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

• US 7,240,150 B1
• Filed: 04/30/2004
• Issued: 07/03/2007
• 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
• Assistant Examiner(s): SCHLIE, PAUL W
• Application Number:
  US10/836,415
• Time in Patent Office: 1,159 Days
• Field of Search: 711/122, 711/108, 709213-238
• US Class Current: 711/108
• CPC Class Codes: G06F 16/10 File systems; File servers

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 data in a computer system comprising at least one host and at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, wherein the at least one host identifies the content units on the at least one CAS system using content addresses generated based, at least in part, on the content of the content units, wherein the computer system includes a CAS layer of software, disposed outside of the CAS system, that is adapted to communicate with the at least one CAS system, and wherein the method comprises acts, performed by the CAS layer of software, of:

(A) receiving an input/output (I/O) request from the at least one host to perform an access on at least one of the content units on the at least one CAS system, wherein the I/O request includes at least one field that specifies at least one of the plurality of operations to be performed on the at least one content unit by the at least one CAS system;

(B) determining which of the plurality of operations is specified in the at least one field; and

(C) based on the at least one of the plurality of operations determined in the act (B) to be specified in the at least one field, performing at least one action selected from the group consisting of;

re-directing the I/O request;

denying the I/O request;

logging information concerning the I/O request;

when the at least one operation specified in the at least one field includes a write request to store a content unit on the at least one CAS system, mirroring the write request to a different location; and

concatenating the I/O request with at least one other I/O request.

2. The method of claim 1, wherein the CAS layer is disposed on the at least one host.

3. The method of claim 1, wherein the computer system further comprises an appliance that is separate from the at least one host and the at least one CAS system, and wherein the CAS layer is disposed on the at least one appliance.

4. The method of claim 1, wherein the I/O request does not specify an address to identify a storage location for the at least one content unit on the at least one CAS system.

5. The method of claim 1, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to retrieve the at least one content unit, and wherein the method further comprises:

determining, at the CAS layer, on which of the first CAS system and the second CAS system the at least one content unit is stored; and

forwarding, from the CAS layer, the I/O request to the one of the first CAS system and the second CAS system on which the at least one content unit is stored.

6. The method of claim 5, further comprising:

receiving, at the CAS layer, the at least one unit of content from the one of the first CAS system and the second CAS system; and

in response to receiving the at least one unit of content, returning, from the CAS layer, the at least one unit of content to the at least one host computer.

7. The method of claim 1, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to store the at least one content unit, and wherein the method further comprises:

selecting, at the CAS layer, one of the first CAS system and the second CAS system to store the unit of content; and

forwarding, from the CAS layer, the I/O request to the selected one of the first CAS system and the second CAS system.

8. The method of claim 1, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a read operation to read the at least one content unit stored on the at least one CAS system.

9. The method of claim 1, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a write operation to store the at least one content unit on the at least one CAS system.

10. The method of claim 1, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a query operation to identify a set of content units stored on the at least one CAS system that meet at least one specified criterion.

11. The method of claim 10, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

12. 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, at the CAS layer, a number of previously established connections to the at least one CAS system; and

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

13. The method of claim 12, 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, adding the I/O request to a queue of I/O requests to be serviced a later time.

14. The method of claim 12, 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.

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

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

16. The method of claim 15, further comprising an act of denying the I/O request when it is determined that the processing of the I/O request would cause the processing load of the at least one CAS system to meet or exceed the threshold.

17. The method of claim 8, further comprising acts of:

forwarding 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, in response to the at least one CAS system receiving the I/O request; and

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

18. The method of claim 17, further comprising an act of:

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

19. The method of claim 17, 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.

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

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

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

forwarding the I/O request to the at least one CAS system to the store the unit of content.

22. The method of claim 21, 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.

23. The method of claim 9, further comprising 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.

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

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

25. The method of claim 8, further comprising acts 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.

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

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

27. The method of claim 26, wherein the act of storing information about the I/O request comprises storing information that identifies the at least one host that issued the I/O request.

28. The method of claim 26, wherein the act of storing information about the I/O request comprises storing information that identifies at least one application program, executing on the at least one host, that issued the I/O request.

29. The method of claim 26, wherein the act of storing information about the I/O request comprises storing information that indicates a quantity of data associated with the I/O request.

