Patent Publication Number: US-7908515-B1

Title: Methods and apparatus for action regulation for continuous data replication systems

Description:
BACKGROUND 
     Computer data is vital to today&#39;s organizations, and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data. 
     Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. Such systems suffer from several drawbacks. First, they require a system shutdown during backup, since the data being backed up cannot be used during the backup operation. Second, they limit the points in time to which the production site can recover. For example, if data is backed up on a daily basis, there may be several hours of lost data in the event of a disaster. Third, the data recovery process itself takes a long time. 
     Another conventional data protection system uses data replication, by creating a copy of the organization&#39;s production site data on a secondary backup storage system, and updating the backup with changes. The backup storage system may be situated in the same physical location as the production storage system, or in a physically remote location. Data replication systems generally operate either at the application level, at the file system level, or at the data block level. 
     Current data protection systems try to provide continuous data protection, which enable the organization to roll back to any specified point in time within a recent history. Continuous data protection systems aim to satisfy two conflicting objectives, as best as possible; namely, (i) minimize the down time, in which the organization production site data is unavailable, during a recovery, and (ii) enable recovery as close as possible to any specified point in time within a recent history. 
     Continuous data protection typically uses a technology referred to as “journaling,” whereby a log is kept of changes made to the backup storage. During a recovery, the journal entries serve as successive “undo” information, enabling rollback of the backup storage to previous points in time. Journaling was first implemented in database systems, and was later extended to broader data protection. 
     One challenge to continuous data protection is the ability of a backup site to keep pace with the data transactions of a production site, without slowing down the production site. The overhead of journaling inherently requires several data transactions at the backup site for each data transaction at the production site. As such, when data transactions occur at a high rate at the production site, the backup site may not be able to finish backing up one data transaction before the next production site data transaction occurs. If the production site is not forced to slow down, then necessarily a backlog of un-logged data transactions may build up at the backup site. Without being able to satisfactorily adapt dynamically to changing data transaction rates, a continuous data protection system chokes and eventually forces the production site to shut down. 
     Another challenge in continuous data replication systems is to avoid entering a loop of repetitive actions that can negatively impact a production site or other system. For example, a system can constantly transition between a first state in which the system has reacted to a detected problem and a second state in which the system has recovered from the problem. If the problem keeps occurring the system can become stuck in a loop. 
     SUMMARY 
     The present invention concerns methods and systems for providing continuous data replication under dynamically changing I/O rates, by implementing an optimized asynchronous journaling policy that adapts dynamically to changing data transaction rates. A nominal journaling policy operates asynchronously, allowing production site data transactions to proceed prior to completion of logging of previous data transactions. The nominal journaling policy changes to an accelerated policy when a backlog of data transactions exceeds a predetermined size, and changes back to the nominal policy when the backlog is suitably relieved. The nominal journaling policy includes five stages of I/O, and the accelerated journal policy omits two of the five stages. 
     In addition, the present invention uses a near real-time journaling policy, when the data transaction rate at the production site is slow enough to enable the backup site to keep pace without need for a journal backlog buffer. The near real-time policy is used until the data transaction rate increases to the point that the journal backlog buffer is necessary. 
     There is thus provided in accordance with an embodiment of the present invention a method for continuous data replication, including receiving a write transaction from a production site, the write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ 1 , and a start address for writing the raw data, designated ADD 1 , copying raw data RWNEW into a first journal data stream, starting at a current journal address, designated JADD 1 , and copying size indicator SZ 1 , start address ADD 1  and journal address JADD 1  into a first journal meta-data stream, at the end of the first journal meta-data stream. 
     There is moreover provided in accordance with an embodiment of the present invention a method for continuous data replication, including receiving a write transaction from a production site, the write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ, and a start address for writing the raw data, designated ADD, reading raw data, designated RWOLD, from a storage unit beginning at start address ADD and extending to the size of size indicator SZ, copying raw data RWOLD to a journal data stream at a current journal address, designated JADD, copying size indicator SZ, start address ADD, and journal address JADD into a journal meta-data stream, at the end of the journal meta-data stream, and copying raw data RWNEW to the storage unit, beginning at start address ADD. 
     There is further provided in accordance with an embodiment of the present invention a method for continuous data replication, including receiving a plurality of write transactions from a production site, each write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ 1 , and a start address for writing the raw data, designated ADD 1 , if the plurality of write transactions are received at a high I/O rate, then copying raw data RWNEW into a first journal data stream, starting at a current journal address, designated JADD 1 , and copying size indicator SZ 1 , start address ADD 1  and journal address JADD 1  into a first journal meta-data stream, at the end of the first journal meta-data stream, else reading raw data, designated RWOLD, from a storage unit beginning at start address ADD 1  and extending to the size of size indicator SZ 1 , copying raw data RWOLD to a second journal data stream at a current journal address, designated JADD 2 , copying size indicator SZ 1 , start address ADD 1 , and journal address JADD 2  into a second journal meta-data stream, at the end of the second journal meta-data stream, and copying raw data RWNEW to the storage unit, beginning at start address ADD 1 . 
     There is yet further provided in accordance with an embodiment of the present invention a computer-readable storage medium storing program code for causing a computing device to receive a write transaction from a production site, the write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ 1 , and a start address for writing the raw data, designated ADD 1 , copy raw data RWNEW into a first journal data stream, starting at a current journal address, designated JADD 1 , and copy size indicator SZ 1 , start address ADD 1  and journal address JADD 1  into a first journal meta-data stream, at the end of the first journal meta-data stream. 
     There is additionally provided in accordance with an embodiment of the present invention a computer-readable storage medium storing program code for causing a computing device to receive a write transaction from a production site, the write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ, and a start address for writing the raw data, designated ADD, read raw data, designated RWOLD, from a storage unit beginning at start address ADD and extending to the size of size indicator SZ, copy raw data RWOLD to a journal data stream at a current journal address, designated JADD, copy size indicator SZ, start address ADD, and journal address JADD into a journal meta-data stream, at the end of the journal meta-data stream, and copy raw data RWNEW to the storage unit, beginning at start address ADD. 
     There is moreover provided in accordance with an embodiment of the present invention a computer-readable storage medium storing program code for causing a computing device to receive a plurality of write transactions from a production site, each write transaction including raw data, designated RWNEW, a size indicator for the size of the raw data, designated SZ 1 , and a start address for writing the raw data, designated ADD 1 , if the plurality of write transactions are received at a high I/O rate, then copy raw data RWNEW into a first journal data stream, starting at a current journal address, designated JADD 1 , and copy size indicator SZ 1 , start address ADD 1  and journal address JADD 1  into a first journal meta-data stream, at the end of the first journal meta-data stream, else read raw data, designated RWOLD, from a storage unit beginning at start address ADD 1  and extending to the size of size indicator SZ 1 , copy raw data RWOLD to a second journal data stream at a current journal address, designated JADD 2 , copy size indicator SZ 1 , start address ADD 1 , and journal address JADD 2  into a second journal meta-data stream, at the end of the second journal meta-data stream, and copy raw data RWNEW to the storage unit, beginning at start address ADD 1 . 
     There is further provided in accordance with an embodiment of the present invention a method for continuous data replication, including receiving a plurality of write transactions from a data production site, for application at a data backup site, dynamically selecting one of a plurality of journaling processes based on at least one processing state transition rule, wherein each of the plurality of journaling processes controls the queuing, recording and application of write transactions received at the backup site, and wherein each of the at least one processing state transition rule relates directly or indirectly to rates of incoming write transactions at the data backup site, temporarily queuing write transactions at the backup site within a queue, in accordance with the selected journaling process, recording write transactions at the backup site, for data recovery purposes, in accordance with the selected journaling process, and applying the write transactions at the backup site, in accordance with the selected journaling process. 
