Methods and apparatus for transferring data in a content addressable computer system

• US 7,240,151 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
• Application Number:
  US10/836,502
• Time in Patent Office: 1,159 Days
• Field of Search: 711/122, 711/108, 709/213, 709/238
• US Class Current: 711/108
• CPC Class Codes: G06F 11/2071 using a plurality of controllers
• CPC Class Codes: G06F 16/152 using file content signatures, e.g. hash values
• CPC Class Codes: G11C 15/00 Digital stores in which information comprising one or more characteristic parts is written into the store and in which information is read-out by searching for one or more of these characteristic parts, i.e. associative or content-addressed stores

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 input/output (I/O) requests in a computer system comprising at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the CAS system, wherein the at least one host identifies a content unit on the at least one CAS system using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the appliance, of:

(A) receiving a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit and involves the transmission of the first content unit between the at least one host and the at least one CAS system;

(B) examining information relating to the first I/O request;

(C) when it is determined, based on the information relating to the first I/O request, that at least one first condition is met, sending an instruction, to the at least one host that issued the first I/O request and/or the at least one CAS system, that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to not pass through the appliance; and

(D) when it is determined, based on the information relating to the first I/O request, that the at least one first condition is not met, sending an instruction to the at least one host and/or to the at least one CAS system that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to pass through the appliance.

2. The method of claim 1, wherein the act (A) comprises an act of receiving a first I/O that comprises a request to perform a write of the first content unit to the at least one CAS system.

3. The method of claim 1, wherein the act (A) comprises an act of receiving a first I/O that comprises a request to perform a read of the first content unit from the at least one CAS system.

4. The method of claim 1, wherein the act (B) comprises an act of examining information including a size of the first content unit, and wherein the first condition comprises the size of the first content unit meeting or exceeding a specified size.

5. The method of claim 2, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system.

6. The method of claim 2, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least one CAS instructing the at least one CAS to instruct the at least one host to transmit the first content unit to the at least one CAS system.

7. The method of claim 2, wherein the act (D) comprises an act of, when it is determined that at least one first condition is not met, sending an instruction to the at least one host instructing the at least one host to transmit the first content unit to the appliance.

8. The method of claim 3, wherein the act (D) comprises an act of, when it is determined that at least one first condition is not met, sending an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the appliance.

9. The method of claim 3, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host.

10. At least one computer readable medium encoded with instructions that, when executed on a computer system, perform a method of processing input/output (I/O) requests, wherein the computer system comprises at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the CAS system, wherein the at least one host identifies a content unit on the at least one CAS system using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the appliance, of:

(A) receiving a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit and involves the transmission of the first content unit between the at least one host and the at least one CAS system;

(B) examining information relating to the first I/O request;

(C) when it is determined, based on the information relating to the first I/O request, that at least one first condition is met, sending an instruction, to the at least one host that issued the first I/O request and/or the at least one CAS system, that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to not pass through the appliance; and

(D) when it is determined, based on the information relating to the first I/O request, that the at least one first condition is not met, sending an instruction to the at least one host and/or to the at least one CAS system that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to pass through the appliance.

11. The at least one computer readable medium of claim 10, wherein the act (A) comprises an act of receiving a first I/O that comprises a request to perform a write of the first content unit to the at least one CAS system.

12. The at least one computer readable medium of claim 10, wherein the act (A) comprises an act of receiving a first I/O that comprises a request to perform a read of the first content unit from the at least one CAS system.

13. The at least one computer readable medium of claim 10, wherein the act (B) comprises an act of examining information including a size of the first content unit, and wherein the first condition comprises the size of the first content unit meeting or exceeding a specified size.

14. The at least one computer readable medium of claim 11, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system.

15. The at least one computer readable medium of claim 11, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least one CAS instructing the at least one CAS to instruct the at least one host to transmit the first content unit to the at least one CAS system.

16. The at least one computer readable medium of claim 11, wherein the act (D) comprises an act of, when it is determined that at least one first condition is not met, sending an instruction to the at least one host instructing the at least one host to transmit the first content unit to the appliance.

17. The at least one computer readable medium of claim 12, wherein the act (D) comprises an act of, when it is determined that at least one first condition is not met, sending an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the appliance.

18. The at least one computer readable medium of claim 12, wherein the act (C) comprises an act of, when it is determined that at least one first condition is met, sending an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host.

