Fibre channel port by-pass selector section for dual ported disk drives

[0002] This invention relates generally to data storage systems and more particularly to data storage systems having a plurality of magnetic storage disk drives in a redundancy arrangement whereby the disk drives are controllable by first disk controllers and second disk controllers. Still more particularly, the invention also relates to systems of such type wherein the disk drives are coupled to the disk controllers through a series, unidirectional, “ring” or, fibre channel protocol, communication system.

[0003] As is known in the art, in one type of data storage system, data is stored in a bank of magnetic storage disk drives. The disk drives, and their coupled interfaces, are arranged in sets, each set being controlled by a first disk controller and a second disk controller. More particularly, in order to enable the set of disk drives to operate in the event that there is a failure of the first disk controller, each set is also coupled to a second, or redundant disk controller. Therefore, if either the first or second disk controller fails, the set can be accessed by the other one of the disk controllers.

[0004] While today, most disk storage systems of this type use a Small Computer System Interconnection (SCSI) protocol, in order to operate with higher data rates, other protocols are being introduced. One higher data rate protocol is sometimes referred to as a fibre channel (FC) protocol. Such FC channel protocol uses a series, unidirectional, “ring” communication system. In order to provide for redundancy, that is, to enable use of the set of disk drives in the event that the first disk controller fails, as discussed above, the set is coupled to the second, or redundant disk controller, using a separate, independent, “ring”, or fibre channel communication protocol. Thus, two fibre channels are provided for each set of disk drives and their disk interfaces; a first fibre channel and a second fibre channel.

[0005] As is also known, when using the fibre channel communication protocol, if any element in the channel becomes inoperative, the entire channel becomes inoperative. That is, if the first disk controller becomes inoperative, or if any one of the disk drives in the set coupled to the first channel becomes inoperative (i.e., as where the disk interface fails, the disk interface is inoperative, or removed with its coupled disk drive, or where the disk drive coupled thereto fails, or is removed), the first fibre channel, is “broken”, or open, and becomes inoperative. The data stored in the entire portion of the set of disk drives coupled to the first disk channel is therefore unavailable until the inoperative first disk controller or inoperative disk drive is replaced. This is true with either the first channel or the second channel. One technique suggested to solve this problem is through the use of a switch, sometimes referred to as an LRC (i.e., a loop resiliency circuit) switch. Such LRC switch is used to remove an inoperative disk drive from its channel.

[0006] In one suggested arrangement, a printed circuit board is provided for each disk drive. The printed circuit board has a pair of LRCs, one for the first channel and one for the second channel. Thus, the open channel may be “closed” in the event of an inoperative disk drive by placing the LRC thereof in a by-pass condition. While such suggested technique solves the inoperative disk drive, or open channel problem, if one of the pair of LRCs fails, the entire printed circuit board having the pair of LRCs must be replaced thereby disrupting both the first and second channels; and, hence, disrupting the operation of the entire data storage system.

[0007] One technique suggested to solve this disruption problem requires n LRC switches (where n is the number of disk drives in the set) in the first channel, i.e., one LRC for each one the n disk drives in the set and another n LRC switches in the second channel for each one of the n disk drives in the second channel. The first channel set of n LRCs is mounted on one printed circuit board and the second channel set of n LRCs is mounted on a different printed circuit board. A backplane is used to interconnect the two LRC printed circuit boards, the associated multiplexers, and the disk drives. In order to provide the requisite serial, or sequential, fibre channel connections, an elaborate, complex, fan-out wiring arrangement has been suggested for the backplane. Further, the slots provided for the two LRC boards eliminates two disk drives, and the disk interfaces which would otherwise be plugged into these two slots of the backplane.

[0008] Another fibre channel arrangement is described in U.S. Pat. No. 5,729,763 entitled “Data Storage System”, inventor Eli Leshem, issued Mar. 17, 1998, assigned to the same assignee as the present invention.

[0009] In accordance with one feature of the invention, a data storage system is provided having a plurality of disk drives. Each one has a pair of ports. A pair of directors is included for controlling the flow of information (i.e, data) to and from the disk drives. A first fibre channel port by-pass selector section is provided. The first fibre channel selector section includes: an input/output port coupled to a first one of the directors; and, a plurality of output/input ports connected between a first one of the ports of the plurality of disk drives through a first plurality of fibre channel links. The first fibre channel port by-pass selector section is adapted to couple the first one of the directors serially to one, or ones, of the first ports of the plurality of disk drives through a first fibre channel selectively in accordance with a control signal fed to the first fibre channel by-pass selector. The first fibre channel includes one, or more, of the first plurality of fibre channel links. A second fibre channel port by-pass selector section is provided having an input/output port coupled to a second one of the directors and a plurality of output/ports serially connected between a second one of the pair of ports of the plurality of disk drives through a second plurality of fibre channel links. The second fibre channel port by-pass selector section is adapted to couple the second one of the directors serially to one, or ones, of the second ports of the plurality of disk drives through a second fibre channel selectively in accordance with the control signal. The second fibre channel includes one, or more, of the second plurality of fibre channel links.