     There is yet further provided in accordance with an embodiment of the present invention a system for continuous data replication, including a receiver for receiving a plurality of write transactions from a data production site, for application at a data backup site, a state machine for dynamically selecting one of a plurality of journaling processes based on at least one processing state transition rule, wherein each of the plurality of journaling processes controls the queuing, recording and application of write transactions received at the backup site, and wherein each of the at least one processing state transition rule relates directly or indirectly to rates of incoming write transactions at the data backup site, a queue manager coupled with the receiver and the state machine for temporarily queuing write transactions at the backup site within a queue until they can be applied, in accordance with the selected journaling process, and a memory manager coupled with the receiver and the state machine for recording write transactions at the backup site, for data recovery purposes, in accordance with the selected journaling process, and a storage manager coupled with the receiver and the state machine for applying the write transactions at the backup site, in accordance with the selected journaling process. 
     There is additionally provided in accordance with an embodiment of the present invention a computer-readable storage medium storing program code for causing a computing device to receive a plurality of write transactions from a data production site, for application at a data backup site, select one of a plurality of journaling processes based on at least one processing state transition rule, wherein each of the plurality of journaling processes controls the queuing, recording and application of write transactions received at the backup site, and wherein each of the at least one processing state transition rule relates directly or indirectly to rates of incoming write transactions at the data backup site, temporarily queue write transactions at the backup site within a queue, in accordance with the selected journaling process, record write transactions at the backup site, for data recovery purposes, in accordance with the selected journaling process, and apply the write transactions at the backup site, in accordance with the selected journaling process. 
     In another aspect of the invention, methods and apparatus provide a continuous data replication system including journaling and replication wherein the system detect conditions under which a regulation state should be entered by monitoring actions that occur within a given time interval. With this arrangement, processing loops due to repetitive internal or external problems can be avoided. 
     In one aspect of the invention, an exemplary method, comprises, in a continuous data protection system including replication and journaling, the system having a processor and memory, detecting a first problem, reacting to the first problem, recovering from the first problem, detecting the first problem again, reacting to the first problem again, recovering from the first problem again, determining whether to enter a regulation state due to repetition of the first problem, entering the regulation state, and maintaining the regulation state after detecting the first problem again. 
     The method can further include one of more of the following features: the step of determining whether to enter the regulation state includes determining whether a first action occurred in a series of time windows more than a first number of times with a first time duration, a count of the first action occurring can only increase one during a time window in the series of time windows, the first action is a split action, the regulation state is maintained for a given time duration, and the regulated state is maintained until receiving a configuration change, 
     In a further aspect of the invention, an article comprises a storage medium comprising computer-executable instructions to enable a machine to perform: in a continuous data protection system including replication and journaling, the system having a processor and memory, detecting a first problem, reacting to the first problem, recovering from the first problem, detecting the first problem again, reacting to the first problem again, recovering from the first problem again, determining whether to enter a regulation state due to repetition of the first problem, entering the regulation state, and maintaining the regulation state after detecting the first problem again. 
     In a further aspect of the invention, an exemplary continuous data protection system comprises at least one processor, memory coupled to the at least one processor containing computer-executable instructions, a replication module, and a regulation manager module, wherein the replication module and the regulation manager module execute at least one of the computer-executable instructions, the replication module and the regulation manager module being operative to: detect a first problem, react to the first problem, recover from the first problem, detect the first problem again, react to the first problem again, recover from the first problem again, determine whether to enter a regulation state due to repetition of the first problem, enter the regulation state, and maintain the regulation state after detecting the first problem again. 
     The following definitions are employed throughout the specification and claims. 
     BACKUP SITE—a facility where replicated production site data is stored; the backup site may be located in a remote site or at the same location as the production site; 
     DPA—a computer or a cluster of computers that serve as a data protection appliance, responsible for data protection services including inter alia data replication of a storage system, and journaling of I/O requests issued by a host computer to the storage system; 
     HOST—at least one computer or networks of computers that runs at least one data processing application that issues I/O requests to one or more storage systems; a host is an initiator with a SAN; 
     HOST DEVICE—an internal interface in a host, to a logical storage unit; 
     IMAGE—a copy of a logical storage unit at a specific point in time; 
     INITIATOR—a node in a SAN that issues I/O requests; 
     JOURNAL—a record of write transactions issued to a storage system; used to maintain a duplicate storage system, and to rollback the duplicate storage system to a previous point in time; 
     LOGICAL UNIT—a logical entity provided by a storage system for accessing data from the storage system; 
     LUN—a logical unit number for identifying a logical unit; 
     PHYSICAL STORAGE UNIT—a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address; 
     PRODUCTION SITE—a facility where one or more host computers run data processing applications that write data to a storage system and read data from the storage system; 
     SAN—a storage area network of nodes that send and receive I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target; 
     SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side; 
     STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators 
     TARGET—a node in a SAN that replies to I/O requests; 
     TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side; 
     WAN—a wide area network that connects local networks and enables them to communicate with one another, such as the Internet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a simplified block diagram of a data protection system, in accordance with an embodiment of the present invention; 
         FIG. 2  is a simplified illustration of a journal history of write transactions for a storage system, in accordance with an embodiment of the present invention; 
         FIG. 3A  is a simplified illustration of a first stage of a journal and four data streams stored therein, after recording three write transactions, in accordance with an embodiment of the present invention; 
         FIG. 3B  is a simplified illustration of a second stage of a journal and four data streams stored therein, after applying a first write transactions to a storage system, in accordance with an embodiment of the present invention; 
         FIG. 3C  is a simplified illustration of a third stage of a journal history and four data streams stored therein, after applying a second write transactions to a storage system, in accordance with an embodiment of the present invention; 
         FIG. 3D  is a simplified illustration of a fourth stage of a journal history and four data streams stored therein, after rolling back a write transaction, in accordance with an embodiment of the present invention; 
         FIG. 4  is a simplified flowchart of a data protection method during a normal production mode, in accordance with an embodiment of the present invention; 
         FIG. 5  is a simplified flowchart of a data protection method during a data recovery mode, prior to completion of rollback, in accordance with an embodiment of the present invention; 
         FIG. 6  is a simplified flowchart of a data protection method during a data recovery mode, after completion of rollback, in accordance with an embodiment of the present invention; 
         FIG. 7  is a simplified illustration of a time-line for tracking new processing of old data, in accordance with an embodiment of the present invention; 
         FIG. 8  is a simplified illustration of a five-stage journaling process for continuous data replication, in accordance with an embodiment of the present invention; 
         FIG. 9  is a simplified illustration of a four-stage journaling process for continuous data replication, for use when an I/O data rate is low, in accordance with an embodiment of the present invention; 
         FIG. 10  is a simplified illustration of a three-stage journaling process for continuous data replication, for use when an I/O data rate is high, in accordance with an embodiment of the present invention; 
         FIG. 11  is a simplified state diagram of transitions between 5-stage, 4-stage and 3-stage journal processing, in accordance with an embodiment of the present invention; 
         FIG. 12  is a simplified illustration of a variant of the three-stage journaling process shown in  FIG. 10 , which may be used in an alternative embodiment of the present invention; 