30. The method of claim 26, wherein the act of storing information about the I/O request comprises storing, after the I/O request is processed, information that specifies a time taken by the CAS system to process the I/O request.

31. The method of claim 26, wherein the act of storing information about the I/O request comprises storing, after the I/O request is processed, information that specifies a quantity of network bandwidth consumed by the I/O request.

32. The method of claim 26, 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 at least one CAS system.

33. The method of claim 9, 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 the at least one CAS system;

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 the at least one CAS system.

34. The method of claim 33, further comprising an act of:

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

35. The method of claim 34, 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 from the at least one CAS system;

generating, at the CAS layer, a fifth I/O request to retrieve the third content unit from the at least one CAS system;

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

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.

36. The method of claim 1, 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 the at least one CAS system.

37. The method of claim 36, wherein the at least one quality of service parameter relates to a processing load of the at least one CAS system.

38. The method of claim 36, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

39. The method of claim 1, wherein the at least one CAS system comprises a first CAS system and a second CAS system, and wherein the method further comprises acts, performed by the CAS layer of software, 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.

40. The method of claim 1, 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 the at least one CAS system.

41. The method of claim 1, further comprising acts of:

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

42. The method of claim 8, 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.

43. At least one computer readable medium encoded with instructions that, when executed on a computer system, perform a method of processing data, wherein the computer system comprises at least one host and at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, wherein the at least one host identifies the content units on the at least one CAS system using content addresses generated based, at least in part, on the content of the content units, wherein the computer system includes a CAS layer of software, disposed outside of the CAS system, that is adapted to communicate with the at least one CAS system, and wherein the method comprises acts, performed by the CAS layer of software, of:

(A) receiving an input/output (I/O) request from the at least one host to perform an access on at least one of the content units on the at least one CAS system, wherein the I/O request includes at least one field that specifies at least one of the plurality of operations to be performed on the at least one content unit by the at least one CAS system;

(B) determining which of the plurality of operations is specified in the at least one field; and

(C) based on the at least one of the plurality of operations determined in the act (B) to be specified in the at least one field, performing at least one action selected from the group consisting of;

re-directing the I/O request;

denying the I/O request;

logging information concerning the I/O request;

when the at least one operation specified in the at least one field includes a write request to store a content unit on the at least one CAS system, mirroring the write request to a different location; and

concatenating the I/O request with at least one other I/O request.

44. The at least one computer readable medium of claim 43, wherein the CAS layer is disposed on the at least one host.

45. The at least one computer readable medium of claim 43, wherein the computer system further comprises an appliance that is separate from the at least one host and the at least one CAS system, and wherein the CAS layer is disposed on the at least one appliance.

46. The at least one computer readable medium of claim 43, wherein the I/O request does not specify an address to identify a storage location for the at least one content unit on the at least one CAS system.

47. The at least one computer readable medium of claim 43, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to retrieve the at least one content unit, and wherein the method further comprises:

determining, at the CAS layer, on which of the first CAS system and the second CAS system the at least one content unit is stored; and

forwarding, from the CAS layer, the I/O request to the one of the first CAS system and the second CAS system on which the at least one content unit is stored.

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

receiving, at the CAS layer, the at least one unit of content from the one of the first CAS system and the second CAS system; and

in response to receiving the at least one unit of content, returning, from the CAS layer, the at least one unit of content to the at least one host computer.

49. The at least one computer readable medium of claim 43, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to store the at least one content unit, and wherein the method further comprises:

selecting, at the CAS layer, one of the first CAS system and the second CAS system to store the unit of content; and

forwarding, from the CAS layer, the I/O request to the selected one of the first CAS system and the second CAS system.

50. The at least one computer readable medium of claim 43, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a read operation to read the at least one content unit stored on the at least one CAS system.

51. The at least one computer readable medium of claim 43, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a write operation to store the at least one content unit on the at least one CAS system.

52. The at least one computer readable medium of claim 43, wherein the act of determining further comprises an act of:

determining, at the CAS layer, that the at least one field specifies a query operation to identify a set of content units stored on the at least one CAS system that meet at least one specified criterion.

53. The at least one computer readable medium of claim 52, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

54. The at least one computer readable medium of claim 43, 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, at the CAS layer, a number of previously established connections to the at least one CAS system; and

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

55. The at least one computer readable medium of claim 54, where 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.

56. The at least one computer readable medium of claim 43, wherein the method further comprises an act of:

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

57. The at least one computer readable medium of claim 56, wherein the method further comprises an act of denying the I/O request when it is determined that the processing of the I/O request would cause the processing load of the at least one CAS system to meet or exceed the threshold.