[0010] These and other features of the invention will become more readily apparent from the follow detailed description when read together with the accompanying drawings, in which:

[0011] FIG. 1 is a block diagram of a data storage system according to the invention;

[0012] FIG. 2 is a block diagram of a redundant fibre channel network used in the system of FIG. 1 according to the invention;

[0013] FIG. 3 is a block diagram of a port by-pass section used in the redundant fibre channel network of FIG. 3 coupled to a bank of disk drives used in the system of FIG. 1 according to the invention;

[0014] FIGS. 4, 4A, and 4B are block diagrams of the system of FIG. 1 arranged in an expanded configuration according to the invention;

[0015] FIGS. 5, 5A and 5B are block diagrams of a redundant fibre channel network adapted for use in the system of FIG. 1 according to the invention;

[0016] FIGS. 5C-5G are diagrams of the network of FIGS. 5, 5A and 5B configured in a number of contracted configurations according to the invention.

[0017] Referring now to FIG. 1, a data storage system 10 is shown wherein a host computer 12 is coupled to a bank 14 of disk drives through a system interface 16. The system interface 16 includes a cache memory 18, having high memory address sections 18H and low address memory sections 18L. A plurality of directors 20₀-20₁₅is provided for controlling data transfer between the host computer 12 and the bank 14 of disk drives as such data passes through the cache memory 18. A pair of high address busses TH, BH is electrically connected to the high address memory sections 18H. A pair of low address busses TL, BL electrically connected to the low address memory sections 18L. The cache memory 18 has a plurality of storage location addresses. Here, the storage locations having the higher addresses are in the high address memory sections 18H and the storage locations having the lower addresses are in the low address memory sections 18L. It should be noted that each one of the directors 20₀-20₁₅is electrically connected to one of the pair of high address busses TH, BH and one of the pair of low address busses TL, BL. Thus, each one of the directors 20₀-20₁₅is able to address all locations in the entire cache memory 18 (i.e., to both the high address memory sections 18H and the low address memory sections 18L) and is therefore able to store data in and retrieve data from any storage location in the entire cache memory 18.

[0018] More particularly, a rear-end portion of the directors, here directors 20₀-20₃and 20₁₂-20₁₅, is electrically connected to the bank 14 of disk drives through fibre channel (FC) port by-pass sections 23₁-23₈(described in more detail in connection with FIG. 3), respectively. and a front-end portion of the directors, here directors 20₄-20₁₁, is electrically connected to the host computer 12 through I/O adapter cards 22₁-22₈, respectively, as indicated. It should also be noted that each end of the busses TH, TL, BH, BL terminates in a pair of master and slave arbiters bus arbiters, not shown, as described in co-pending patent application Ser. No. 09/224,194 filed Dec. 30, 1998, entitled DATA STORAGE SYSTEM, inventor Mark Zani, assigned to the same assignee as the present invention, now abandoned, the entire subject matter thereof being incorporated herein by reference.

[0019] In operation, when the host computer 12 wishes to store data, the host computer 12 issues a write request to one of the front-end directors 20₄-20₁₁to perform a write command. One of the front-end directors 20₄-20₁₁replies to the request and asks the host computer 12 for the data. After the request has passed to the requesting one of the front-end directors 20₄-20₁₁, the director determines the size of the data and reserves space in the cache memory 18 to store the request. The front-end director then produces control signals on either a high address memory bus (TH or BH) or a low memory address bus (TL, BL) connected to such front-end director depending on the location in the cache memory 18 allocated to store the data and enable the transfer to the cache memory 18. The host computer 12 then transfers the data to the front-end director. The front-end director then advises the host computer 12 that the transfer is complete. The front-end director looks up in a Table, not shown, stored in the cache memory 18 to determine which one of the rear-end directors 20₀-20₃and 20₁₂-20₁₅is to handle this request. The Table maps the host computer 12 address into an address in the bank 14 of disk drives. The front-end director then puts a notification in a “mail box” (not shown and stored in the cache memory 18) for the rear-end director which is to handle the request, the amount of the data and the disk address for the data. Other rear-end directors poll the cache memory 18 when they are idle to check their “mail boxes”. If the polled “mail box” indicates a transfer is to be made, the rear-end director processes the request, addresses the disk drive in the bank, reads the data from the cache memory and writes it into the addresses of a disk drive in the bank 14. When data is to be read from the disk drive to the host computer 12 the system operates in a reciprocal manner.