         FIG. 13  is a simplified block diagram of a data protection system, in accordance with an embodiment of the present invention having action regulation; 
         FIG. 14  is a state diagram for a system without action regulation; 
         FIG. 15  is a state diagram for a system having action regulation for a repetitive internal problem; 
         FIG. 16  is a state diagram for a system having action regulation for a repetitive external problem; and 
         FIG. 17  is a flow diagram showing an exemplary sequence of steps for action regulation. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to  FIG. 1 , which is a simplified illustration of a data protection system  100 , in accordance with an embodiment of the present invention. Shown in  FIG. 1  are two sites; Site I, which is a production site, on the right, and Site II, which is a backup site, on the left. Under normal operation the production site is the source side of system  100 , and the backup site is the target side of the system. The backup site is responsible for replicating production site data. Additionally, the backup site enables rollback of Site I data to an earlier pointing time, which may be used in the event of data corruption of a disaster, or alternatively in order to view or to access data from an earlier point in time. 
     During normal operations, the direction of replicate data flow goes from source side to target side. It is possible, however, for a user to reverse the direction of replicate data flow, in which case Site I starts to behave as a target backup site, and Site II starts to behave as a source production site. Such change of replication direction is referred to as a “failover”. A failover may be performed in the event of a disaster at the production site, or for other reasons. In some data architectures, Site I or Site II behaves as a production site for a portion of stored data, and behaves simultaneously as a backup site for another portion of stored data. In some data architectures, a portion of stored data is replicated to a backup site, and another portion is not. 
     The production site and the backup site may be remote from one another, or they may both be situated at a common site, local to one another. Local data protection has the advantage of minimizing data lag between target and source, and remote data protection has the advantage is being robust in the event that a disaster occurs at the source side. 
     The source and target sides communicate via a wide area network (WAN)  128 , although other types of networks are also adaptable for use with the present invention. 
     In accordance with an embodiment of the present invention, each side of system  100  includes three major components coupled via a storage area network (SAN); namely, (i) a storage system, (ii) a host computer, and (iii) a data protection appliance (DPA). Specifically with reference to  FIG. 1 , the source side SAN includes a source host computer  104 , a source storage system  108 , and a source DPA  112 . Similarly, the target side SAN includes a target host computer  116 , a target storage system  120 , and a target DPA  124 . 
     Generally, a SAN includes one or more devices, referred to as “nodes”. A node in a SAN may be an “initiator” or a “target”, or both. An initiator node is a device that is able to initiate requests to one or more other devices; and a target node is a device that is able to reply to requests, such as SCSI commands, sent by an initiator node. A SAN may also include network switches, such as fiber channel switches. The communication links between each host computer and its corresponding storage system may be any appropriate medium suitable for data transfer, such as fiber communication channel links. 
     In an embodiment of the present invention, the host communicates with its corresponding storage system using small computer system interface (SCSI) commands. 
     System  100  includes source storage system  108  and target storage system  120 . 
     Each storage system includes physical storage units for storing data, such as disks or arrays of disks. Typically, storage systems  108  and  120  are target nodes. In order to enable initiators to send requests to storage system  108 , storage system  108  exposes one or more logical units (LU) to which commands are issued. Thus, storage systems  108  and  120  are SAN entities that provide multiple logical units for access by multiple SAN initiators. 
     Logical units are a logical entity provided by a storage system, for accessing data stored in the storage system. A logical unit is identified by a unique logical unit number (LUN). In an embodiment of the present invention, storage system  108  exposes a logical unit  136 , designated as LU A, and storage system  120  exposes a logical unit  156 , designated as LU B. 
     In an embodiment of the present invention, LU B is used for replicating LU A. As such, LU B is generated as a copy of LU A. In one embodiment, LU B is configured so that its size is identical to the size of LU A. Thus for LU A, storage system  120  serves as a backup for source side storage system  108 . Alternatively, as mentioned hereinabove, some logical units of storage system  120  may be used to back up logical units of storage system  108 , and other logical units of storage system  120  may be used for other purposes. Moreover, in certain embodiments of the present invention, there is symmetric replication whereby some logical units of storage system  108  are used for replicating logical units of storage system  120 , and other logical units of storage system  120  are used for replicating other logical units of storage system  108 . 
     System  100  includes a source side host computer  104  and a target side host computer  116 . A host computer may be one computer, or a plurality of computers, or a network of distributed computers, each computer may include inter alia a conventional CPU, volatile and non-volatile memory, a data bus, an I/O interface, a display interface and a network interface. Generally a host computer runs at least one data processing application, such as a database application and an e-mail server. 
     Generally, an operating system of a host computer creates a host device for each logical unit exposed by a storage system in the host computer SAN. A host device is a logical entity in a host computer, through which a host computer may access a logical unit. In an embodiment of the present invention, host device  104  identifies LU A and generates a corresponding host device  140 , designated as Device A, through which it can access LU A. Similarly, host computer  116  identifies LU B and generates a corresponding device  160 , designated as Device B. 
     In an embodiment of the present invention, in the course of continuous operation, host computer  104  is a SAN initiator that issues I/O requests (write/read operations) through host device  140  to LU A using, for example, SCSI commands. Such requests are generally transmitted to LU A with an address that includes a specific device identifier, an offset within the device, and a data size. Offsets are generally aligned to 512 byte blocks. The average size of a write operation issued by host computer  104  may be, for example, 10 kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB) per second, this corresponds to approximately 5,000 write transactions per second. 
     System  100  includes two data protection appliances, a source side DPA  112  and a target side DPA  124 . A DPA performs various data protection services, such as data replication of a storage system, and journaling of I/O requests issued by a host computer to source side storage system data. As explained in detail hereinbelow, when acting as a target side DPA, a DPA may also enable rollback of data to an earlier point in time, and processing of rolled back data at the target site. Each DPA  112  and  124  is a computer that includes inter alia one or more conventional CPUs and internal memory. 
     For additional safety precaution, each DPA is a cluster of such computers. Use of a cluster ensures that if a DPA computer is down, then the DPA functionality switches over to another computer. The DPA computers within a DPA cluster communicate with one another using at least one communication link suitable for data transfer via fiber channel or IP based protocols, or such other transfer protocol. One computer from the DPA cluster serves as the DPA leader. The DPA cluster leader coordinates between the computers in the cluster, and may also perform other tasks that require coordination between the computers, such as load balancing. 
     In the architecture illustrated in  FIG. 1 , DPA  112  and DPA  124  are standalone devices integrated within a SAN. Alternatively, each of DPA  112  and DPA  124  may be integrated into storage system  108  and storage system  120 , respectively, or integrated into host computer  104  and host computer  116 , respectively. Both DPAs communicate with their respective host computers through communication lines such as fiber channels using, for example, SCSI commands. 
     In accordance with an embodiment of the present invention, DPAs  112  and  124  are configured to act as initiators in the SAN; i.e., they can issue I/O requests using, for example, SCSI commands, to access logical units on their respective storage systems. DPA  112  and DPA  124  are also configured with the necessary functionality to act as targets; i.e., to reply to I/O requests, such as SCSI commands, issued by other initiators in the SAN, including inter alia their respective host computers  104  and  116 . Being target nodes, DPA  112  and DPA  124  may dynamically expose or remove one or more logical units. 
     As described hereinabove, Site I and Site II may each behave simultaneously as a production site and a backup site for different logical units. As such, DPA  112  and DPA  124  may each behave as a source DPA for some logical units, and as a target DPA for other logical units, at the same time. 
     In accordance with an embodiment of the present invention, host computer  104  and host computer  116  include protection agents  144  and  164 , respectively. Protection agents  144  and  164  intercept SCSI commands issued by their respective host computers, via host devices to logical units that are accessible to the host computers. In accordance with an embodiment of the present invention, a data protection agent may act on an intercepted SCSI commands issued to a logical unit, in one of the following ways: 
     Send the SCSI commands to its intended logical unit. 
