Abstract:
Obtaining mirrored data so that the original data can be recovered after failure without transmitting the entire mirrored data between computers. A write request at a primary computer is stored in a delay buffer and a copy is transmitted to a backup computer, where it is stored in a delta queue. The backup computer executes the copy of the write request to the mirrored data and transmits an acknowledgement to the primary computer that the copy of the write request has been received. In response to the acknowledgement, the primary computer executes the write request stored in the delay buffer. The computers send to each other subsequent acknowledgements of the write request execution, enabling the computers to delete the write requests. If the primary computer fails, the primary computer can recover the original data by receiving only the copies of write requests that remain stored in the delta queue.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/455,072, filed Dec. 6, 1999, now U.S. Pat. No. 6,338,126 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to data storage associated with computers and data processing systems. Specifically, the present invention relates to methods used to recover from a computer failure in a system having a plurality of computer systems, each with its own mass storage device. 
     2. Background and Related Art 
     Computer networks have greatly enhanced mankind&#39;s ability to process and exchange data. Unfortunately, on occasion, computers partially or completely lose the ability to function properly in what is termed a “crash” or “failure”. Computer failures may have numerous causes such as power loss, computer component damage, computer component disconnect, software failure, or interrupt conflict. Such computer failures can be quite costly as computers have become an integral part of most business operations. In some instances, computers have become such an integral part of business that when the computers crash, business operation cannot be conducted. 
     Almost all larger businesses rely on computer networks to store, manipulate, and display information that is constantly subject to change. The success or failure of an important transaction may turn on the availability of information which is both accurate and current. In certain cases, the credibility of the service provider, or its very existence, depends on the reliability of the information maintained on a computer network. Accordingly, businesses worldwide recognize the commercial value of their data and are seeking reliable, cost-effective ways to protect the information stored on their computer networks. In the United States, federal banking regulations also require that banks take steps to protect critical data. 
     One system for protecting this critical data is a data mirroring system. Specifically, the mass memory of a secondary backup computer system is made to mirror the mass memory of the primary computer system. Write requests executed in the primary mass memory device are transmitted also to the backup computer system for execution in the backup mass memory device. Thus, under ideal circumstances, if the primary computer system crashes, the backup computer system may begin operation and be connected to the user through the network. Thus, the user has access to the same files through the backup computer system on the backup mass memory device as the user had through the primary computer system. 
     However, the primary computer system might crash after a write request is executed on the primary mass memory device, but before the request is fully transmitted to the backup computer system. In this case, a write request has been executed on the primary mass memory device without being executed on the backup mass memory device. Thus, synchronization between the primary and backup mass memory devices is lost. In other words, the primary and backup mass memory devices are not perfectly mirrored, but are slightly different at the time of the crash. 
     To illustrate the impact of this loss in synchronization, assume that the primary and backup mass memory devices store identical bank account balances. Subsequently, a customer deposits money into an account and then shortly thereafter changes his mind and withdraws the money back from the account. The primary computer system crashes just after the account balance in the primary mass memory device is altered to reflect the deposit, but before the write request reflecting the deposit is transferred to the backup computer system. Thus, the account balance in the backup mass memory device does not reflect the deposit. When the customer changes his mind and withdraws the money back out from the account, the account balance in the backup memory device is altered to reflect the withdrawal. When the primary computer system is brought back into operation, the account balance from the backup mass memory device is written over the account balance in the primary mass memory device. Thus, the account balance reflects the withdrawal, but does not reflect the deposit. 
     Another disadvantage of this system is that when that primary computer system is brought back into operation, the entire backup mass storage device is copied back to the primary mass storage device in what is termed a “remirror”. The copying of such large amounts of data can occupy a significant time and be disruptive to transactional operations. 
     Therefore, a backup computer system and method are desired that do not result in the above-described loss of synchronization, and that do not require a complete remirror. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and system are provided in which data from a primary computer system is mirrored in a secondary backup computer system. This system maintains complete synchronization between the primary and backup memory devices even should the primary computer system fail after a write request was executed in the memory of the primary computer system, but before the request is fully transmitted to the backup computer system. 
     For each write request, a copy of the request is written into a delay buffer associated with the primary computer system, and a copy is transmitted to the backup computer system. After the write request has been fully transmitted to the backup computer system, the backup computer system informs the primary computer system (e.g., by sending an acknowledgement signal) that the request has been received at the backup computer system. The write request in the delay buffer of the primary computer system is executed only after the primary computer system receives the acknowledgement signal indicating that the backup computer system also received a copy of the write request. Thus, if the primary computer system fails before a copy of the write request is transmitted to the backup computer system, the primary computer system will not have executed the write request since the write request was left unexecuted in the delay buffer. Therefore, synchronization is not lost between the primary and backup computer systems. 