[0020] Each one of the rear-end portion of the directors 20₀-20₃is identical in construction and are described in detail in the above-referenced co-pending patent application Ser. No. 09/224,194 (now abandoned) to include a pair of central processing sections, CPU X and CPU Y, a dual port random access memory (RAM), and shared resources (Flash memories, etc,) coupled to the bank 14 of disk drives (FIG. 1) through the fibre channel (FC) port by-pass sections (FIG. 3), as indicated and to a high memory address bus, here TH, and low memory address bus, here BL. It should be noted that the director 20₀-20₃and 20₁₂-20₁₅has a first output port, A, and a second output port, B. Further, it should be noted that different pairs of the rear-end directors 20₀, 20₁; 20₂, 20₃; 20₁₂, 20₁₃(not shown); and, 20₁₄, 20₁₅are arranged in redundant fibre channel (FC) networks 25₁-25₄, respectively, as indicated. Still further, it is noted that each one of the redundant fibre channel (FC) networks 25₁-25₄also includes pairs of the fibre channel (FC) port by-pass sections 23₁, 23₂; 23₃, 23₄; 23₅(not shown), 23₆(not shown); and, 23₇, 23₈, respectively, as indicated and disk drive sets 14₁, 14₂; 14₃, 14₄; 14₅(not shown), 14₆(not shown); and, 14₇, 14₈, respectively, as indicated. Each one of the pairs of the redundant fibre channel (FC) networks 25₁-25₄is identical in construction, an exemplary one thereof, here redundant fibre channel (FC) networks 25₁is shown in detail in FIG. 2.

[0021] Thus, as shown in FIG. 2, the first port A and second port B of director 20₀is connected to both FC port by-pass section 23₁and to FC port by-pass section 23₂. Likewise, the first port A and second port B of director 20₁is connected to both FC port by-pass section 23₁and to FC port by-pass section 23₂. More particularly, both directors 20₀and 20₁of the redundant FC network 25₁are connected to the pair of FC port by-pass sections 23₁, 23₂. Each one of the FC port by-pass sections 23₁, 23₂includes a pair of FC port by-passes 34₁and 34₂. Each one of the FC port by-passes 34₁and 34₂is connected to a corresponding one of a plurality of sets 14₁, 14₂of the disk drives in the bank of disk drives 14 (FIG. 1). Each one of the FC port by-passes 34₁, 34₂and is identical in construction, an exemplary one thereof, here FC port by-pass 34₁being shown in detail in FIG. 3 connected to set 14₁of the disk drives and to directors 20₀and 20₁. It is noted that director 20₀is connected to busses TH and BL and that director 20₁is connected to busses TL and BH. Thus, the redundant FC network 25₁(FIG. 1) is connected to all four busses TH, BH, TL, and BL.

[0022] More particularly, and referring again also to FIG. 2, it is noted that each one of the disk drive sets 14₁, 14₂includes a plurality of, here for example eight, disk drives 19₁-19₈, it being understood that the number of disk drives 19 in a set can be selected in accordance with the requisite storage requirements. It is also noted that each one of the disk drives 19₁-19₈includes a pair of input/output ports A, B. As noted above, each one of the FC port by-pass sections 23₁-23₈is identical in construction. An exemplary one thereof, here FC port by-pass section 23₁is shown in FIG. 2 to include: an input/output port 30_Acoupled to the A port of director 20, and a plurality of, here in this example, eight, output/input ports 32_A1-32_A8serially connected between a first one of the ports, here port A, of the plurality of disk drives 19₁-19₈through a plurality of fibre channel links 29_A1-29_A8. The fibre channel port by-pass selector section 23₁is adapted to couple port A of director 20₀serially to a selected one, or ones, of port A of the plurality of disk drives 19₁-19₈in set 14₁through a first fibre channel comprising one, or more, of the plurality of fibre channel links 29_A1-29_A8during a normal mode of operation. The fibre channel port by-pass selector section 23₁also has an input/output port 30_Bcoupled to the A port of director 20₁and a plurality of output/ports 32_B1-32_B8serially connected between the B ports of the plurality of disk drives 19₁-19₈through fibre channel links 29_B1-29_B8, as indicated. The fibre channel port by-pass selector section 23₁is adapted to couple the A port of director 20₁serially to a selected one, or ones, of the B ports of the plurality of disk drives 19₁-19_Bin set 14₁through a second fibre channel comprising one, or more, of the second plurality of fibre channel links 29_B1-29_B8in the normal operating mode.