19. A computer system comprising:

at least one host;

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 a content unit on the at least one CAS system using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system; and

an appliance that is separate from the at least one host and the at least one CAS system comprising at least one controller that;

receives a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit and involves the transmission of the first content unit between the at least one host and the at least one CAS system;

examines information relating to the first I/O request;

when it is determined, based on the information relating to the first I/O request, that at least one first condition is met, sends an instruction, to the at least one host that issued the first I/O request and/or the at least one CAS system, that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to not pass through the appliance; and

when it is determined, based on the information relating to the first I/O request, that the at least one first condition is not met, sends an instruction to the at least one host and/or to the at least one CAS system that causes the path for transmission of the first content unit between the at least one host and the at least one CAS system to pass through the appliance.

20. The computer system of claim 19, wherein the at least one controller receives a first I/O that comprises a request to perform a write of the first content unit to the at least one CAS system.

21. The computer system of claim 19, wherein the at least one controller receives a first I/O that comprises a request to perform a read of the first content unit from the at least one CAS system.

22. The computer system of claim 19, wherein the at least one controller examines information including a size of the first content unit, and wherein the first condition comprises the size of the first content unit meeting or exceeding a specified size.

23. The computer system of claim 20, wherein at least one controller, when it is determined that at least one first condition is met, sends an instruction to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system.

24. The computer system of claim 20, wherein the at least one controller, when it is determined that at least one first condition is met, sends an instruction to the at least one CAS instructing the at least one CAS to instruct the at least one host to transmit the first content unit to the at least one CAS system.

25. The computer system of claim 20, wherein the at least one controller, when it is determined that at least one first condition is not met, sends an instruction to the at least one host instructing the at least one host to transmit the first content unit to the appliance.

26. The computer system of claim 21, wherein the at least one controller, when it is determined that at least one first condition is not met, sends an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the appliance.

27. The computer system of claim 21, wherein the at least one controller, when it is determined that at least one first condition is met, sends an instruction to the at least CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host.

28. A method of processing input/output (I/O) requests in a computer system comprising at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the at least one host, of:

(A) transmitting a first I/O to the appliance, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) engaging in a communication session with the at least one CAS system so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

29. The method of claim 28, wherein the act (A) comprises an act of transmitting a first I/O that is a request to perform a read of the first content unit from the at least one CAS system;

and

wherein the act (B) comprises an act of receiving the first content unit from the at least one CAS system without the first content unit passing through the appliance.

30. The method of claim 28, wherein the act (A) comprises an act of transmitting a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of transmitting the first content unit to the at least one CAS system without passing the first content unit to the appliance.

31. The method of claim 30, further comprising an act, after the act (A), of receiving a communication from the at least one CAS system, and wherein the act (B) comprises an act of transmitting the first content unit in response to receipt of the communication from the at least one CAS system.

32. The method of claim 30, further comprising an act, after the act (A), of receiving a communication from the appliance, and wherein the act (B) comprises an act of transmitting the first content unit in response to receipt of the communication from the appliance.

33. At least one computer readable medium encoded with instructions that, when executed on a computer system, perform a method of processing input/output (I/O) requests, wherein the computer system comprises at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the at least one host, of:

(A) transmitting a first I/O to the appliance, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) engaging in a communication session with the at least one CAS system so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

34. The at least one computer readable medium of claim 33, wherein the act (A) comprises an act of transmitting a first I/O that is a request to perform a read of the first content unit from the at least one CAS system;

and

wherein the act (B) comprises an act of receiving the first content unit from the at least one CAS system without the first content unit passing through the appliance.

35. The at least one computer readable medium of claim 33, wherein the act (A) comprises an act of transmitting a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of transmitting the first content unit to the at least one CAS system without passing the first content unit to the appliance.

36. The at least one computer readable medium of claim 35, further comprising an act, after the act (A), of receiving a communication from the at least one CAS system, and wherein the act (B) comprises an act of transmitting the first content unit in response to receipt of the communication from the at least one CAS system.

37. The at least one computer readable medium of claim 35, further comprising an act, after the act (A), of receiving a communication from the appliance, and wherein the act (B) comprises an act of transmitting the first content unit in response to receipt of the communication from the appliance.