     Another advantage of this invention is that complete remirroring (i.e., recopying) of data from the backup computer system to the primary computer system is not needed when the primary computer system is brought back into operation after a failure. Both the primary and backup computer systems have a memory queue to which a copy of the write request is forwarded. When the primary computer system determines that the write request has been executed in the memory device of the backup computer system, the primary computer system deletes that request from its memory queue. Likewise, when the backup computer system determines that the primary computer system has executed the write request, the backup computer system deletes the write request from its memory queue. Thus, the memory queue includes write requests which have been generated, but which are not confirmed to have been executed by the opposite computer system. 
     Should the opposite computer system experience a failure, the memory queue will accumulate all the write requests that need to be executed within the failed computer system to once again mirror the memory of the operational computer system. Only the write requests in the memory queue, rather than the entire memory, are forwarded to the failed computer system once it becomes operational. Thus, complete remirroring is avoided. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a schematic drawing of a network configuration that represents a suitable operating environment for the invention; 
     FIG. 2 is a more detailed drawing of the network configuration of FIG. 1; 
     FIG. 3 is a flowchart of a method for synchronizing the primary and backup mass memory devices of FIGS. 1 and 2; and 
     FIG. 4 is a flowchart of an alternative method for synchronizing the primary and backup mass memory devices of FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a computer configuration  100  that represents a suitable operating environment for the invention. The configuration  100  includes two computer systems  110 ,  120 , both running a computer server operating system such as Novell NetWare®. The backup computer system  120  monitors the primary computer system  110  to verify that the primary computer system  110  is operational. Should the primary computer system  110  cease to operate, the backup computer system  120  takes over operations. 
     The primary computer system  110  includes a computer  112  connected to a network  101  through an interface  111  and its associated software. The computer  112  is connected to a mass storage device  114  through a mass storage controller  113  and its associated software. In the case of Novell NetWare®, the computer  112  may be a standard PC-compatible computer, the network  101  may be an Ethernet, and the mass storage device  114  may be a SCSI or IDE magnetic disk. The network interface  111  may be an Ethernet network interface and the mass storage controller  113  may be a SCSI or IDE magnetic disk controller. Network  101  could also be implemented using a token ring, Arcnet, or any other network technology. 
     The backup computer system  120  has components which can be similar to computer system  110 . For example, a computer  122  can be connected to the network  101  through a network interface  121 , although it is not necessary for computer  122  to be connected to the network  101  as long as there is available some means for communication between the computers  112  and  122 . Computer  122  is connected to a backup mass storage device  124  through a mass storage controller  123 . 
     While it is not necessary for the computer system  120  to have identical components to the computer system  110 , many times that will be the case. In other cases, the computer system  120  may be an older, slower system previously used as a filer server but replaced with the computer system  110 . All that is required of computer system  120  is that it be capable of running the file server operating system in case of the failure of computer system  110 , and that its mass memory  124  be of sufficient capacity to hold that data mirrored from the mass storage device  114 . In this description and in the claims, “primary” means associated with the primary computer system  110 , and “backup” means associated with the backup computer system  120 . The term “backup” is used herein to conveniently distinguish certain elements and components from “primary” components, and does not necessarily require full, traditional backup capabilities other than those specifically enumerated herein. Indeed, in one embodiment, the primary computer system  110  and the backup computer system  120  can be interchangeable, in that backup computer system  120  can be used as desired to provide network services to network  101  and can exhibit the functionality described herein in reference to primary computer system, and vice versa. 
     U.S. Pat. No. 5,978,565, entitled “Method for Rapid Recovery From a Network File Server Failure Including Method for Operating Co-Standby Servers,” is incorporated herein by reference and discloses components that correspond generally to those of FIG. 1 of the present application, and which can be adapted as taught herein to perform the functionality and operations associated with the present invention. 
     The primary and backup mass storage devices  114 ,  124  of the invention may include any mass memory capable of handling the read and write requests of the computer systems  110 ,  120 . Such memories may include optical disks, magnetic tape drives, magnetic disk drives, and the like. 