[0023] Likewise, fibre channel port by-pass selector section 23₂in FIG. 2 is shown to includes: an input/output port 30_Acoupled here, however, coupled to the B port of director 20₀and a plurality of, here in this example, eight, output/input ports 32_A1-32_A8serially connected between port A of the plurality of disk drives 19₁-19_Bin set 14₂through a plurality of fibre channel links 29_A1-29_A8. The fibre channel port by-pass selector section 23₂is adapted to couple port B of director 20₀serially to a selected one, or ones, of port A of the plurality of disk drives 19₁-19₈in set 142 through a first fibre channel comprising one, or more, of the plurality of fibre channel links 29_A1-29_A8in section 23₂during a normal node of operation. The fibre channel port by-pass selector section 23₂also has an input/output port 30_Bcoupled to the B port of director 20, and a plurality of output/ports 32_B1-32_B8serially connected between the B ports of the plurality of disk drives 19₁-19₈in set 142 through fibre channel links 29_B1-29_B8, as indicated. The fibre channel port by-pass selector section 23₂is adapted to couple the B port of director 20₁serially to a selected one, or ones, of the B ports of the plurality of disk drives 19₁-19₈in set 14₂through a second fibre channel comprising one, or more, of the second plurality of fibre channel links 29_B1-29_B8in section 23₂in the normal operating mode.

[0024] It is noted that director 20₀is able to access the disk drives 19₁-19₈in set 14₂through its port B in the event of a failure in director 20₁and, likewise, director 20₁is able to access disk drives 19₁-19₈in set 14₁through its A port in the event of a failure in director 20₀.

[0025] It is also noted that in the event of a failure of, or removal of, any one of the port A or port B by-passes 34A, 34B, both sets of disk drives 14₁and 14₂are still accessible from one of the directors 20₀and 20₁. For example, if the port A by-pass 34A of fibre channel port by-pass section 23₁fails or is removed, the set 14₁of disk drives is accessible from director 20₁, via the path between port A of director 20₁, the port B by-pass 34B of fibre channel by-pass section 23₁, and the port B of the disk drives in set 14₁. In like manner, if the port B by-pass 34B of fibre channel port by-pass section 23₁fails or is removed, the set 14₁of disk drives is accessible from director 20₁, via the path between port A of director 20₀, the port A by-pass 34A of fibre channel by-pass section 23₁, and the port A of the disk drives in set 14₁. If the port A by-pass 34A of fibre channel port by-pass section 23₂fails or is removed, the set 14₂of disk drives is accessible from director 20₀, via the path between port B of director 20₁, the port B by-pass 34B of fibre channel by-pass section 23₁, and the port B of the disk drives in set 14₂. In like manner, if the port B by-pass 34B of fibre channel port by-pass section 23₂fails or is removed, the set 14₂of disk drives is accessible from director 20₀, via the path between port B of director 20₀, the port A by-pass 34A of fibre channel by-pass section 23₂, and the port A of the disk drives in set 14₂.

[0026] Referring now to FIG. 3, an exemplary one of the FC port by-pass sections 23₁-23₈, here section 23₁is shown in detail connected to set 14₁of disk drives 19₁-19₈and to ports A of directors 20₀and 20₁. As shown also in FIG. 2, the FC port by-pass section 23₁includes a pair of FC port by-passes 34_A, 34_B. The FC port by-pass 34_Ais coupled between input/output port 30_Aand the A ports of the disk drives 19₁-19₈in set 14₁through fibre channel links 29_A1-29_A8and the FC port by-pass 34_Bis coupled between input/output port 30_Band the B ports of the disk drives 19₁-19₈in set 14₁through fibre channel links 29_B1-29_B8.

[0027] Referring again to FIG. 3, the FC port by-pass 34_Aincludes selectors 36_A1-36_A12and a control section 40_A. (It should be understood that the number of selectors is determined in accordance with the requisite storage requirements). Each one of the selectors 36_A1-36_A12has a pair of input ports (i.e., an A input and a B input) and an output port, one of the input ports A or B being coupled to the output port selectively in accordance with a control signal C_A1-CA₁₂, respectively, fed thereto, as indicated, by the control section 40_A. It is also noted that input/output port 30_Ahas an input port 30_A, and an output port 30_AO.

[0028] It is noted that the selectors 36_A1-36_A5; and, 36_A8-36_A11are arranged in two selector sections 37_A1and 37_A2, Section 37_A1is coupled to disk drives 19₁-19₄and section 37_A2is coupled to disk drives 19₅-19₈. Each section 37_A1, 37_A2is used to control whether one, or more, of the disk drives coupled thereto should be by-passed. In like manner. the selectors 36_B1-36_B5; and, 36_B8-36_B11, are arranged in two selector sections 37_B1and 37_B2, Section 37_B1is coupled to disk drives 19₁-19₄and section 37_B2is coupled to disk drives 19₅-19₈. Each section 37_B1, 37_B2is used to control whether one, or more, of the disk drives 19₁-19₄, 19₅-19₈coupled thereto, respectively, should be by-passed.