     Redirect the SCSI command to another logical unit. 
     Split the SCSI command by sending it first to the respective DPA. After the DPA returns an acknowledgement, send the SCSI command to its intended logical unit. 
     Fail a SCSI command by returning an error return code. 
     Delay a SCSI command by not returning an acknowledgement to the respective host computer. 
     A protection agent may handle different SCSI commands, differently, according to the type of the command. For example, a SCSI command inquiring about the size of a certain logical unit may be sent directly to that logical unit, while a SCSI write command may be split and sent first to a DPA associated with the agent. A protection agent may also change its behavior for handling SCSI commands, for example as a result of an instruction received from the DPA. 
     Specifically, the behavior of a protection agent for a certain host device generally corresponds to the behavior of its associated DPA with respect to the logical unit of the host device. When a DPA behaves as a source site DPA for a certain logical unit, then during normal course of operation, the associated protection agent splits I/O requests issued by a host computer to the host device corresponding to that logical unit. Similarly, when a DPA behaves as a target device for a certain logical unit, then during normal course of operation, the associated protection agent fails I/O requests issued by host computer to the host device corresponding to that logical unit. 
     Communication between protection agents and their respective DPAs may use any protocol suitable for data transfer within a SAN, such as fiber channel, or SCSI over fiber channel. The communication may be direct, or via a logical unit exposed by the DPA. In an embodiment of the present invention, protection agents communicate with their respective DPAs by sending SCSI commands over fiber channel. 
     In an embodiment of the present invention, protection agents  144  and  164  are drivers located in their respective host computers  104  and  116 . Alternatively, a protection agent may also be located in a fiber channel switch, or in any other device situated in a data path between a host computer and a storage system. 
     What follows is a detailed description of system behavior under normal production mode, and under recovery mode. 
     In accordance with an embodiment of the present invention, in production mode DPA  112  acts as a source site DPA for LU A. Thus, protection agent  144  is configured to act as a source side protection agent; i.e., as a splitter for host device A. Specifically, protection agent  144  replicates SCSI I/O requests. A replicated SCSI I/O request is sent to DPA  112 . After receiving an acknowledgement from DPA  124 , protection agent  144  then sends the SCSI I/O request to LU A. Only after receiving a second acknowledgement from storage system  108  will host computer  104  initiate another I/O request. 
     When DPA  112  receives a replicated SCSI write request from data protection agent  144 , DPA  112  transmits certain I/O information characterizing the write request, packaged as a “write transaction”, over WAN  128  to DPA  124  on the target side, for journaling and for incorporation within target storage system  120 . 
     DPA  112  may send its write transactions to DPA  124  using a variety of modes of transmission, including inter alia (i) a synchronous mode, (ii) an asynchronous mode, and (iii) a snapshot mode. In synchronous mode, DPA  112  sends each write transaction to DPA  124 , receives back an acknowledgement from DPA  124 , and in turns sends an acknowledgement back to protection agent  144 . Protection agent  144  waits until receipt of such acknowledgement before sending the SCSI write request to LU A. 
     In asynchronous mode, DPA  112  sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     In snapshot mode, DPA  112  receives several I/O requests and combines them into an aggregate “snapshot” of all write activity performed in the multiple I/O requests, and sends the snapshot to DPA  124 , for journaling and for incorporation in target storage system  120 . In snapshot mode DPA  112  also sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     For the sake of clarity, the ensuing discussion assumes that information is transmitted at write-by-write granularity. 
     While in production mode, DPA  124  receives replicated data of LU A from DPA  112 , and performs journaling and writing to storage system  120 . When applying write operations to storage system  120 , DPA  124  acts as an initiator, and sends SCSI commands to LU B. 
     During a recovery mode, DPA  124  undoes the write transactions in the journal, so as to restore storage system  120  to the state it was at, at an earlier time. 
     As described hereinabove, in accordance with an embodiment of the present invention, LU B is used as a backup of LU A. As such, during normal production mode, while data written to LU A by host computer  104  is replicated from LU A to LU B, host computer  116  should not be sending I/O requests to LU B. To prevent such I/O requests from being sent, protection agent  164  acts as a target site protection agent for host Device B and fails I/O requests sent from host computer  116  to LU B through host Device B. 
     In accordance with an embodiment of the present invention, target storage system  120  exposes a logical unit  176 , referred to as a “journal LU”, for maintaining a history of write transactions made to LU B, referred to as a “journal”. Alternatively, journal LU  176  may be striped over several logical units, or may reside within all of or a portion of another logical unit. DPA  124  includes a journal processor  180  for managing the journal. 
     Journal processor  180  functions generally to manage the journal entries of LU B. Specifically, journal processor  180  (i) enters write transactions received by DPA  124  from DPA  112  into the journal, by writing them into the journal LU, (ii) applies the journal transactions to LU B, and (iii) updates the journal entries in the journal LU with undo information and removes already-applied transactions from the journal. As described below, with reference to FIGS.  2  and  3 A- 3 D, journal entries include four streams, two of which are written when write transaction are entered into the journal, and two of which are written when write transaction are applied and removed from the journal. 
     Reference is now made to  FIG. 2 , which is a simplified illustration of a write transaction  200  for a journal, in accordance with an embodiment of the present invention. The journal may be used to provide an adaptor for access to storage  120  at the state it was in at any specified point in time. Since the journal contains the “undo” information necessary to rollback storage system  120 , data that was stored in specific memory locations at the specified point in time may be obtained by undoing write transactions that occurred subsequent to such point in time. 
     Write transaction  200  generally includes the following fields: 
     one or more identifiers; 
     a time stamp, which is the date &amp; time at which the transaction was received by source side DPA  112 ; 
     a write size, which is the size of the data block; 
     a location in journal LU  176  where the data is entered; 
     a location in LU B where the data is to be written; and 
     the data itself. 
     Write transaction  200  is transmitted from source side DPA  112  to target side DPA  124 . As shown in  FIG. 2 , DPA  124  records the write transaction  200  in four streams. A first stream, referred to as a DO stream, includes new data for writing in LU B. A second stream, referred to as an DO METADATA stream, includes metadata for the write transaction, such as an identifier, a date &amp; time, a write size, a beginning address in LU B for writing the new data in, and a pointer to the offset in the do stream where the corresponding data is located. Similarly, a third stream, referred to as an UNDO stream, includes old data that was overwritten in LU B; and a fourth stream, referred to as an UNDO METADATA, include an identifier, a date &amp; time, a write size, a beginning address in LU B where data was to be overwritten, and a pointer to the offset in the undo stream where the corresponding old data is located. 
     In practice each of the four streams holds a plurality of write transaction data. As write transactions are received dynamically by target DPA  124 , they are recorded at the end of the DO stream and the end of the DO METADATA stream, prior to committing the transaction. During transaction application, when the various write transactions are applied to LU B, prior to writing the new DO data into addresses within the storage system, the older data currently located in such addresses is recorded into the UNDO stream. 
     By recording old data, a journal entry can be used to “undo” a write transaction. To undo a transaction, old data is read from the UNDO stream in a reverse order, from the most recent data to the oldest data, for writing into addresses within LU B. Prior to writing the UNDO data into these addresses, the newer data residing in such addresses is recorded in the DO stream. 
     The journal LU is partitioned into segments with a pre-defined size, such as 1 MB segments, with each segment identified by a counter. The collection of such segments forms a segment pool for the four journaling streams described hereinabove. Each such stream is structured as an ordered list of segments, into which the stream data is written, and includes two pointers—a beginning pointer that points to the first segment in the list and an end pointer that points to the last segment in the list. 
     According to a write direction for each stream, write transaction data is appended to the stream either at the end, for a forward direction, or at the beginning, for a backward direction. As each write transaction is received by DPA  124 , its size is checked to determine if it can fit within available segments. If not, then one or more segments are chosen from the segment pool and appended to the stream&#39;s ordered list of segments. 
     Thereafter the DO data is written into the DO stream, and the pointer to the appropriate first or last segment is updated. Freeing of segments in the ordered list is performed by simply changing the beginning or the end pointer. Freed segments are returned to the segment pool for re-use. 
     When a write transaction is received, journaling is thus advanced as indicated in TABLE I below. 
     