     A communication means  102  provides a link between the primary computer system  110  and the backup computer system  120 . Primary computer  112  is connected to the communication means  102  through a primary communication means attachment  115 , and the backup computer  122  is connected to the communication means  102  through a backup communication means attachment  125 . Communication means  102  can be implemented using a variety of techniques, well known to those skilled in the art. In one embodiment, a high-speed serial point-to-point link is used. Alternatively, the serial communication ports of the computers  112 ,  122  are used after being programmed to run at a high data rate. As another alternative, the parallel ports of the computers  112 ,  122  are used. 
     The communication means  102  provides data transfer at rates comparable to the data transfer rate of the mass storage device  124  so that the communication means  102  does not limit the performance of the configuration  100 . The method of this invention is not dependent on the particular implementation of the communication means  102 , although a communication means  102  dedicated only to the method of the invention will generally result in more efficient operation and simpler programs. 
     FIG. 2 shows a more detailed schematic diagram of the configuration  100  of FIG. 1 in which the primary computer  112  includes an I/O module  211  and mirroring code  212 . The primary mass storage device  114  includes a delta queue  213 , a delay buffer  214 , and a memory portion  215 ; and the backup mass storage device  124  includes a delta queue  223  and a memory portion  225 . The interrelationship of these components may best be understood by describing the operation of the network configuration  100 . 
     A read operation is performed by the primary computer  112  issuing a read request through the primary mass storage controller  113  to the primary mass storage device  114 . The corresponding data is transmitted from the primary mass storage device  114  to the primary computer  112 . If the backup computer system  120  is operating instead, the backup computer  122  issues a read request through the backup mass storage controller  123  to the backup mass storage device  124 . 
     A write operation in accordance with the invention may be performed as shown in the flow chart of FIG.  3 . In this description and in the claims, a write operation (or request) includes any operation (or request) that alters mass memory such as a write, delete, destructive read, or initialization. 
     A method in accordance with the invention will now be described in detail with respect to FIGS. 2 and 3. First, the I/O module  211  of the primary computer  112  provides a write request REQ to the mirroring code  212  (step  305  of FIG.  3 ). The mirroring code  212  then duplicates the request REQ (step  310 ) and causes a copy of the request REQ to be forwarded to the primary mass storage controller  113  (step  315 ). The mirroring code  212  also causes another copy of the request REQ to be forwarded to the primary communication means attachment  115  (step  320 ). Each copy is to be executed on the corresponding mass storage device  114 ,  124  so that mass storage devices  114 ,  124  are synchronized. 
     The primary mass storage controller  113  writes the request REQ to the primary delta queue  213  of the primary mass storage device  114  (step  325 ). The primary delta queue  213  includes requests that are not confirmed by the primary computer system  110  to have been executed in the backup computer system  120 . If the primary computer system  110  receives confirmation or learns by other means that the request was executed in the backup mass storage device  124 , the request is deleted from the primary delta queue  213  of the primary mass storage device  114  as described further below. The primary mass storage controller  113  also writes the request REQ to the delay buffer  214  of the primary mass storage device  114  (also step  325 ). 
     A copy of the request REQ is forwarded from the primary communication means attachment  115  over the communication means  102  to the backup communication means attachment  125  (step  330 ). The request REQ is then forwarded from the backup communication means attachment  125  through the backup mass storage controller  123  (step  335 ) and to the backup delta queue  223  (step  340 ). The delta queue  223  includes requests that are not confirmed by the backup computer system  120  to have been executed in the primary computer system  110 . If the backup computer system  120  receives confirmation or learns by other means that the request was executed in the primary mass storage device  114 , the request is deleted from the backup delta queue  223 . 
     As soon as the request REQ is received in the backup delta queue  223 , the backup computer system  120  sends an acknowledgement signal ACK 1  back to the delay buffer  214  in the primary mass storage device  114  (step  345 ). Thus, the acknowledgement signal ACK 1  indicates that the backup computer system  120  has properly received the write request REQ. Upon receipt of the acknowledgement signal ACK 1 , the primary computer system  110  executes the request REQ stored in the delay buffer  214  by performing the associated operation in the memory portion  215  of the primary mass storage device  114  (step  350 ). Thus, the primary computer system  110  does not execute a write request until it has confirmation that the backup computer system  120  has received a copy of the write request. Hence, there are no synchronization problems caused a primary computer system  110  failure after the write request REQ has been executed in the primary mass storage device  114 , but before a copy of the write request REQ has been fully transmitted to the backup computer system  120 . 
