Abstract:
Loop interface failure is managed. A first device on a loop is identified as a potential cause of the loop interface failure. The loop is tested with the first device functionally removed from the loop. Depending on the results of the test, it is determined that the first device is not the cause of the loop interface failure and a second device on the loop is identified as the cause of the loop interface failure.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to managing loop interface failure. 
     BACKGROUND OF THE INVENTION 
     Computers, computer networks, and other computer-based systems are becoming increasingly important as part of the infrastructure of everyday life. Networks are used for sharing peripherals and files. In such systems, complex components are the most common sources of failure or instability. The proliferation of multiple interacting components leads to problems that are difficult or impossible to predict or prevent. The problems are compounded by the use of networks, which introduce the added complexity of multiple machines interacting in obscure and unforeseen ways. 
     Most complex electronic devices, including computer systems and networked hardware, are designed with built-in diagnostics. These diagnostics are specifically designed for the system and usually detect a fairly wide range of problems. Sometimes they can also implement fixes or workarounds, or at least pinpoint a problem to speed its repair. 
     The use of interconnected components, although advantageous for performance and expandability, increases the risk of an error propagating through the system and causing widespread harm in the system. 
     For example, Fibre Channel (“FC”) is a high performance, serial interconnect standard for bi-directional, point-to-point communications between servers, storage systems, workstations, switches, and hubs. Fibre Channel standards are described by the Fibre Channel Industry Association (FCIA) (http://www.fibrechannel.org). FC supports a variety of upper-level protocols, including the small computer systems interface (“SCSI”) protocol. A device is linked to the network through an FC port and copper wires or optical fibres. An FC port includes a transceiver and an interface controller, which conducts lower-level protocol exchanges between the FC channel and the device in which the FC port resides. 
     Because of the high bandwidth and flexible connectivity provided by FC, FC is a common medium for interconnecting devices within multi-peripheral-device enclosures, such as redundant arrays of inexpensive disks (“RAIDs”), and for connecting multi-peripheral-device enclosures with one or more host computers. These multi-peripheral-device enclosures economically provide greatly increased storage capacities and built-in redundancy that facilitates mirroring and fail over strategies needed in high-availability systems. Although FC is well-suited for this application with regard to capacity and connectivity, FC is a serial communications medium. Malfunctioning peripheral devices and enclosures can, in certain cases, degrade or disable communications. FC-based multi-peripheral-device enclosures are expected to isolate and recover from malfunctioning peripheral devices. 
     In particular, an FC interface which connects devices in a loop such as a Fibre Channel Arbitrated Loop (FC-AL) is widely used in disk array apparatuses and the like, since it has a simple connecting configuration of cables and can easily accommodate device extensions. In this type of interface, when signals cannot propagate in the loop because of failures or the like in interface circuits of connected devices (this is called, for example, loop abnormality or link down), the whole loop cannot be used. That is, even though a failure occurs in only one device, all devices connected to the loop cannot be used. Thus, disk array apparatuses usually have interface circuits for two ports, so that these devices are connected to two independent loops. With this configuration, even when one loop of the dual loop interfaces is out of use because of a failure or the like, accesses can be performed using the other loop, to thereby improve reliability. 
     SUMMARY OF THE INVENTION 
     Loop interface failure is managed. A first device on a loop is identified as a potential cause of the loop interface failure. The loop is tested with the first device functionally removed from the loop. Depending on the results of the test, it is determined that the first device is not the cause of the loop interface failure and a second device on the loop is identified as the cause of the loop interface failure. 
     One or more implementations of the invention may provide one or more of the following advantages. 
     A bad device can be correctly identified and kept off a Fibre Channel Arbitrated Loop, in order to maintain accessibility to other devices on the same Loop. In a data storage system, a drive with a bad transmitter can be correctly identified and put in bypass mode, helping to prevent a data unavailability/data loss scenario. 
     Other advantages and features will become apparent from the following description, including the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of a rack mount system including several storage enclosures. 
         FIG. 2  is a rear view of the rack mount system and storage enclosures of  FIG. 1 . 
         FIGS. 3-4  are block diagrams of components of the rack mount system and storage enclosures of  FIG. 1 . 