       
         
           
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Entering a write transaction in the journal 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Step 1 
                 The new data is written at the end of the DO stream,  
               
               
                   
                 assuming a forward write direction, and corresponding 
               
               
                   
                 metadata is written at the end of the  
               
               
                   
                 DO METADATA stream. 
               
               
                 Step 2 
                 Data is read from the beginning of the DO stream, and 
               
               
                   
                 corresponding metadata is read from the beginning  
               
               
                   
                 of the DO METADATA stream. 
               
               
                 Step 3 
                 Old data to be overwritten is read from LU B. 
               
               
                   
                 The location and size of such old data is determined  
               
               
                   
                 from the DO METADATA stream. 
               
               
                 Step 4 
                 The old data is written at the end of the UNDO stream,  
               
               
                   
                 and corresponding metadata is written at the end 
               
               
                   
                 of the UNDO METADATA stream. 
               
               
                 Step 5 
                 The new data read at Step 2 is written into LU B, 
               
               
                   
                 and the beginning and end pointers of the 
               
               
                   
                 DO and DO METADATA streams are 
               
               
                   
                 moved appropriately. 
               
               
                   
               
            
           
         
       
     
     Conversely, during a rollback to undo a write transaction, the above operations are reversed, as indicated in TABLE II below. 
     
       
         
           
               
             
               
                 TABLE II 
               
               
                   
               
               
                 Undoing a write transaction in the journal 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Step 1 
                 Read the data and metadata from the end  
               
               
                   
                 of the UNDO and UNDO METADATA streams. 
               
               
                 Step 2 
                 Read from LU B the data that is to be overwritten. 
               
               
                   
                 The location and size of such data is determined 
               
               
                   
                 from the UNDO METADATA stream. 
               
               
                 Step 3 
                 Write the data from Step 2 at the beginning 
               
               
                   
                 of the DO stream, and update the DO METADATA 
               
               
                   
                 stream accordingly. 
               
               
                 Step 4 
                 Write the data from Step 1 to LU B, and  
               
               
                   
                 update the beginning and end pointers of the UNDO 
               
               
                   
                 and UNDO metadata streams appropriately. 
               
               
                   
               
            
           
         
       
     
     The following example, in conjunction with  FIGS. 3A-3D , describes specific details of the journaling process, in accordance with an embodiment of the present invention. A journal volume includes a plurality of segments from a segment pool, each segment including 20 data blocks. 
     Three write transactions are received, as indicated in TABLE III. 
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 Example Write Transactions 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 LU B 
                   
                 Journal LU 
               
               
                 Write ID 
                 Time 
                 location 
                 Length 
                 location 
               
               
                   
               
               
                 1 
                 Dec. 3, 2005 
                 LU B 
                 15 blocks 
                 Segment 1, 
               
               
                   
                 10:00:00.00 
                 offset 57 blocks 
                   
                 offset 0 
               
               
                 2 
                 Dec. 3, 2005 
                 LU B 
                 20 blocks 
                 Segment 1, 
               
               
                   
                 10:00:00.05 
                 offset 87 blocks 
                   
                 offset 15 
               
               
                 3 
                 Dec. 3, 2005 
                 LU B 
                 20 blocks 
                 Segment 3, 
               
               
                   
                 10:00:00.18 
                 offset 12 blocks 
                   
                 Offset 15 
               
               
                   
               
            
           
         
       
     
     The following discussion describes four stages of journaling and data storage; namely, 
     Stage #1: Enter the three write transactions as journal entries in the journal LU. 
     Stage #2: Apply the first write transaction to LU B. 
     Stage #3: Apply the second write transaction to LU B. 
     Stage #4: Rollback the second write transaction, to recover data from an earlier point in time. 
     The write transaction with ID=1 is written to the first 15 blocks of Segment #1. The metadata corresponding to this transaction is written to the first block of Segment #2. The second write transaction with ID=2 is written to the last 5 blocks of Segment #1 and the first 15 blocks of Segment #3. The metadata corresponding to this transaction is written to the second block of Segment #2. The third write transaction with ID=3 is written to the last 5 blocks of Segment #3 and the first 15 blocks of Segment #4. The metadata corresponding to this transaction is written to the third block of Segment #2. 
     Thus at stage #1, the DO stream in memory includes a list of segments 1, 3, 4; and a beginning pointer to offset=0 in Segment #1 and an end pointer to offset=−10 in Segment #4. The DO METADATA stream in memory includes a list of one segment, namely Segment #2; and a beginning pointer to offset=0 in Segment #2 and an end pointer to offset=3 in Segment #2. The UNDO stream and the UNDO METADATA stream are empty. The journal and the four streams at the end of stage #1 are illustrated in  FIG. 3A . 
     At stage #2 the write transaction with ID=1 is applied to LU B. New data to be written is read from the journal LU at the offset and length indicated in the DO METADATA; namely, 15 blocks of data located in blocks 0-14 of journal volume Segment #1. Correspondingly, old data is read from LU B at the offset and length indicated in the UNDO METADATA; namely, 15 blocks of data located in blocks 57-71 of LU B. The old data is then written into the UNDO stream in the journal LU, and the associated metadata is written into the UNDO METADATA stream in the journal LU. Specifically, for this example, the UNDO data is written into the first 15 blocks of Segment #5, and the UNDO METADATA is written into the first block of Segment #6. The beginning pointer of the UNDO data stream is set to offset=0 in Segment #5, and the end pointer is set to offset=15 in Segment #5. Similarly, the beginning pointer of the UNDO METADATA stream is set to offset=0 on Segment #6, and the end pointer is set to offset=1 in Segment #6. 
     At this point, the new data that was read from blocks 0-14 of journal LU Segment #1 is written to blocks 57-71 of LU B. The beginning pointer for the DO stream is moved forward to block 15 of journal LU Segment #1, and the beginning pointer for the DO METADATA stream is moved forward to block 1 of journal LU Segment #2. The journal and the four streams at the end of stage #2 are illustrated in  FIG. 3B . 
     At stage #3 the write transaction with ID=2 is applied to the storage system. As above, 20 blocks of new data are read from blocks 15-19 of journal LU Segment #1 and from blocks 0-14 of journal LU Segment #3. Similarly, 20 blocks of old data are read from blocks 87-106 of LU B. The old data is written to the UNDO stream in the last 5 blocks of journal LU Segment #5 and the first 15 blocks of journal LU Segment #7. The associated metadata is written to the UNDO METADATA stream in the second block of Segment #6. The list of segments in the UNDO stream includes Segment #5 and Segment #7. The end pointer of the UNDO stream is moved to block 15 of Segment #7, and the end pointed of the UNDO METADATA stream is moved to block 2 of Segment #6. 
     Finally, the new data from blocks 15-19 of journal LU Segment #1 and blocks 0-14 of journal LU Segment #3 is written into blocks 87-106 of LU B. The beginning pointer for the DO stream is moved forward to block 15 of journal volume Segment #3, and the beginning pointer for the DO METADATA stream is moved forward to block 2 of journal LU Segment #2. Segment #1 is freed from the DO stream, for recycling within the segment pool, and the list of segments for the DO stream is changed to Segment #3 and Segment #4. The journal and the four streams at the end of stage #3 are illustrated in  FIG. 3C . 
     At stage #4 a rollback to time 10:00:00.00 is performed. Since the write transaction with ID=3 was not applied yet, the only write transaction to be undone is the write transaction with ID=2. The last entry is read from the UNDO METADATA stream, the location of the end of the UNDO METADATA stream being determined by its end pointer, i.e., the metadata before block 2 of journal LU Segment #6 is read, indicating two areas each of 20 blocks; namely, (a) the last 5 blocks of journal LU Segment #5 and the first 15 blocks of journal LU Segment #7, and (b) blocks 87-106 of LU B. Area (a) is part of the UNDO stream. 
     The 20 blocks of data from area (b) are read from LU B and written to the beginning of the DO stream. As the beginning pointer of the DO stream is set to offset=15 of journal LU Segment #3, 5 blocks are written at the end of Segment #3, and the remaining 15 blocks are written to Segment #8. The end pointer for the DO stream is set to block 15 of Segment #8. The list of segments for the DO stream is changed to Segment #3, Segment #4 and Segment #8. The metadata associated with the 20 blocks from area (b) is written to block 3 of Segment #2, and the end pointer of the DO METADATA stream is advanced to block 4 of Segment #2. 
     The 20 blocks of data in area (a) of the journal LU are then written to area (b) of the LU B. Finally, Segment #7 is freed for recycling in the segment pool, the UNDO stream ending pointer is moved back to Segment #5 of the journal LU, block 15, and the UNDO METADATA stream ending pointed is moved back to Segment #6 of the journal LU, block 1. The journal and the four streams at the end of stage #4 are illustrated in  FIG. 3D . 
     Thus it may be appreciated that the journal is thus used to rollback LU B to the state that it was in at a previous point in time. The journal is also used to selectively access data from LU B at such previous point in time, without necessarily performing a rollback. Selective access is useful for correcting one or more files that are currently corrupt, or for simply accessing old data. 
     TABLE IV below summarizes the behavior of the special protection components of system  100  during production mode. Reference is also made to  FIG. 4 , which is a simplified flowchart of a data protection method corresponding to TABLE IV.  FIG. 4  is divided into four columns. The leftmost column indicates steps performed by source side protection agent  112 , the middle left column indicates steps performed by source side DPA  144 , the middle right column indicates steps performed by target side DPA  124 , and the rightmost column indicates steps performed by target side protection agent  164 . 
     