[0029] Selector 36_A1has its A input connected to input port 30_A1and its B input port connected to output port 30_AO. The outputs of selectors 36_A1-36_A11are connected to the B inputs of selectors 36_A2-36_A12, respectively. The outputs of selectors 36_A1-36_A4and 36_A7-36_A10are also coupled to the A ports of disk drives 19₁-19₈, respectively as shown. The output of selector 36_A12is connected to the A input of selector 36_A6. The output of selector 36_A6is connected to both the B input of selector 36_A1and the input port 30_AIof director 20₀, as indicated. The output of selector 36_A12is also connected to the A input of selector 36_A7. The A inputs of selectors 36_A2-36_A5and 36_A8-36_A11are fed thereto from the A ports of disk drives 19₁-19₈, respectively, as indicated. It is noted that the output of selector 36_A11, in addition to being fed to the B input of selector 36_A12is connected to an expansion port 30_EXAOand that the A input of selector 36A₁₂is connected to an expansion port 30_EXAI, for reasons to be discussed in more detail below in connection with FIGS. 4, 4A and 4B.

[0030] The FC port by-pass 34_Bincludes selectors 36_B1-36_B12and a control section 40_B. Each one of the selectors 36_B1-36_B12has a pair of input ports (i.e., an A input and a B input) and an output port, one of the input ports A or B being coupled to the output port selectively in accordance with a control signal C_B1-CB₁₂, respectively, fed thereto, as indicated, by the control section 40_B, It is also noted that input/output port 30_Bhas an input port 30_B, and an output port 30_BO.

[0031] It is noted that the selectors 36_B1-36_B5; and, 36_B8-36_B11are arranged in two selector sections 37_B1and 37_B2. Section 37_B1is coupled to disk drives 19₁-19₄and section 37_B2is coupled to disk drives 19₅-19₈. Each section 37_B1, 37_B2is used to control whether one, or more, of the disk drives coupled thereto should be by-passed. In like manner. the selectors 36_A1-36_B5; and, 36_A8-36_A11are arranged in two selector sections 37_A1and 37_A2. Each section 37_B1, 37_B2is used to control whether one, or more, of the disk drives 19₁-19₄, 19₅-19₈coupled thereto, respectively, should be by-passed. Selector 36_B1has its A input connected to input port 30_B1and its B input port connected to output port 30_BO. The outputs of selectors 36_B1-36_B11are connected to the B inputs of selectors 36_B2-36_B12, respectively. The outputs of selectors 36_B1-36_B4and 36_B7-36_B10are also coupled to the B ports of disk drives 19₁-19₈, respectively as shown. The output of selector 36_B12is connected to the A input of selector 36_B6. The output of selector 36_B6is connected to both the B input of selector 36_B1and the input port 30_BIof director 20₁, as indicated. The output of selector 36_B12is also connected to the A input of selector 36_B7. The A inputs of selectors 36_B2-36_B5and 36_B8-36_B11are fed thereto from the B ports of disk drives 19₁-19₈, respectively, as indicated. It is noted that the output of selector 36_B11, in addition to being fed to the B input of selector 36_B12is connected to an expansion port 30_EXBOand that the A input of selector 36_B12is connected to an expansion port 30_EXBI, for reasons to be discussed in more detail below in connection with FIGS. 4, 4A and 4B.

[0032] During the normal operating mode, port 30_Aof director 20₀is coupled serially through disk drives 19₁-19₄of set 14₁via ports A of such disk drives 19₁-19₄and port 30_Bof director 20₁is coupled serially through disk drives 19₅-19₈of set 14₁via ports B of such disk drives 19₅-19₈. Such is accomplished by the control signals C_A1-C_A12and C_B1-C_B12which couple one of the A and B ports of the selectors coupled to the outputs of such selectors in the following TABLE:

[0033] In the event of a failure in one of the disk drives 19₁-19₈, the control sections 40A and 40B are advised of such failure by the directors 20₀and 20₁via control lines 45₀, 45₁, respectively. For example, assume there is a failure in disk drive 19₃. Once such a failure is detected during the normal operating mode, control section 40A changes the logic state on control line C_A4to thereby de-couple input port A of selector 36_A4from its output and couples input port B of selector 36_A4to its output; thereby by-passing disk drive 19₃from the first fibre channel (i.e., the fibre channel through the input and output ports 30_AIand 30_AOof director 20₀). In like manner, if there is a failure in disk drive 19₇, once such a failure is detected during the normal operating mode, control section 40B changes the logic state on control line CB_A10to thereby de-couple input port A of selector 36_B10from its output and couples input port B of selector 36_B10to its output; thereby by-passing disk drive 19₇from the second fibre channel (i.e., the fibre channel through the input and output ports 30_BIand 30_BOof director 20₁).