         FIG. 5  is a flowchart of a procedure that may be used with the rack mount system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Described below is a technique for use in managing loop interface failure, particularly for use in identifying a drive with a bad transmitter. On a loop interface, if a drive has a bad transmitter, the drive can disable an entire data storage enclosure and/or drives after the bad drive. As described below, use of the technique helps identify the drive with the bad transmitter and therefore improves the chances of keeping other drives and other components available on the loop. In accordance with the technique, after a loop interface failure, all the drives on the loop are removed from the loop and one drive is added at a time in order to the loop. If adding a drive causes the loop to start to fail again, the previous drive on the loop may be put in bypass mode as described below. If the loop is operable after such previous drive is put in bypass mode, it is determined that the drive with a bad transmitter has been correctly identified, and that drive is kept in bypass mode. The rest of the drives can then be added to the loop, and this helps prevent a data unavailability/data loss scenario. 
     Previously, when it was suspected that a drive was bad such that the entire loop was disabled, all the drives on the loop were removed from the loop and one drive was added at a time to the loop as described above, but subsequent steps only handled the case of a drive with a bad receiver, not a bad transmitter. Therefore, previously, the drive with the bad transmitter was added successfully, but the loop would fail when the subsequent drive was added to the loop, and it would be determined that the subsequent drive had a bad receiver and thus needed to be put in bypass mode. Accordingly, previously, the subsequent (good) drive was put in bypass mode, and the next drive was added, but adding that drive would also result in loop failure, causing that next drive also to be put in bypass mode. In this way, previously, all drives after the drive with the bad transmitter would be put in bypass mode but the actual bad-transmitter drive itself would not be put in bypass mode. 
     Therefore, use of the technique helps cause the actual drive with the bad transmitter to be put in bypass mode rather than the good drives, thus helping to prevent a data unavailability/data loss scenario. 
     Referring to  FIG. 1  of the present application, there is shown an example of a storage system  10  in which the present invention may be employed. A rack mount cabinet  12  includes several storage enclosures  14 . Each storage enclosure  14  includes several disk drives  16 . The disk drives and the enclosures are preferably interconnected via a serial bus loop or ring architecture, e.g., Fibre Channel Arbitrated Loop (FC-AL). In  FIG. 2  there is shown a rear view of the rack mount cabinet  12  and the storage enclosure  14 . Each storage enclosure includes two power supplies  18 , and two link control cards (“LCCs”)  20 . The power supplies  18  and link control cards  20  are coupled to the disk drives  16  via a midplane within the chassis (not shown in  FIG. 2 ). The link control card  20  serves to interconnect the disks and enclosures on the FC-AL. 
     Each link control card  20  includes a primary port  22  and an expansion port  24 . These ports are used to link the storage enclosures together on a single FC-AL. A cable  26  may come from a host or from another storage system, and plugs into the primary port  22 . The FC-AL extends from the primary port  22 , is coupled to the disk drives  16 , and continues out the expansion port  24 . A cable  28  couples the expansion port  24  of a first storage enclosure  14  to the primary port  22  of a second storage enclosure  14 . All the storage enclosures  14  are interconnected in this manner in a daisy chain to form the FC-AL. Thus, all the disk drives  16  are interconnected on the same FC-AL. 
     Each link control card  20  is capable of controlling all the disks  16  in a given enclosure. 
       FIG. 3  illustrates communication among drives  16 , midplane  30 , LCCA and LCCB  20 , and storage processors (SP)  80 . In at least some embodiments, storage processors  80  are controllers within the storage system that control the storage system&#39;s access to the storage enclosure, and are configured to communicate with each of drives  16  (exemplified by drive  0  in  FIG. 3 ) over respective Fibre Channel links (loops)  74 ,  70 , and over respective diplexing links  76 ,  72  as described in U.S. Pat. No. 5,901,151 to Bleiweiss, et al. entitled “System for orthogonal signal multiplexing”, which is hereby incorporated herein by reference in its entirety. 
     The system may include a diagnostic section (which may be included within the SPs) which regularly polls the enclosures at a rate of typically 500 milliseconds, and can communicate with an enclosure not yet added to the FC-AL, e.g., by using the diplexing links. In a particular example, such communication could use a diplexed signal which is a transmission comprising two distinct signals that have been modulated together and transmitted over a single transmission wire. The signals are generally transmitted at frequencies and may also be transmitted at different voltage levels. One example of a diplexed signal is the piggybacking of an RS232 protocol signal over Fibre Channel protocol signal lines, which may be done in storage area networked environments. The RS232 protocol is a standard for serial transmission of data between two devices, normally carrying between ±5V and ±12V on both data and control signal lines. The Fibre Channel signals generally have a lower voltage. The diplexed signals are typically separated at their destinations by a filter device into the RS232 and Fibre Channel signals, and forwarded as appropriate. 