       
         
           
               
             
               
                 TABLE IV 
               
             
            
               
                   
               
               
                 Normal Production Mode Functionality 
               
            
           
           
               
               
            
               
                 System 
                   
               
               
                 Component 
                 Behavior 
               
               
                   
               
               
                 Source Side  
                 Intercept SCSI commands issued to LU A by 
               
               
                 Agent 144 
                 source side host via Device A (step 404). Replicate write 
               
               
                   
                 commands, and route write commands to DPA (steps 408 
               
               
                   
                 and 412). Wait for first acknowledgement, from DPA 
               
               
                   
                 (step 416), and then route replicate I/O command to LU A 
               
               
                   
                 (step 420). Wait for second acknowledgement, from 
               
               
                   
                 storage system (step 424), and then process next 
               
               
                   
                 intercepted SCSI command (step 404). 
               
               
                 Source Side  
                 Receive write command from agent (step 428). 
               
               
                 DPA 112 
                 Format write command as write transaction, and send to 
               
               
                   
                 target DPA (step 428). In synchronous mode, wait for 
               
               
                   
                 acknowledgement from target DPA (step 432), and then 
               
               
                   
                 send acknowledgement to agent (step 436). In 
               
               
                   
                 asynchronous mode and in snapshot mode, send 
               
               
                   
                 acknowledgement to agent without waiting for 
               
               
                   
                 acknowledgement from target DPA (step 436). 
               
               
                 Target Side  
                 Receive write transaction from source DPA (step 
               
               
                 DPA 124 
                 444). Enter write transaction in journal DO and DO 
               
               
                   
                 METADATA streams (step 444), and send back 
               
               
                   
                 acknowledgement to source DPA (step 448). Process 
               
               
                   
                 journal entries by applying them to LU B, and enter undo 
               
               
                   
                 information in UNDO and UNDO METADATA streams 
               
               
                   
                 (step 440). 
               
               
                 Target Side 
                 Fail SCSI commands issued to LU B (step 452). 
               
               
                 Agent 164 
               
               
                   
               
            
           
         
       
     
     Only steps with arrows connecting them in  FIG. 4  are necessarily sequential. Thus steps  432  and  436 , which do not have arrows connecting them, are not necessarily sequential. In synchronous mode these steps are sequential, but in asynchronous mode and in snapshot mode they are not sequential. In particular, DPA  112  may send an acknowledgement to protection agent  144  before receiving an acknowledgement back from DPA  124 . 
     It is also noted in  FIG. 4  that the steps performed by target side DPA  124  include two non-sequential groups; namely, (i) step  440 , and (ii) steps  444  and  448 . 
     Recovery mode is generally triggered as a result of a disaster at the source side. The source side data may become corrupt, or may not exist at all. In such case, after recovery is completed at the backup site, a user may perform a failover operation by switching the roles of the production site and backup site. The original backup site becomes a current production site, and the original production site becomes a current backup site. Alternatively, recovery mode can be triggered without a failover, in order to access data from a previous point in time. 
     While in recovery mode, target site DPA  124  continues to receive new write transactions from DPA  112  and enter them at the ends of the DO and DO METADATA streams. However, unlike production mode behavior, DPA  124  stops applying journal entries received from DPA  112  to LU B. Instead, DPA  124  uses the UNDO stream of the journal to rollback LU B, as described hereinabove. 
     During recovery, after or possibly before rollback of LU B is complete, a user may wish to access data from the target site. To this end, protection agent  164  stops failing I/O requests issued by host computer  160  and begins redirecting them to DPA  124 . The processing of data by host computer  160  during recovery mode is referred to as “target side processing (TSP)”. 
     To manage TSP write commands that are received by target side DPA  124 , journal processor  180  uses two additional data streams, referred to as TSP DO and TSP METADATA streams. When a TSP write command is received by DPA  124 , it is entered at the end of the TSP DO stream and the end of the TSP DO METADATA stream. Since TSP writes relate to the state of LU B after the rollback is complete, the TSP DO stream writes are only applied to LU B after rollback is complete. Journal processor  180  applies TSP writes to LU B in a way similar to the way it applies write transactions deceiver from DPA  112 ; namely, journal processor  180  maintains the undo information for each write applied to LU B, in TSP UNDO and TSP UNDO METADATA streams. 
     When TSP read commands are received by target site DPA  124 , DPA  124  returns the data to be read by identifying locations of the read command, and finding the most recent TSP write command or commands that were applied at these locations. The data is searched for (i) first in the TSP DO stream, and (ii) then in the journal UNDO data that was not yet applied to LU B and (iii) finally, if the data was not found in (i) and (ii), then the data is taken from LU B itself. In order to perform such a search efficiently, DPA  124  generates and stores in its memory a virtual image of the UNDO METADATA storage locations by using an efficient data structure, such as a binary search tree. 
     After rollback is completed, the TSP writes that were performed during the rollback are applied to LU B, and DPA  124  begins applying TSP writes synchronously; i.e., TSP writes are applied to LU B when they are received by DPA  124 , without keeping them in the TSP DO stream. As such, when a read command is received after rollback is complete, it is sent directly to LU B instead of being redirected through DPA  124 . 
     TABLES V and VI below summarize the behavior of the special protection components of system  100  during recovery mode, before and after the rollback is complete, in accordance with an embodiment of the present invention. Reference is also made to  FIGS. 5 and 6 , which are simplified flowcharts of data protection methods corresponding to TABLES V and VI, respectively.  FIGS. 5 and 6  are divided into four columns. The leftmost column indicates steps performed by target side protection agent  164 , the middle left column indicates steps performed by target side DPA  124 , the middle right column indicates steps performed by source side DPA  112 , and the rightmost column indicates steps performed by source side protection agent  144 . 
     
       
         
           
               
             
               
                 TABLE V 
               
             
            
               
                   
               
               
                 Recovery Functionality prior to Completion of Rollback 
               
            
           
           
               
               
            
               
                 System  
                   
               
               
                 Component  
                 Behavior 
               
               
                   
               
               
                 Target Side  
                 Intercept SCSI commands issued to LU B (step 
               
               
                 Agent 164 
                 576). Redirect commands to DPA (step 580). 
               
               
                 Target Side  
                 Use UNDO stream of journal to roll back target 
               
               
                 DPA 124 
                 storage system (step 540). Continue receiving write 
               
               
                   
                 transactions from DPA 112 and enter these transactions 
               
               
                   
                 into DO and DO METADATA streams without applying 
               
               
                   
                 them to LU B (step 548). Enter TSP write transactions to 
               
               
                   
                 TSP DO and TSP DO METADATA streams (step 564). 
               