[0034] As noted above, during normal operation, director 20₀is coupled to the A ports of disk drives 19₁-19₄and director 20₁is coupled to the B ports of disk drives 19₅-19₈. In the event of a failure in director 20₀, director 20₀is de-coupled from disk drives 19₁-19₄and director 20₁is coupled to the B ports of disk drives 19₁-19₄in addition to remaining coupled to the B ports of disk drives 19₅-19₈. Likewise, in the event of a failure in director 20₁, director 20₁is de-coupled from disk drives 19₅-19₈and director 20₀is coupled to the A ports of disk drives 19₅-19₈in addition to remaining coupled to the A ports of disk drives 19₁-19₄. Such is accomplished (i.e., removal of failed director 20₁, for example) by the control signals C_A1-C_A12and C_B1-C_B12which couple one of the A and B ports of the selectors coupled to the outputs of such selectors in the following TABLE:

[0035] As described above in connection with FIG. 1, for each redundant FC network 25₁-25₄the A port of one of the rear-end directors thereof is coupled in the normal mode to one disk drive set and the A port of the other one of the rear-end directors thereof is coupled such set of disk drives. Thus, for example, considering redundant FC network 25₁shown more clearly in FIG. 2, during the normal operating mode, the A port of rear-end director 20, and the A port of rear-end director 20₁are coupled to disk drive set 14₁through port by-passes 34_Aand 34_B, respectively, of port by-pass section 23₁. Each one of the rear-end directors 20₀-20₁₅(FIG. 1) may, however, may be coupled to more than one disk drive set through the use of additional port by-passes. For example, referring to FIG. 4 and considering port A of the rear-end directors 20₀and 20₁, an additional disk drive set 14′₁is shown coupled to the A ports of rear-end directors 20₀and 20₁through FC port by-passes 34′_Aand 34′_B, respectively, as shown. It is first noted that the additional FC port by-passes 34′_Aand 34′_Bare identical in construction to FC port by-passes 34_Aand 34_Bshown in FIG. 3. Thus, referring also to FIGS. 4 and 4A, the same numerical designation is used for the elements in FC port by-pass 34′_Aas was used in FC port by-pass 34_Aexcept that the elements in FC port by-pass 34′_Ahave a prime (′) numerical designation. It is next noted in FIG. 4, that the A port of director 20₀is connected to the A ports of the disk drives 19₁-19₈, 19′₁-19′₈in both disk drive sets 14₁and 14′₁through FC port by-passes 34_Aand 34′_A, respectively, and that the A port of director 20₁is connected to the B ports of the disk drives 19₁-19₈and 19′₁-19′₈, respectively, in both disk drive sets 14₁and 14′₁, respectively, through FC port by-passes 34_Band 34′_B, respectively. More specifically, and referring also to FIG. 4A, the output of port A of director 20₀is coupled to port 30_AIof FC port by-pass 34_Aand the input of port A of director 20₀is coupled to port 30_AO. The output of selector 36_A6is coupled to the B input of selector 36_A1of port A by-pass 34_A, and to port 30_A1. Port 30_AIis coupled to the A input of selector 36_A1of the port A by-pass 34_A. The details of the port A by-pass have been described above in connection with FIG. 3. The output of selector 36_A11is connected, through expansion port 30_EXAOto A input of selector 36′_A1of port A by-pass 34′_Aand the output of selector 36′_A6of selector 34′_Ais connected to the B input of selector 36′_A1and through port 30′_AO, to the A input of selector 36_A12of port A by-pass 34_A. Thus, port A by-pass 34_Ais adapted to service disk drive set 14 and, if expansion is desired, the additional port A by-pass 34′_Ais adapted to service the additional disk drive set 14′₁.

[0036] Referring now also to FIG. 4B, the port A by-pass 34_Aand the additional port A by-pass 34′_Aare shown in more detail. Thus, the FC port by-pass 34_Aincludes selectors 36_A1-36_A12and a control section 40_A. Each one of the selectors 36_A1-36_A12has a pair of input ports (i.e., an A input and a B input) and an output port, one of the input ports A or B being coupled to the output port selectively in accordance with a control signal C_A1-C_A12, respectively, fed thereto, as indicated, by the control section 40_A. It is also noted that input port 30_AIis coupled to port A of director 20₀and the output of selector 36_A6of by-pass 34_Ais coupled to port 30_AO, as in FIG. 4A.

[0037] During the normal mode of operation, port 30_Aof director 20₀is coupled serially through disk drives 19₁-19₄of set 14₁via ports A of such disk drives 19₁-19₄and through disk drives 19′₁-19′₄and then back to port 30′_A0. Such is accomplished by the control signals C_A1-C_A12and C′_A1-C′_A12which couple one of the A and B ports of the selectors coupled to the outputs of such selectors in the following TABLE:

[0038] In the event of a failure in director 20, director 20₀is coupled to the A ports of disk drives 19′₁-19′₈and 19₁-19₈. Such is accomplished by the control signals C_A1-C_A12and C′_A1-C′_A12which couple one of the A and B ports of the selectors coupled to the outputs of such selectors in the following TABLE:

[0039] It is understood that appropriate control signals are produced by FC port by-passes 34_B, 34′_B(FIGS. 4, 4A and 4B) to de-couple failed director 20₁from the B ports of disk drives 19₄-19₈and 19′₄-19′₈.

