Storage system and storage control method to select path for transmitting command via SAS expanders

A storage control method of a storage system including an expander sequence including a plurality of expanders coupled in series, a plurality of storage devices directly coupled to the expander sequence, a first initiator device directly coupled to a first expander which is one of the expanders in the expander sequence, and a second initiator device directly coupled to a second expander which is one of the expanders in the expander sequence and different from the first expander, the storage control method comprising:

• selecting a path, with respect to a transmission destination device of an I/O command among the plurality of storage devices and the plurality of expanders, between the optimal path which is an initiator device with fewer expanders in a distance to the transmission destination device and a roundabout path which is an initiator device with more expanders in a distance to the transmission destination device,
• transmitting the command to the transmission destination device through the selected path;
• comparing, with respect to the transmission destination device, response time of the optimal path with response time of the roundabout path;
• comparing, with respect to the transmission destination device, the number of I/O commands issued through the optimal path with the number of I/O commands issued through the roundabout path;
• and
• selecting the optimal path with fewer expanders in the distance to the transmission destination device than the roundabout path, when the number of I/O commands of the optimal path is equal to or greater than the number of I/O commands of the roundabout path with more expanders in the distance to the transmission destination device than the optimal path, without regard to whether the response time of the optimal path is smaller than or equal to or longer than the response time of the roundabout path.

A storage system comprising:

• an expander sequence including a plurality of expanders coupled in series;
• a plurality of storage devices directly coupled to the expander sequence;
• a first initiator device directly coupled to a first expander which is one of the expanders in the expander sequence;
• a second initiator device directly coupled to a second expander which is one of the expanders in the expander sequence and different from the first expander;
• and
• a processor configured to transmit a command through one of the first and second initiator devices to a transmission destination device of the command among the plurality of storage devices and the plurality of expanders,
• wherein the processor is configured to select a path, with respect to the transmission destination device, between the optimal path which is an initiator device with fewer expanders in a distance to the transmission destination device and a roundabout path which is an initiator device with more expanders in a distance to the transmission destination device, and to transmit the command to the transmission destination device through the selected path;
• wherein the command is an I/O command;
• wherein the processor is configured to compare, with respect to the transmission destination device, response time of the optimal path with response time of the roundabout path;
• and
• wherein the processor is configured to compare, with respect to the transmission destination device, the number of I/O commands issued through the optimal path with the number of I/O commands issued through the roundabout path, and to select the roundabout path with more expanders in the distance to the transmission destination device than the optimal path, when the response time of the roundabout path is shorter than the response time of the optimal path and the number of I/O commands of the roundabout path is greater than the number of I/O commands of the optimal path.