               
                   
                 Create a virtual image, to reply to read commands issued 
               
               
                   
                 during the recovery process (step 572). 
               
               
                 Source Side  
                 As in production mode. 
               
               
                 DPA 112 
                   
               
               
                 Source Side 
                 As in production mode. 
               
               
                 Agent 144 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE VI 
               
             
            
               
                   
               
               
                 Recovery Functionality after Completion of Rollback 
               
            
           
           
               
               
            
               
                 System  
                   
               
               
                 Component 
                 Behavior 
               
               
                   
               
               
                 Target Side  
                 Intercept SCSI commands issued to LU B (step 
               
               
                 Agent 164 
                 664). Redirect write transactions to DPA (step 672), and 
               
               
                   
                 route read commands directly to LU B (step 680). 
               
               
                 Target Side  
                 Apply TSP write transactions to LU B, in the same 
               
               
                 DPA 124 
                 manner that write transactions received from DPA 112 are 
               
               
                   
                 applied in production mode; i.e., by entering data into TSP 
               
               
                   
                 UNDO and TSP UNDO METADATA streams (step 640). 
               
               
                   
                 Enter DO information and write transactions received 
               
               
                   
                 from DPA 112 into DO and DO METADATA streams, 
               
               
                   
                 without applying them to LU B (step 644). Apply TSP 
               
               
                   
                 write transactions to LU B as they are received (step 656). 
               
               
                 Source Side  
                 As in production mode. 
               
               
                 DPA 112 
                   
               
               
                 Source Side 
                 As in production mode. 
               
               
                 Agent 144 
               
               
                   
               
            
           
         
       
     
     It is also noted in  FIG. 5  that the steps performed by target side DPA  124  include three non-sequential groups; namely, (i) step  540 , (i) steps  548  and  552 , and (iii) steps  556 ,  560 ,  564 ,  568  and  572 . Similarly in  FIG. 6  target side DPA performs three non-sequential groups of steps; namely, (i) step  640 , (ii) steps  644  and  648 , and (iii) steps  652 ,  656  and  660 . 
     Reference is now made to  FIG. 7 , which is a simplified illustration of a time-line for tracking new processing of old data, in accordance with an embodiment of the present invention.  FIG. 7  illustrates journal processor  180  bringing the timeline back to a previous time, TOLD, and journal processor  180  applying TSP writes to bring the timeline forward from time TCURRENT to time TNEW. As shown in  FIG. 7 , current data at time ( 1 ) is rolled back to old data at time ( 2 ). After rolling back the data to time ( 2 ), the rolled back data becomes the image upon which target side processing advances to new data at time ( 3 ); i.e., the target side processing is applied to data ( 2 ) and not to data ( 1 ). 
     The data at time ( 1 ) is a common image for LU A and LU B at the same point in time, TCURRENT. Similarly, the data at time ( 2 ) is a common image for LU A and LU B at time TOLD. Rolled back data at time ( 2 ) may be processed by TSP writes, while at the same time current data at time ( 1 ) is being processed by source side writes. As such, the data evolves along the path from time ( 2 ) to time ( 3 ) as it is processed by the target side, and along the path from time ( 2 ) to time ( 4 ) as it is processed by the source side. The data images at the source and target sides at time TNEW are thus different. 
     When the recovery process is completed, the user may (i) return to a normal production mode, or (ii) perform a failover by switching the replication direction. In case (i), LU B is rolled back to its state at time ( 2 ), and the write transactions along the path from ( 2 ) to ( 4 ) are applied to LU B, so as to bring LU B to the same image as LU A. Conversely, in case (ii), LU B is maintained at its state at time ( 3 ), and its data is copied from the target side to the source side so as to bring LU A to the same image as LU B. 
     It may be appreciated that after rolling back the UNDO data stream to LU B, the state of the target side storage is substantially identical to the state that LU A was in at an earlier point in time. However, after applying TSP writes, the state of LU B is then in a new state that is different from the earlier state of LU A. As such, in order to return to a normal production mode, and ensure that LU B is a copy of LU A, DPA  124  undoes the TSP writes that were written to LU B using the TSP undo stream, and then returns to its normal production mode and begins applying the data that was written into the DO stream. The DO stream includes all write transactions that were undone while LU B was rolled back. Additionally, the DO stream includes new journal entries that were received from DPA  112  while DPA was in recovery mode. Similarly, protection agent  164  returns to its production mode by beginning to fail I/O requests issued by host  116 . 
     Alternatively, the user want to perform a failover; i.e., to make LU B in its current state a production LU and ensure that LU A is a copy of LU B. In this case the write transactions in the DO stream that correspond to a point in time subsequent to the recovered point in time are ignored. Additionally, the TSP writes that were applied to LU B during the recovery process are applied to LU A. Thereafter, the replication direction changes. Specifically, DPA  124  and protection agent  164  begin behaving in accordance with source site behavior, and DPA  112  and protection agent  144  begin behaving in accordance with target site behavior. 
     It may be appreciated that in order to provide failover capability, in which the roles of the production site and the backup site are switched, it is desirable that the source side has the necessary system components to function as a target side, and vice versa. Thus, in an embodiment of the present invention, the source side includes its own journal LU  184  and journal processor  188 , as indicated with dotted lines in  FIG. 1 . 
     Referring back to TABLE I, it may be appreciated that during normal data replication, for each write transaction received from a production site, there are five I/O operations performed at a backup site. Reference is now made to  FIG. 8 , which is a simplified illustration of a 5-stage journaling process for continuous data replication, in accordance with an embodiment of the present invention. The five steps shown in  FIG. 8  correspond respectively to the five steps listed in TABLE I. For the sake of clarity,  FIG. 8  only shows three meta-data elements; namely, a size, a journal address and a storage address. It may be appreciated that the meta-data in the DO METADATA and UNDO METADATA streams includes an ID, a time, and other attributes. 
     In accordance with an embodiment of the present invention, the meta-data for each transaction is of a fixed size, typically 30 bytes. The raw data varies in size, typically averaging around 10 KB per transaction. 
     As write transactions performed at a production site vary in frequency, and as each write transaction at the production site normally requires five I/O transactions at the backup site, it may be appreciated that the size of the DO stream grows and shrinks accordingly. When the I/O rate is low, the beginning of the DO stream is close to the end of the DO stream. In such case, it is possible to keep all write transactions between the beginning and the end of the DO stream in memory, and there is no need to read the beginning of the DO stream for every new transaction received in the backup site. As such, step  2  may be skipped. 
     Reference is now made to  FIG. 9 , which is a simplified illustration of a 4-stage journaling process for continuous data replication, for use when an I/O data rate is low, in accordance with an embodiment of the present invention. The first step in  FIG. 9  copies the write transaction to the end of the DO stream and the end of the DO METADATA stream, as in the 5-stage journaling process. Unlike, the 5-stage journaling process, though, instead of reading write transaction data from the beginning of the DO and DO METADATA streams, the 4-stage journaling process takes advantage of the fact that the write transaction that was just received at the backup site is still resident in memory. For this write transaction, steps  3 - 5  are performed, as indicated in  FIG. 9 . 
     However, during the steps  3 - 5  distribution of the write transaction that was just received, it is possible that a new transaction arrives at the backup site. In order to keep pace with the arriving transaction, as each write transaction is entered into the ends of the DO and DO METADATA streams, the write transaction is written into the end of a queue in memory. In accordance with an embodiment of the present invention, the queue in memory is handled similar to the way the DO stream is handled; namely, each received write is appended to the end of the queue, and when a write transaction is distributed according to steps  3 - 5 , a subsequent write transaction is taken from the beginning of the queue. Effectively, the queue corresponds to a cached DO stream. 
     The 4-stage journaling process is used until the queue in memory is full, at which point the normal 5-stage journal processing is resumed. Also in the event of a disaster, the normal 5-stage journal processing is resumed. In order to resume the 5-stage journal processing, it is important to identify the last write in the DO stream that was written. As such, even during the 4-stage journal processing, the pointers to the first and last write transactions in the DO stream are updated. 
     Conversely, when the I/O rate is high, in order to control the size of the DO stream and ensure that it does not overflow its disk allotment, the present invention switches from the normal 5-stage mode to a faster 3-stage mode whenever the DO stream reaches a large percentage of its maximum capacity, typically 80%. The present invention afterwards switches back from the faster 3-stage mode to the normal 5-stage mode whenever the DO stream is reduced to a smaller percentage of its maximum capacity, typically 75%. 
     The 3-stage mode eliminates steps  3  and  4  from the normal mode; namely, the steps that record the UNDO information. As such, rollback of the backup storage unit to its state at the times of those transactions processed with the 3-stage mode is not possible. 
     Reference is now made to  FIG. 10 , which is a simplified illustration of a 3-stage journaling process for continuous data replication, for use when the DO stream is near its maximum capacity, in accordance with an embodiment of the present invention. 
     TABLE VII summarizes the relative pros and cons of each of the journaling processes described hereinabove. 
     