       FIG. 4  illustrates an example implementation of system  10  having enclosures  14  (specifically  14 X,  14 Y) in communication with SPs  80  (specifically  80 A,  80 B) using FC-AL loops  74 ,  70 . (Mechanically, SPs  80 A,  80 B may or may not be included in one of enclosures  14 X,  14 Y.) Enclosure  14 X has LCCs  20 AX,  20 BX and disks  16 X- 0  through  16 X- 14 , and enclosure  14 Y has LCCs  20 AY,  20 BY and disks  16 Y- 0  through  16 Y- 14 . Loop  74  allows SP  80 A to communicate with disks  16 X- 0  through  16 X- 14  via LCC  20 AX, and with disks  16 Y- 0  through  16 Y- 14  via LCC  20 AY. Loop  70  allows SP  80 B to communicate with disks  16 X- 0  through  16 X- 14  via LCC  20 BX, and with disks  16 Y- 0  through  16 Y- 14  via LCC  20 BY. 
     Each of disks  16 X- 0  through  16 X- 14  and  16 Y- 0  through  16 Y- 14  has a FC receiver and a FC transmitter for each loop connection. For example, disk  16 X- 0  has receiver  102  and transmitter  104 . Thus, a communication directed from SP  80 A to disk  16 Y- 14  is first received at disk  16 X- 0  by receiver  102  and is transmitted by transmitter  104  to disk  16 X- 1 , which in turn passes the communication along to disk  16 X- 2 , and so on. The communication passes between enclosures when disk  16 X- 14  passes it to disk  16 Y- 0 , which in turn passes it to disk  16 Y- 1 , and so on. Ultimately the communication reaches its destination, disk  16 Y- 14 . As shown in  FIG. 4 , a communication directed from disk  16 Y- 14  to SP  80 A travels directly to SP  80 A since there are no disks between disk  16 Y- 14  and SP  80 A in that direction on the loop. 
     With respect to each loop, one or more disks or enclosures may be set, e.g., using the diplex signals, to a bypass mode such that FC signals travel along the loop as if the disks or enclosures were not on the loop. For example, if enclosure  14 Y is set to bypass mode, a communication directed from disk  16 X- 14  to SP  80 A travels directly to SP  80 A since there are no disks between disk  16 X- 14  and SP  80 A in that direction on the loop when enclosure  14 Y is not on the loop. In another example, if disk  16 X- 2  is set to bypass mode, a communication directed from SP  80 A to disk  16 X- 4  passes directly from disk  16 X- 1  to disk  16 X- 3  on its way to disk  16 X- 4 . 
     Bypass mode only affects FC signals, not diplex signals. 
     A characteristic of an FC loop is that if any device, e.g., LCC, drive, or cable, on the loop has a problem passing communications along the loop, it is possible for the problem to disable the entire loop (i.e., cause the entire loop to fail) so that none of the devices on the loop is discoverable or can receive or transmit FC communications, i.e., the loop ends up completely broken. 
     Diplex signaling works largely independently of FC communications, such that devices may still be discoverable via diplex signaling even if the FC loop is broken. For example, with respect to the system of  FIG. 4 , if the FC loop is working, the same number of disks, i.e., 30 disks, should be discovered both by diplex signal discovery and by FC discovery. In another example with respect to  FIG. 4 , if the FC loop is working but enclosure  14 Y is set to bypass mode, diplex signal discovery should still discover 30 disks but FC discovery will discover only 15 disks (the disks of enclosure  14 X). Similarly with respect to  FIG. 4 , if the FC loop is working and only disk  16 X- 2  is set to bypass mode, diplex signal discovery should still discover 30 disks but FC discovery will discover only 29 disks. 
     If a device (e.g., drive) on the loop has a problem with its transmitter or receiver, this problem may prevent the device from passing communications along the loop and therefore may disable the entire loop unless the device is bypassed. If the entire loop is disabled due to this problem, FC discovery will fail completely, and therefore its results will not match the results of diplex signal discovery. 
     When a problem exists that causes the entire loop to be disabled, technique including a trial and error process is to be used to help identify the device having the problem, so that the device can be put in bypass mode to prevent the problem from disabling the entire loop. If a device has a bad receiver, the loop will become disabled when the device is added. If a device has a bad transmitter, the loop will become disabled when the next device is added. 
       FIG. 5  illustrates a procedure which is an example implementation of the technique. 
     It is determined that the loop has failed (step  510 ). 
     All enclosures on the loop are put in bypass mode (step  520 ). 