[0040] Referring now to FIGS. 5, and 5A, an arrangement is shown where the disk drive set 14₁in bank 14 (FIG. 1) has 24 disk drives 19″₁-19″₂₄instead of the 8 disk drive set 14₁-14₈discussed above. Further, a redundant FC network 25″₁includes, in addition to the rear-end directors 20₀and 20₁described above in connection with redundant FC network 25₁(FIG. 1), a pair of additional rear-end directors 20″₀and 20″₁. Director 20″₀is coupled to busses TL and BH (FIG. 1) and director 20″₁is coupled to busses TH and BL (FIG. 1). The A port of director 20″₀is coupled to the A ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₁through port A by-pass 34″_Aof port by-pass section 23″₁and the B port of director 20″₀is coupled to the A ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₂through port A by-pass 34″_Aof port by-pass section 23″₂. The A port of director 20″₁is coupled to the B ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₁₁through port B by-pass 34″_Aof port by-pass section 23″₁and the B port of director 20″₁is coupled to the B ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₂through port B by-pass 34″_Bof port by-pass section 23″₂. In a manner similar to the redundant FC network 25₁described above in connection with FIGS. 1 and 2, the A port of director 20₀is coupled to the A ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₁through port A by-pass 34″_Aof port by-pass section 23″₁and the B port of director 20₀is coupled to the A ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₂through port A by-pass 34″_Aof port by-pass section 23″₂. Likewise, the A port of director 20₁is coupled to the B ports of the disk drives 19″₁-19″₂₄in disk drive set 14″₁through port A by-pass 34″_Aof port by-pass section 23″₁and the B port of director 20₁is coupled to the B ports of the disk drives 19″₁-19″₂₄in disk drive set 14₁t2 through port B by-pass 34″_Bof port by-pass section 23″₂.

[0041] Thus, here each set of disk drives 14″₁and 14″₂may be coupled to four rear-end directors 20₀, 20₁, 20″₀, 20″₁. The connections of rear-end directors 20₀and 20₁is at the input/output ports 30″_A, 30″_Bof the port A by-passes 34″_A, 341″_Bof port by-pass sections 23″₁and 23″₂, respectively, in a manner similar to that described above in connection with FIG. 2. The connections of rear-end directors 20″₀and 20″₁is at the expansion input/output ports 30″_EXA, 30″_EXBof the port A by-passes 34″_A, 34″_Bof port by-pass sections 23″₁and 23″₂, respectively, as shown more clearly in FIGS. 5A and 5B.

[0042] It is first noted that each of the port A and port B by-passes 34″_Aand 34″_Bis identical in construction. An exemplary one thereof, here port B by-pass 34″_Bis shown in detail in FIGS. 5B and 5C. More particularly, and referring to FIGS. 5B and 5C, the disk drives 19″₁-19″₂₄are arranged in four sets, each set being coupled to a corresponding one of four selector sections 37″₁-37″₄, as indicated. Thus, disk drives 19″₇-19″₁₂are coupled to selector section 37″₁, disk drives 19″₁-19″₁₆are coupled to selector section 37″₂₁, disk drives 19″₁₉-19″₂₄are coupled to selector section 37″₃, and disk drives 19″₁₃-19″₁₈are coupled to selector section 37″₄, as indicated. The selectors: 36″_B2-36″_B7; 36″_B9-36″_B14; 36″_B17-36″_B22; and, 36″₂₅-36″_B30, in the sections 37″₁; 37″₂; 37″₃; and 37″₄, respectively, are controlled by control signals fed to the selectors therein by control 40″_Bin a manner similar to that described above in FIG. 3 in connection with controls 40_Aand 40_B. Referring also to FIG. 5C, the port B by-pass 34″ is shown to include, in addition to the selectors in the four selectors sections 37″₁-37″₄, described above in connection with FIG. 5B, selectors 36″_B1, 36″_B8, 36″_B15, 36″₁₆, 36″₂₃, 36″_B24, 36″_B31, and 36″_B32, arranged as shown.

[0043] Referring again also to FIGS. 5 and 5A, and the exemplary port B by-pass 34″_B, it is noted that port A of rear-end director 20₁is connected to ports 30_BIand 30″_BOof input/output port 30″_Band that port A of rear-end director 20″₁is connected to ports 30_EXIand 30″_EXOof expansion port 30″_EXB. This arrangement is useful where one has a system with 24 disk dives in each set and then wants to reduce the number of disk drives to either 18, or 12, or 6 disk drives.

