated to store content units stored by the at least one host that issued the I/O request.

70. The at least one computer readable medium of claim 38, wherein the method further comprises acts of:

maintaining, at the CAS layer, an index that maps at least one content address to at least one location of a corresponding content unit on at least one of the first and second CAS systems.

71. The at least one computer readable medium of claim 38, wherein the method further comprises an act of:

maintaining, at the CAS layer, an index that maps metadata associated with content unit to a content address for the content unit.

72. The at least one computer readable medium of claim 40, wherein the computer system is a first computer, wherein the first computer system is operably coupled to at least one second computer system, wherein the CAS layer of software directly manages a plurality of content units, and wherein the method further comprises acts of:

determining if the at least one content unit is directly managed by the CAS layer; and

when the at least one content unit is not directly managed by the CAS layer, forwarding the request to the at least one second computer system.

73. A first content addressable storage (CAS) system for use in a computer system that comprises the first content addressable storage system and a second CAS system, each of which processes content units using content addresses, wherein the content address for each one of the content units is generated based, at least in part, on content of the one of the content units that is to be provided when the one of the content units is read from one of the CAS systems, and wherein the first content addressable storage system comprises:

at least one storage device; and

;

a controller, coupled to the at least one storage device, that, in response to receipt of an I/O request seeking access to at least one content unit;

determines whether the I/O request is to be processed by the first CAS system or the second CAS system; and

when it determines that the I/O request is to be processed by the second CAS system, forwards the I/O request to the second CAS system.

74. The first CAS system of claim 73, wherein the controller determines whether the I/O request is to be processed by the first CAS system or the second CAS system based on which of the first and second CAS systems store the at least one content unit.

75. The first CAS system of claim 73, wherein the controller includes a CAS layer of software that receives the I/O request, determines whether the I/O request is to be processed by the first CAS system or the second CAS system, and forwards the I/O request to the second CAS system when it is determined that the I/O request is to be processed by the second CAS system.

76. The first CAS system of claim 74, wherein the controller:

determines, at the CAS layer, that the I/O request specifies a read operation to read the at least one content unit.

77. The first CAS system of claim 74, wherein the controller:

determines, at the CAS layer, that the I/O request specifies a write operation to store the at least one content unit.

78. The first CAS system of claim 74, wherein controller:

determines, at the CAS layer, that the I/O request specifies a query operation to identify a set of content units that meet at least one specified criterion.

79. The first CAS system of claim 78, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

80. The first CAS system of claim 73, wherein the I/O request seeks to establish a connection between the at least one host and the at least one CAS system, and wherein the controller:

determines a number of previously established connections to the at least one CAS system; and

determines whether the connection sought in the I/O request would cause the number of connections to meet or exceed threshold number.

81. The first CAS system of claim 80, wherein the controller:

denies the I/O request when it is determined that the connection sought in the I/O request would cause the number of connections to meet or exceed the threshold number.

82. The first CAS system of claim 73, wherein the controller:

determines whether processing of the I/O request by the first CAS system would cause a processing load of the first CAS system to meet or exceed a particular threshold.

83. The first CAS system of claim 77, wherein the controller further comprises an execution engine for processing I/O requests, and wherein the controller:

forwards the I/O request from the CAS layer to the execution engine;

receives, at the CAS layer, the at least one content unit from the execution engine, in response to the execution engine receiving the I/O request; and

caches, at the CAS layer, a copy of the at least one content unit.

84. The first CAS system of claim 73, wherein the controller:

sends the at least one content unit from the CAS layer to the at least one host computer.

85. The first CAS system of claim 84, wherein the controller:

receives, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

responds to the second I/O request by reading from the cached copy of the at least one content unit at the CAS layer.

86. The first CAS system of claim 74, wherein the controller:

caches, at the CAS layer, a copy of the at least one content unit.

87. The first CAS system of claim 86, wherein the controller further comprises an execution engine for processing I/O requests, and wherein the controller forwards the I/O request to the execution engine to the store the unit of content.

88. The first CAS system of claim 77, wherein the controller:

receives, at the CAS layer, a second I/O request from the at least one host computer to retrieve the at least one content unit; and

in response to the second I/O request, returns the cached copy of the at least one content unit from the CAS layer to the at least one host computer.