     Enclosures are added back onto the loop one by one until the loop fails (step  530 ). (A device is added back by changing it out of bypass mode.) 
     The last enclosure added is initially identified as the enclosure containing the problem (step  540 ). 
     All disks in the identified enclosure are put in bypass mode (step  550 ). The enclosure is added back onto the loop (step  560 ). 
     If the loop fails with all the disks in the enclosure in bypass mode, the last drive on the previous enclosure is put in bypass mode; if the loop still fails, it is determined that the problem is not a disk problem (e.g., is a cable or an LCC problem), the identified enclosure is put in bypass mode, and the procedure terminates; if the loop does not still fail, the last drive on the previous enclosure is left in bypass mode, and the procedure proceeds from step  590  (step  565 ). 
     Disks in the enclosure, starting with the first disk on the loop in the enclosure, are added one by one in order back onto the loop until the loop fails (step  570 ). 
     The last disk added is initially identified as the source of the problem, and is put in bypass mode (step  580 ). This is because the last disk added is initially identified as having a bad receiver. 
     Disks in the enclosure, starting with the next disk on the loop in the enclosure, are added one by one in order back onto the loop until the loop fails or all remaining disks have been added back (step  590 ). 
     If all remaining disks have been added back, the procedure terminates (step  595 ). 
     If the loop has failed, the last disk added is put in bypass mode (step  600 ). 
     If the number of consecutive disks put in bypass mode is not excessive (e.g., is not more than 3), the procedure returns to step  590  (step  610 ). 
     If the number of consecutive disks put in bypass mode is excessive, the source of the problem is determined to be the disk before the disk initially identified (step  620 ). This is because it is determined that the problem is not bad receivers on all the consecutive disks; rather, a bad transmitter on such earlier disk which prevents such earlier disk from properly communicating with any subsequent disk on the loop. Thus, the last known good disk is determined to be the disk before the disk determined to have a bad transmitter. 
     The disk determined to have a bad transmitter is put in bypass mode (step  630 ). This applies even if the disk determined to have a bad transmitter is in another (previous) enclosure. In such a case, unless this disk truly having a bad transmitter is put in bypass mode, no enclosures can be added after this disk on the loop, and the inability to add such enclosures can cause data unavailability and/or data loss. 
     If the loop does not fail, the procedure returns to step  590  (step  640 ). 
     If the loop still fails, it is determined that this is not a disk problem, the disk determined to have a bad transmitter is probably not bad after all, so the disk is added back onto the loop, and the enclosure is put in bypass mode (step  650 ). 
     Thus, for example, with reference to  FIG. 4 , if loop  74  fails, the procedure may proceed as follows. Enclosures  14 X,  14 Y are put in bypass mode. Enclosure  14 X is added with all its disks in bypass mode. All of the disks of enclosure  14 X are added back one by one onto the loop, without the loop failing. Thus it is determined that the problem is not in enclosure  14 X. Enclosure  14 Y is added with all its disks in bypass mode. The disks of enclosure  14 Y are added back one by one onto the loop until the loop fails when disk  16 Y- 2  is added. Disk  16 Y- 2  is put in bypass mode. All of the remaining disks of enclosure  14 Y are added back one by one onto the loop without the loop failing. Thus, in this example, the problem was with the receiver of disk  16 Y- 2 . 
     A variation of this example is the same up to the point when disk  16 Y- 2  is put in bypass mode, but differs thereafter. When disk  16 Y- 3  is added back onto the loop, the loop fails again. Disk  16 Y- 3  is put in bypass mode. When disk  16 Y- 4  is added back onto the loop, the loop fails again. Disk  16 Y- 4  is put in bypass mode. When disk  16 Y- 5  is added back onto the loop, the loop fails again. Disk  16 Y- 5  is put in bypass mode. Thus it is determined that disk  16 Y- 1  has a bad transmitter, and that the last good disk is disk  16 Y- 0 . Disk  16 Y- 1  is put in bypass mode. All of the remaining disks of enclosure  14 Y are added back one by one onto the loop without the loop failing. 
     In at least some implementations, the technique may be executed in software running on an operating system of system  10  which provides a mechanism for invoking diplex signals to cause drives and enclosures in bypass mode. In at least one implementation of system  10 , to cause a drive to be put in bypass mode, a diplex command is issued causing a register on an LCC to be written to, which closes a corresponding port to the drive so that the FC signal does not reach the drive. 
     Other embodiments are within the scope of the following claims. For example, the technique may be used for non-FC types of loop architectures.