     Also after a copy of the request REQ is sent to the backup delta queue  223  (step  340 ), the request REQ is executed in the memory portion  225  of the backup mass storage device  124  (step  355 ). Another acknowledgement signal ACK 2  is then transmitted from the backup computer system  120  to the primary computer system  110  (step  365 ) indicating that the copy of the write request REQ has been executed by the backup computer system  120 . Once the primary computer system  110  receives the second acknowledgement signal ACK 2  (step  360 ), the primary computer system  110  deletes the request REQ from the primary delta queue  213  (step  370 ). The primary delta queue  213  thus includes all requests that have been sent to the primary mass storage device  114  for execution, but which are not confirmed to have been executed in the backup mass storage device  124 . 
     During normal operation of the backup computer system  120 , write requests in the primary delta queue  213  are steadily deleted as the write requests are executed in the backup mass storage device  124 . Should the backup computer system  110  shut down such that the stream of write requests is no longer being executed in the backup mass storage device  124 , the write requests will accumulate in the primary delta queue  213 . When the backup computer system  120  becomes operational again, the accumulated write requests in the primary delta queue  213  are transmitted to the backup computer system  120  for execution to bring the backup mass storage device  124  back into synchronization with the primary mass storage device  114 . 
     After the request REQ is executed in the primary main memory  215  (step  350 ), a third acknowledgement signal ACK 3  is transmitted from the primary computer system  110  to the backup computer system  120  (step  365 ) indicating that the request REQ has been executed by the primary computer system  110 . The request REQ is then deleted from the backup delta queue  223 . The backup delta queue  223  thus includes all requests that have been sent to the backup mass storage device  124  for execution, but which are not confirmed to have been executed in the primary mass storage device  114 . 
     During normal operation of the primary computer system  110 , write requests in the backup delta queue  223  are steadily deleted as the write requests are executed in the primary mass storage device  114 . Should the primary computer system  110  shut down such that the stream of write requests are no longer being executed in the primary mass storage device  114 , the write requests will accumulate in the backup delta queue  223 . When the primary computer system  110  becomes operational again, the accumulated write requests in the backup delta queue  223  are transmitted to the primary computer system  110  for execution to bring the primary mass memory device  114  back into synchronization with the backup mass memory device  124 . 
     Thus, synchronization is maintained between the mass storage devices  114 ,  124  even should the primary computer system  110  shut down before the request REQ is transmitted to the backup computer system  120 . Furthermore, only the requests in the backup delta queue  223  need to be transmitted upon the primary computer system  110  becoming operational. Likewise, only the requests in the primary delta queue  213  need to be transmitted upon the backup computer system  120  becoming operational. Thus, complete remirroring of the data after one of the computer systems  110 ,  120  becomes operational is avoided. 
     It is noted that the delta queue  213 , the delay buffer  214  and memory portion  215  may all be located within the same memory component or may be implemented in separate memory components as desired. Also, the delta queue  223  and the memory portion  225  may also be implemented in the same or different memory component as desired. 
     The foregoing description relates to a method in which each computer system  110 ,  120  confirms that the opposite computer system  120 ,  110  has executed the request by receiving acknowledgement signals ACK 2  and ACK 3 , respectively. However, other confirmation methods are possible. 
     FIG. 4 shows a flow chart of an alternate synchronization method in which acknowledgement signals ACK 2  and ACK 3  are not used. Steps  305 ,  310 ,  315 ,  320 ,  325 ,  330 ,  335 ,  340 ,  345 ,  350  and  355  are the same in FIG. 4 as they are in FIG.  3 . In FIG. 4, the primary computer system  110  waits during a predetermined time period (e.g., five seconds or any other suitable amount of time) after the acknowledgement signal ACK 1  is received (step  405 ). During this time period, if no incident report is received by the primary computer system  110  indicating that the backup computer system  120  has failed, then the primary computer system  110  assumes that the backup computer system  120  executed the request REQ in the backup mass storage device  124 . In this case, the primary computer system  110  deletes the request REQ from the primary memory queue  213  after the predetermined time period (also step  405 ). 
     Likewise, the backup computer system  120  waits during a predetermined time period after the request REQ is received (step  410 ). During this time period, if no incident report is received in the backup computer system  120  indicating that the primary computer system  110  has failed, then the backup computer system  120  assumes that the primary computer system  110  executed the request REQ in the primary mass storage device  114 . In this case, the backup computer system  120  deletes the request REQ from the backup delta queue  223  after the predetermined time period (also step  410 ). Thus, confirmation is achieved by assuming that the opposite computer system executed the request if the opposite computer system is still operational after a predetermined time period. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.