Unit attention processing in proxy and owner storage systems

A method comprising,

• receiving, by a first computer, a input/output (I/O) request for a logical volume stored on an owner module, the I/O request being received from a given one of multiple initiator ports on a second computer;
• conveying the I/O request to a third computer;
• receiving, in response to the conveyed I/O request, a unit attention code and a respective unique serial number from the third computer;
• and
• upon determining, based on the unique serial number, that the first computer has not yet conveyed the unit attention code to any of the multiple initiator ports, conveying a unit attention message to the given initiator port.

A storage facility, comprising:

• a storage area network (SAN);
• and
• first, second and third computers configured on the SAN, the first computer being coupled to the SAN via multiple ports and configured:
• to receive, via a given one of the multiple ports, an input/output (I/O) request for a logical volume stored on an owner module, the I/O request being received from a given one of multiple initiator ports on the second computer,
• to convey the I/O request to the third computer,
• to receive, in response to the conveyed I/O request, a unit attention code and a respective unique serial number from the third computer, and
• upon determining, based the unique serial number, that the first computer has not yet conveyed the unit attention code to any of the multiple initiator ports, to convey a unit attention message to the given initiator port.