38. A host computer for use in a computer system comprising 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 a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the host computer comprises:

a memory; and

at least one controller, coupled to the memory, that;

transmits a first I/O to the appliance, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

engages in a communication session with the at least one CAS system so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

39. The host computer of claim 38, wherein the at least one controller transmits a first I/O that is a request to perform a read of the first content unit from the at least one CAS system and receives the first content unit from the at least one CAS system without the first content unit passing through the appliance.

40. The host computer of claim 38, wherein the at least one controller transmits a first I/O that is a request to perform a write of the first content unit to the at least one CAS system and transmits the first content unit to the at least one CAS system without passing the first content unit to the appliance.

41. The host computer of claim 40, wherein the at least one controller receives a communication from the at least one CAS system and transmits the first content unit in response to receipt of the communication from the at least one CAS system.

42. The host computer of claim 40, wherein the at least one controller receives a communication from the appliance and transmits the first content unit in response to receipt of the communication from the appliance.

43. A method of processing an input/output (I/O) request in a computer system comprising at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, wherein the I/O request is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system, the method comprising acts, performed by the at least one CAS system, of:

(A) receiving from the appliance a communication instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) engaging in a communication session with the at least one host so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

44. The method of claim 43, wherein the I/O is a request to perform a read of the first content unit from the at least one CAS system, and wherein the act (B) comprises an act of transmitting the first content unit from the at least one CAS system to the at least one host without the first content unit passing through the appliance.

45. The method of claim 43, wherein the I/O is a request to perform a write of the first content unit to the at least one CAS system, and wherein the act (B) comprises an act of receiving the first content unit from the at least one host without the first content unit passing the first content unit to the appliance.

46. The method of claim 45, wherein the act (B) comprises an act, prior to the act of receiving the first content unit, of transmitting to the at least one host a communication that causes the at least one host to transmit the first content unit to the at least one CAS system.

47. 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 at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, wherein the I/O request is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system, the method comprising acts, performed by the at least one CAS system, of:

(A) receiving from the appliance a communication instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) engaging in a communication session with the at least one host so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

48. The at least one computer readable medium of claim 47, wherein the I/O is a request to perform a read of the first content unit from the at least one CAS system, and wherein the act (B) comprises an act of transmitting the first content unit from the at least one CAS system to the at least one host without the first content unit passing through the appliance.

49. The at least one computer readable medium of claim 47, wherein the I/O is a request to perform a write of the first content unit to the at least one CAS system, and wherein the act (B) comprises an act of receiving the first content unit from the at least one host without the first content unit passing the first content unit to the appliance.

50. The at least one computer readable medium of claim 49, wherein the act (B) comprises an act, prior to the act of receiving the first content unit, of transmitting to the at least one host a communication that causes the at least one host to transmit the first content unit to the at least one CAS system.

51. A content address storage (CAS) system, for use in a computer system comprising at least one host, an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, wherein the I/O request is a request to perform an operation that accesses a first content unit on the at least one CAS system, wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system, and wherein that CAS system comprises:

at least one storage device; and

at least one controller, coupled to the at least one storage device, that;

receives from the appliance a communication instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system; and

engages in a communication session with the at least one host so that the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

52. The CAS system of claim 51, wherein the I/O is a request to perform a read of the first content unit from the at least one CAS system, and wherein the at least one controller transmits the first content unit from the at least one CAS system to the at least one host without the first content unit passing through the appliance.

53. The CAS system of claim 51, wherein the I/O is a request to perform a write of the first content unit to the at least one CAS system, and wherein the at least one controller receives the first content unit from the at least one host without the first content unit passing the first content unit to the appliance.

54. The CAS system of claim 53, wherein the at least one controller, prior to the act of receiving the first content unit, transmits to the at least one host a communication that causes the at least one host to transmit the first content unit to the at least one CAS system.

80. The host computer of claim 51 in combination with the computer system.

55. A method of processing input/output (I/O) requests in a computer system comprising at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the appliance, of:

(A) receiving a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) establishing a communication session in which the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

56. The method of claim 55, wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

57. The method of claim 55, wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

58. The method of claim 56, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a read of the first content unit from the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host without passing the first content unit to the appliance.

59. The method of claim 56, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to communicate with the at least one host and request that the at least one host transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

60. The method of claim 57, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

61. The method of claim 56, wherein the at least one CAS system comprises a plurality of CAS systems, and wherein the act (B) comprises acts of:

determining at least one of the plurality of CAS systems as a target for the first I/O, and

sending the communication to the target CAS system.