89. The first CAS system of claim 77, wherein the controller further comprises an execution engine for processing I/O requests, and wherein the controller, in response to the act of determining that the at least one field specifies a write request, issues multiple write requests, from the at least one CAS layer, to the at least one CAS system, to store the at least one content unit in at least two locations, so there is at least one mirrored copy of the at least one content unit.

90. The first CAS system of claim 74, wherein the controller:

performs, at the CAS layer, a virus scan on the at least one content unit.

91. The first CAS system of claim 77, wherein the controller further comprises an execution engine for processing I/O requests, and wherein the controller:

sends the I/O request from the CAS layer to the at least one CAS system;

receives, at the CAS layer, the at least one content unit from the at least one CAS system;

performs, at the CAS layer, virus scanning on the at least one content unit; and

when it determines that the at least one content unit is not infected with a virus, forwards the at least one content unit to the at least one host.

92. The first CAS system of claim 74, wherein the controller:

stores, at the CAS layer, information about the I/O request.

93. The first CAS system of claim 92, wherein the controller stores information that identifies the at least one host that issued the I/O request.

94. The first CAS system of claim 92, wherein the controller stores information that identifies at least one application program, executing on the at least one host, that issued the I/O request.

95. The first CAS system of claim 92, wherein the controller stores information that specifies a quantity of data associated with the I/O request.

96. The first CAS system of claim 92, wherein the controller stores, after the I/O request is processed, information that specifies a time taken by the computer system to process the I/O request.

97. The first CAS system of claim 92, wherein the controller stores, after the I/O request is processed, information that specifies a quantity of network bandwidth consumed by the I/O request.

98. The first CAS system of claim 92, wherein the controller:

uses the information stored in the CAS layer about the I/O request as a basis for charging a customer for use of the first and second CAS systems.

99. The first CAS system claim 77, wherein the I/O request is a first I/O request, wherein the at least one content unit is a first content unit, and wherein the controller:

receives, at the CAS layer, a second I/O request from the at least one host computer to store a second content unit on one of the first and second CAS systems;

forms, at the CAS layer, a third content unit that includes at least the first and second content units; and

generates, at the CAS layer, a third I/O request to store the third content unit on one of the first and second CAS systems.

100. The first CAS system of claim 99, wherein the controller:

sends the third I/O request from the CAS layer to the second CAS system, without sending the first and second I/O requests to the second CAS system.

101. The first CAS system of claim 100, wherein the controller:

receives, at the CAS layer, a fourth I/O request from the at least one host computer to retrieve the first content unit;

generates, at the CAS layer, a fifth I/O request to retrieve the third content unit from the one of the first and second CAS systems on which the third content unit is stored;

receives, at the CAS layer, the third content unit from the second CAS system;

extracts, at the CAS layer, the first content unit from the third content unit; and

sends the first content unit from the CAS layer to the at least one host computer.

102. The first CAS system of claim 101, wherein the controller:

monitors, at the CAS layer, information that is related to at least one quality of service parameter relating to operation of at least one of the first and second CAS systems.

103. The first CAS system of claim 102, wherein the at least one quality of service parameter relates to a processing load of at least one of the first and second CAS systems.

104. The first CAS system of claim 103, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

105. The first CAS system of claim 74, wherein the controller:

determines which of the first and second CAS systems is allocated to store content units stored from the at least one host; and

forwards the I/O request to the one of the first and second CAS systems allocated to store content units stored by the at least one host that issued the I/O request.

106. The first CAS system of claim 74, wherein the controller:

maintains, at the CAS layer, an index that maps at least one content address to at least one location of a corresponding content unit on at least one of the first and second CAS systems.

107. The first CAS system of claim 74, wherein the controller:

maintains, at the CAS layer, an index that maps metadata associated with content unit to a content address for the content unit.

108. The first CAS system of claim 76, wherein the computer system is a first computer, wherein the first computer system is operably coupled to at least one second computer system, wherein the CAS layer directly manages a plurality of content units, and wherein the controller:

determines if the at least one content unit is directly managed by the CAS layer; and

when the at least one content unit is not directly managed by the CAS layer, forwards the request to the at least one second computer system.