[0044] More particularly, and referring also to FIG. SD, rear-end director 20₁a configurations is shown where director 20₁is coupled to the B ports of all 24 disk drives 19″₁-19″₂₄in set 14″₁and director 20″₁is not coupled to any of the 24 disk drives 19″₁-19″₂₄in set 14′₁. Thus, the output of selector 36″_B1is coupled to its input port A (as indicated by the arrows 50), the output of selector 36″_B8is coupled to its A input port, the output of selector 36″_B15is coupled to its A port, the output of selector 36″₁₆is coupled to its B port, the output of selector 36″₂₃is coupled to its A port, the output of selector 36″_B24is coupled to its B port, the output of selector 36″_B31is coupled to its B port, and the output of selector 36″_B32is coupled to its A port.

[0045] Thus, the A port of director 20₁connected to the port 30″_BIpasses through selector 36″_B1to the input of selector section 37″₁. The output of selector section 37″₁then passes successively through: the selector 36″_B8; selector section 37″₂; selector 36″_B16; selector section 37″₃; selector 36″_B24; selector section 37″₄; selector 36″_B31; selector 36″_B23; selector 36″_B15; selector 36″_B32, then to port 30″_BOand back to the A port of director 20₁. It is noted that director 20″₁is by-passed and not connected to the selector sections 37″₁-37″₂and hence not connectable to the disk drives connected to such selector sections 37″₁-37″₂.

[0046] If the user wishes to reduce the number of disk drives coupled to director 20₁from 24 to 18 with director 20″₁coupled to the other 6 disk drives, such a configuration is shown in FIG. 5E. Thus, the output of selector 36″_B1remains coupled to its input port A (as indicated by the arrows 50), the output of selector 36″_B8remains coupled to its A input port, the output of selector 36″_B15remains coupled to its A port, the output of selector 36″₁₆remains coupled to its B port, the output of selector 36″₂₃switches to its B port, the output of selector 36″_B24switches to its A port, the output of selector 36″_B31switches to its A port, and the output of selector 36″_B32remains coupled to its A port.

[0047] Thus, the A port of director 20₁coupled to port 30″_BIis then coupled through selector 36″_B1to the input of selector section 37″₁. The output of selector section 37″₁then passes successively through: the selector 36″_B8; selector section 37″₂; selector 36″_B16; selector section 37″₃; selector 36″_B23; selector 36″_B15; selector 36″_B32and then to port 30″_BOand back to the A port of director 20₁. The A port of director 20″₁is connected through selector 36″_B31and through selector 36″_B24to selector section 37″₄and the back to the A port of selector 20″₁.

[0048] If the user wishes to reduce the number of disk drives coupled to director 20″₁from 18 to 12 with the director 20″₁being coupled to the other 12 disk drives, such a configuration is shown in FIG. 5F. Thus, the output of selector 36″_B1remains coupled to its input port A (as indicated by the arrows 50), the output of selector 36″_B8remains coupled to its A port, the output of selector 36″_B15switches to its B port, the output of selector 36″₁₆switches to its A port, the output of selector 36″₂₃switches to its A port, the output of selector 36″_B24switches to its B port, the output of selector 36″_B31remains at its A port, and the output of selector 36″_B32remains at its B port.

[0049] Thus, the A port of director 20₁coupled to port 30″_BIis coupled through selector 36″_B1to the input of selector section 37″₁. The output of selector section 37″₁then passes successively through: the selector 36″_B8; selector section 37″₂; selector 36″_B15; selector 36″_B32and then to port 30″_BOand back to the A port of director 20₁. The A port of director 20″₁, connected through selector 36″_B31is coupled through selector 36″_B31then through selector 36″_B23; selector 36_B16; selector section 37″₃, then through selector 36″_B24, selector section 37″₄and the back to the A port of selector 20″₁.

[0050] If the user wishes to reduce the number of disk drives coupled to director 20₁from 12 to 6 with the director 20″₁being coupled to the other 18 disk drives, such a configuration is shown in FIG. 5G. Thus, the output of selector 36″_B1remains coupled to its input port A (as indicated by the arrows 50), the output of selector 36″_B8is switched to its B port, the output of selector 36″_B15is switched to its B port, the output of selector 36″₁₆is switched to its B port, the output of selector 36″₂₃remains at its A port, the output of selector 36″_B24remains at its B port, the output of selector 36″_B31remains at its A port, and the output of selector 36″_B32is switched to its A port.

[0051] Thus, the A port of director 20₁coupled to port 30″_BIis coupled through selector 36″_B1to the input of selector section 37″₁. The output of selector section 37″₁then passes through selector 36″_B32and then to port 30″_BOand back to the A port of director 20₁. The A port of director 20″₁connected through selector 36″_B31is coupled through selector 36″_B31then through successively through selector 36″_B23; selector 36_B15, selector section 37₂, selector 36_B16, selector section 37″₃, selector 36_B24, to selector section 37″₃, then through selector 36″_B24to selector section 37″₄and the back to the A port of selector 20″₁.

[0052] Other embodiments are within the spirit and scope of the appended claims.