62. The method of claim 55, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a read of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to transmit the first I/O request to the at least one CAS system with passing the first I/O request to the appliance.

63. At least one computer readable medium encoded with instructions that, when executed on a computer system, perform a method of processing input/output (I/O) requests, wherein the computer system comprises at least one host, at least one content addressable storage (CAS) system that performs a plurality of operations on content units for the at least one host, and an appliance that is separate from the at least one host and the at least one CAS system, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts, performed by the appliance, of:

(A) receiving a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

(B) establishing a communication session in which the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

64. The at least one computer readable medium of claim 63, wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

65. The at least one computer readable medium of claim 63, wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

66. The at least one computer readable medium of claim 64, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a read of the first content unit from the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host without passing the first content unit to the appliance.

67. The at least one computer readable medium of claim 64, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one CAS system instructing the at least one CAS system to communicate with the at least one host and request that the at least one host transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

68. The at least one computer readable medium of claim 65, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a write of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

69. The at least one computer readable medium of claim 64, wherein the at least one CAS system comprises a plurality of CAS systems, and wherein the act (B) comprises acts of:

determining at least one of the plurality of CAS systems as a target for the first I/O, and

sending the communication to the target CAS system.

70. The at least one computer readable medium of claim 63, wherein the act (A) comprises an act of receiving a first I/O that is a request to perform a read of the first content unit to the at least one CAS system;

and

wherein the act (B) comprises an act of sending a communication to the at least one host instructing the at least one host to transmit the first I/O request to the at least one CAS system with passing the first I/O request to the appliance.

71. An appliance 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 identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, and wherein the appliance comprises:

an input that receives a first I/O from the at least one host; and

a controller, coupled to the input, that;

receives the first I/O from the input, wherein the first I/O is a request to perform an operation that accesses a first content unit on the at least one CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the at least one CAS system; and

establishes a communication session in which the first content unit is transmitted between the at least one host and the at least one CAS system without passing through the appliance.

72. The appliance of claim 71, wherein the at least one controller sends a communication to the at least one CAS system instructing the at least one CAS system to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

73. The appliance of claim 71, wherein the act (B) at least one controller sends a communication to the at least one host instructing the at least one host to initiate transmission of the first content unit between the at least one host and the at least one CAS system.

74. The appliance of claim 72, wherein the at least one controller receives a first I/O that is a request to perform a read of the first content unit from the at least one CAS system and sends a communication to the at least one CAS system instructing the at least one CAS system to transmit the first content unit to the at least one host without passing the first content unit to the appliance.

75. The appliance of claim 72, wherein the at least one controller receives a first I/O that is a request to perform a write of the first content unit to the at least one CAS system and sends a communication to the at least one CAS system instructing the at least one CAS system to communicate with the at least one host and request that the at least one host transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

76. The appliance of claim 73, wherein the at least one controller receives a first I/O that is a request to perform a write of the first content unit to the at least one CAS system and sends a communication to the at least one host instructing the at least one host to transmit the first content unit to the at least one CAS system without passing the first content unit to the appliance.

77. The appliance of claim 72, wherein the at least one CAS system comprises a plurality of CAS systems, and wherein the at least one controller determines at least one of the plurality of CAS systems as a target for the first I/O and sends the communication to the target CAS system.

78. The appliance of claim 71, wherein the at least one controller receives a first I/O that is a request to perform a read of the first content unit to the at least one CAS system and sends a communication to the at least one host instructing the at least one host to transmit the first I/O request to the at least one CAS system with passing the first I/O request to the appliance.

81. The appliance of claim 71 in combination with the computer system.

79. A method of processing input/output (I/O) requests in a computer system comprising at least one host, a first content addressable storage (CAS) system and a second CAS system, wherein the first and second CAS systems each performs a plurality of operations on content units for the at least one host, wherein the at least one host identifies a content unit using a content address generated based, at least in part, on the content of the content unit that is to be provided when the content unit is read from the CAS system, the method comprising acts of:

(A) receiving, at the first CAS system, a first I/O from the at least one host, wherein the first I/O is a request to perform an operation that accesses a first content unit on the second CAS system, and wherein the operation involves the transmission of the first content unit between the at least one host and the second CAS system; and

(B) establishing a communication session in which the first content unit is transmitted between the at least one host and the second CAS system without passing through the first CAS system.