Method and apparatus for grouping multiple SAS expanders to form a single cohesive SAS expander

1. A SAS (Serial Attached SCSI or Serial Attached Small Computer System Interface) switch, comprising:

a master SAS expander;

a plurality of slave expanders connected to the master SAS expander, each of the plurality of slave expanders having a distinct SAS address and not being directly connected to other slave expanders, each slave SAS expander being configured by the master SAS expander;

a first SAS wide port having a first plurality of SAS links each associated with a different one of the plurality of slave expanders; and

a second SAS wide port having a second plurality of SAS links each associated with a different one of the plurality of slave expanders, wherein the plurality of slave expanders are configured to route SAS traffic in accordance with routing tables established by the master SAS expander, the master SAS expander not being directly connected to either the first or the second SAS wide ports,

wherein, in a SAS link identification sequence, the plurality of slave SAS expanders are configured to send IDENTIFY address frames which include the master SAS expander's SAS address and a plurality of PHY identifiers globally indexed within the SAS switch.

2. The SAS switch of claim 1 wherein the master SAS expander and the plurality of slave SAS expanders are configured to operate and respond as a single coherent SAS expander with a unified SAS address, wherein the first and the second wide ports are configured to connect to external SAS devices.

3. The SAS switch of claim 1 wherein said master expander is configured to:

detect a failure with a first one of the plurality of SAS links; and

reroute a SAS traffic bound for the detected first one of the plurality of SAS links.

4. The SAS switch of claim 1 wherein each of the first plurality of SAS links in the first SAS wide port and each of the second plurality of SAS links in the second SAS wide port is configured to have a distinct non-overlapping external PHY identifier.

5. The SAS switch of claim 1 wherein each of the plurality of slave SAS expanders communicates with the master SAS expander using the slave expander's SAS address and a local PHY identifier.

6. The SAS switch of claim 1 further comprising a third and a fourth wide ports, each link in each of the wide port is associated with a different one of the plurality of slave expanders, wherein the third SAS wide port is configured to connect the SAS switch to a third external SAS device, and the fourth SAS wide port is configured to connect the SAS switch to a fourth external SAS device.

7. The SAS switch of claim 1 wherein each of the slave SAS expanders is configured to:

route SAS traffic among external SAS devices; and

route SAS management traffic to and from the master SAS expander.

8. A method for forming a single coherent SAS (Serial Attached SCSI or Serial Attached Small Computer System Interface) expander, the method comprising:

assigning a SAS expander as a master SAS expander;

assigning a plurality of SAS expanders as SAS slave expanders, the SAS master expander and each of the plurality of slave SAS expanders having distinct SAS addresses;

connecting each of the slave SAS expanders to the master SAS expander through at least one internal SAS link;

inhibiting direct connection between the plurality of slave SAS expanders;

forming a first SAS wide port having a first plurality of SAS links each associated with a different one of the plurality of slave expanders;

forming a second SAS wide port having a second plurality of SAS links each associated with a different one of the plurality of slave expanders;

configuring the plurality of slave expanders to route SAS traffic in accordance with routing tables established by the master SAS expander; and

configuring the slave SAS expanders such that the IDENTIFY address frames sent to external SAS devices by the slave expanders include a SAS ADDRESS field set to the master SAS expander's SAS address and a PHY IDENTIFIER field set to a PHY identifier that is uniquely indexed within the single coherent SAS expander.

9. The method of claim 8 further comprising:

inhibiting direct connection between the master expander and the wide port.

10. The method of claim 8 further comprising:

configuring the slave SAS expanders such that SAS traffic between two external SAS devices is routed via external SAS wide ports.

11. The method of claim 8 wherein the master SAS expander is configured to:

detect a failure with a first one of a plurality of SAS links; and

reroute a SAS traffic bound for the first one of the plurality of SAS links.