58. The at least one computer readable medium of claim 50, wherein the method further comprises acts of:

forwarding 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, in response to the at least one CAS system receiving the I/O request; and

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

59. The at least one computer readable medium of claim 58, 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.

60. The at least one computer readable medium of claim 58, 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.

61. The at least one computer readable medium of claim 43, wherein the method further comprises an act of:

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

62. The at least one computer readable medium of claim 61, wherein the method further comprises an act of:

forwarding the I/O request to the at least one CAS system to the store the unit of content.

63. The at least one computer readable medium of claim 62, 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.

64. The at least one computer readable medium of claim 51, wherein the 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.

65. The at least one computer readable medium of claim 51, wherein the method further comprises an act of:

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

66. The at least one computer readable medium of claim 50, wherein the method further comprises acts 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.

67. The at least one computer readable medium of claim 43, wherein the method further comprises an act of:

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

68. The at least one computer readable medium of claim 67, wherein the act of storing information about the I/O request comprises storing information that identifies the at least one host that issued the I/O request.

69. The at least one computer readable medium of claim 67, wherein the act of storing information about the I/O request comprises storing information that identifies at least one application program, executing on the at least one host, that issued the I/O request.

70. The at least one computer readable medium of claim 67, wherein the act of storing information about the I/O request comprises storing information that specifies a quantity of data associated with the I/O request.

71. The at least one computer readable medium of claim 67, wherein the act of storing information about the I/O request comprises storing, after the I/O request is processed, information that specifies a time taken by the CAS system to process the I/O request.

72. The at least one computer readable medium of claim 67, wherein the act of storing information about the I/O request comprises storing, after the I/O request is processed, information that specifies a quantity of network bandwidth consumed by the I/O request.

73. The at least one computer readable medium of claim 67, 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 at least one CAS system.

74. The at least one computer readable medium of claim 51, 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 the at least one CAS system;

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 the at least one CAS system.

75. The at least one computer readable medium of claim 74, wherein the method further comprises an act of:

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

76. The at least one computer readable medium of claim 75, 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 from the at least one CAS system;

generating, at the CAS layer, a fifth I/O request to retrieve the third content unit from the at least one CAS system;

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

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.

77. The at least one computer readable medium of claim 43, 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 the at least one CAS system.

78. The at least one computer readable medium of claim 77, wherein the at least one quality of service parameter relates to a processing load of the at least one CAS system.

79. The at least one computer readable medium of claim 77, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

80. The at least one computer readable medium of claim 43, wherein the at least one CAS system comprises a first CAS system and a second CAS system, and wherein the method further comprises acts, performed by the CAS layer of software, 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.

81. The at least one computer readable medium of claim 43, 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 the at least one CAS system.

82. The at least one computer readable medium of claim 43, wherein the method further comprises acts of:

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

83. The at least one computer readable medium of claim 50, 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.

84. An apparatus for use in a computer system comprising at least one host and at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, wherein the at least one host executes at least one application program identifies the content units on the at least one CAS system using content addresses generated based, at least in part, on the content of the content units, and wherein the apparatus comprises:

an input for receiving input/output (I/O) requests from the application program before the I/O request is received by the at least one CAS system; and

a controller that includes a CAS layer of software, disposed outside of the CAS system, that is adapted to communicate with the at least one CAS system and that;

receives, from the input, an input/output (I/O) request to perform an access on at least one of the content units on the at least one CAS system, wherein the I/O request includes at least one field that specifies at least one of the plurality of operations to be performed on the at least one content unit by the at least one CAS system;

determines which of the plurality of operations is specified in the at least one field; and

based on the at least one of the plurality of operations determined to be specified in the at least one field, performs at least one action selected from the group consisting of;

re-directs the I/O request;

denies the I/O request;

logs information concerning the I/O request;

when the at least one operation specified in the at least one field includes a write request to store a content unit on the at least one CAS system, mirrors the write request to a different location; and

concatenates the I/O request with at least one other I/O request.

85. The apparatus of claim 84, wherein the apparatus is a driver disposed on the at least one host.

86. The apparatus of claim 84, wherein the apparatus is an appliance that is separate from the at least one host and the at least one CAS system.

87. The apparatus of claim 84, wherein the I/O request does not specify an address to identify a storage location for the at least one content unit on the at least one CAS system.

88. The apparatus of claim 84, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to retrieve the at least one content unit, and wherein the controller:

determines, at the CAS layer, on which of the first CAS system and the second CAS system the at least one content unit is stored; and

forwards, from the CAS layer, the I/O request to the one of the first CAS system and the second CAS system on which the at least one content unit is stored.

89. The apparatus of claim 88, wherein the controller:

receives, at the CAS layer, the at least one unit of content from the one of the first CAS system and the second CAS system; and

in response to receiving the at least one unit of content, returns, from the CAS layer, the at least one unit of content to the at least one host computer.

90. The apparatus of claim 84, wherein the at least one CAS system comprises a first CAS system and a second CAS system, wherein the I/O request is a request to store the at least one content unit, and wherein the controller:

selects, at the CAS layer, one of the first CAS system and the second CAS system to store the unit of content; and

forwards, from the CAS layer, the I/O request to the selected one of the first CAS system and the second CAS system.

91. The apparatus of claim 84, wherein the controller:

determines, at the CAS layer, that the at least one field specifies a read operation to read the at least one content unit stored on the at least one CAS system.

92. The apparatus of claim 84, wherein the controller:

determines, at the CAS layer, that the at least one field specifies a write operation to store the at least one content unit on the at least one CAS system.

93. The apparatus of claim 84, wherein the controller:

determines, at the CAS layer, that the at least one field specifies a query operation to identify a set of content units stored on the at least one CAS system that meet at least one specified criterion.

94. The apparatus of claim 84, wherein the at least one specified criterion includes a time period during which content units were written to the at least one CAS system.

95. The apparatus of claim 84, 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, at the CAS layer, a number of previously established connections to the at least one CAS system; and

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

96. The apparatus of claim 95, where the controller:

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, denies the I/O request.

97. The apparatus of claim 84, wherein the controller:

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

98. The apparatus of claim 97, wherein the controller denies the I/O request when it is determined that the processing of the I/O request would cause the processing load of the at least one CAS system to meet or exceed the threshold.

99. The apparatus of claim 91, wherein the controller:

forwards 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, in response to the at least one CAS system receiving the I/O request; and

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

100. The apparatus of claim 99, wherein the controller:

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

101. The apparatus of claim 99, 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.

102. The apparatus of claim 84, wherein the controller:

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

103. The apparatus of claim 102, wherein the controller:

forwards the I/O request to the at least one CAS system to the store the unit of content.

104. The apparatus of claim 103, 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.

105. The apparatus of claim 92, 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.

106. The apparatus of claim 92, wherein the controller:

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

107. The apparatus of claim 91, 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 is determined 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.

108. The apparatus of claim 84, wherein the controller:

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

109. The apparatus of claim 108, wherein the controller stores information that identifies the at least one host that issued the I/O request.

110. The apparatus of claim 108, 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.

111. The apparatus of claim 108, wherein the controller stores information that specifies a quantity of data associated with the I/O request.

112. The apparatus of claim 108, wherein the controller stores, after the I/O request is processed, information that specifies a time taken by the CAS system to process the I/O request.

113. The apparatus of claim 108, 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.

114. The apparatus of claim 108, 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 at least one CAS system.

115. The apparatus of claim 92, 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 the at least one CAS system;

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 the at least one CAS system.

116. The apparatus of claim 115, wherein the controller:

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

117. The apparatus of claim 116, 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 from the at least one CAS system;

generates, at the CAS layer, a fifth I/O request to retrieve the third content unit from the at least one CAS system;

receives, at the CAS layer, the third content unit from the at least one 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.

118. The apparatus of claim 84, wherein the controller:

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

119. The apparatus of claim 118, wherein the at least one quality of service parameter relates to a processing load of the at least one CAS system.

120. The apparatus of claim 118, wherein the at least one quality of service parameter relates to a number of connections to the at least one CAS system.

121. The apparatus of claim 84, wherein the at least one CAS system comprises a first CAS system and a second CAS system, and 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.

122. The apparatus of claim 84, 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 the at least one CAS system.

123. The apparatus of claim 84, wherein the controller:

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

124. The apparatus of claim 91, 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 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.