       
         
           
               
             
               
                 TABLE VII 
               
             
            
               
                   
               
               
                 Pros and Cons of Journaling Processes 
               
            
           
           
               
               
               
            
               
                 Journaling  
                   
                   
               
               
                 Process 
                 Pros 
                 Cons 
               
               
                   
               
               
                 3-Stage  
                 Fastest replication time 
                 Long time to recover to 
               
               
                 Journaling 
                   
                 current time 
               
               
                 4-Stage  
                 Moderate replication time; 
                 Only able to be used as long 
               
               
                 Journaling 
                 Full data recovery capability 
                 as the beginning and the end 
               
               
                   
                   
                 of the DO stream are close 
               
               
                 5-Stage 
                 Full data recovery capability 
                 Slowest replication time 
               
               
                 Journaling 
               
               
                   
               
            
           
         
       
     
     One data replication strategy is the set of automated rules for controlling when a data replication system transitions between 5-stage, 4-stage and 3-stage journal processing. As mentioned hereinabove, transitions from 5-stage to 3-stage journaling, and from 3-stage back to 5-stage journaling, may be controlled based on the current size of the DO stream. Transitions from 5-stage to 4-stage journaling may be automated to occur when the beginning and end of the DO stream are close; and transitions from 4-stage back to 5-stage journaling may be automated to occur when the memory queue reaches its capacity. 
     Reference is now made to  FIG. 11 , which is a simplified state diagram of transitions between 5-stage, 4-stage and 3-stage journal processing, in accordance with an embodiment of the present invention. Shown in  FIG. 11  are three nodes, representing each of the journaling processes, and directed edges between the nodes corresponding to rules that govern transitions therebetween. As shown in  FIG. 11 , a 5-stage to 3-stage transition occurs when the size of the DO stream exceeds 80% of its allotted capacity, and a 3-stage to 5-stage transition occurs when the size of the DO stream falls under 75% of its allotted capacity. Similarly, a 5-stage to 4-stage transition occurs when the beginning and end of the DO stream are close; and a 4-stage to 5-stage transition occurs when the memory queue reaches its capacity. 
     It will be appreciated by those skilled in the art that using 4-stage journaling enables a data replication system to keep pace with higher I/O rates than can be handled when using 5-stage journaling. If the system is currently using 5-stage journaling and the I/O rate is higher than can be handled, a lag increases until the system necessarily transitions to the 3-stage journaling process. However, if the system can catch up with the lag, empty the DO stream and transition to a 4-stage journaling process, then the system can accommodate higher I/O rates before transitioning back to the 5-stage journaling process. 
     In this regard, it is noted that in general, if the system cannot keep pace with I/O rates using a 4-stage journaling process, then it most probably cannot keep pace using a 5-stage journaling process, and in either case the system would have to transition to a 3-stage journaling process. However, since the I/O rate changes continuously, a transition from 4-stage journaling to 5-stage journaling does not necessarily push the system to 3-stage journaling. 
     Reference is now made to  FIG. 12 , which is a simplified illustration of a variant of the three-stage journaling process shown in  FIG. 10 , which may be used in an alternative embodiment of the present invention. The alternative 3-stage journaling proceeds according to the last three stages of the 4-stage journaling process. That is, the stage of writing to the DO stream is skipped within 4-stage journaling, for the alternative embodiment of 3-stage journaling. When performing 4-stage journaling, the backup site DPA (element  124  of  FIG. 1 ) can return an acknowledgement to the production site DPA (element  112  of  FIG. 1 ) immediately after the first stage, when the write transaction is written to the DO stream. However, when performing the alternative 3-stage journaling, the backup site DPA must wait until the write transaction is written to storage, before it can return an acknowledgement to the production site DPA. Since the last three stages of 4-stage journaling can be performed in a separate thread than the thread that performs the first stage, the alternative 3-stage journaling may result in a longer time lag between the source and target sites. 
     Each acknowledgement received by the production site DPA relates to a specific time, and thus the production site DPA can determine from the acknowledgement the amount of time lag between the backup site and the production site. A user may set a control parameter to configure the data replication system to constrain the time lag between the production and backup sites. When the time lag constraint is exceeded, the system regulates the lag by slowing down the production site application. As such, the alternative 3-stage journaling will generally hit the time lag constraint, and thereby cause a slow down in the production site application, more often than will the 4-stage journaling. 
     In another aspect of the invention, exemplary embodiments of a continuous data replication system include an action regulation module to detect conditions under which a regulation state should be entered. With this arrangement, processing loops due to repetitive internal or external problems can be avoided. 
       FIG. 13  shows an exemplary continuous data replication system  100 ′ including a replication module  800  and a regulation control manager module  802  on the target DPA  124 . Similarly, the source side DPA  112  can include a replication module  801  and a regulation manager module  803 . It is understood that the system of  FIG. 13  has commonality with the system of  FIG. 1 , where like reference numbers indicate like elements. In general, the replication module  800  and the regulation manager module  802  provide detection of repetitive actions and control entry into a regulation state. 
     The system can include tangible media/memory  850 , e.g., disks, access memory, etc, to store computer-executable instructions  852  for providing at least some of the system functionality. The instructions can be executed by the journal or other processor to achieve desired operation of the system. In other embodiments, digital and/or analog circuitry can provide some or all of the system functionality. 
     In general, control actions can occur for a variety of reasons, such as user actions, disasters, and the like. It is understood that control action represents any action the control-path of the system performs in reaction to settings (configuration/operation) changes or a disaster that occurred. These actions are intended to bring the system to a new state that reflects the correct behavior considering the settings/disaster. For example, a significant number of repetitive actions can be performed on the same copy operation, due to jittering of a parameter in the environment/disaster in the system/bug. These repetitive actions may have a negative impact on the customer&#39;s production site on their own. An action, as defined above, reflects the operations that the system performed. 
     It is understood that the term “state” represents the current “disaster state” and “working state” of the system. The “disaster state” represents the status of any component outside that the system interacts on/dependant on. For example, any LUN in the user&#39;s setup. “Working state” represents the current status of any component that is part of the system. “Control Actions” are intended to affect the “working state” of the system, given the external “disaster state” and the system&#39;s settings. It is understood that there are many more possible combinations of “steps” than certain “control actions”. 
     Consider the splitter in the protection agent  144  that is instructed to split again and again due to a failure to split caused by a hardware fault in the DPA. This may result in a crash of the application if I/O operations are not successful for a given period of time. Performing a full sweep again and again without ending causes a negative impact on customer storage area networks and wide area networks. A sweep refers to the process of re-synchronizing all the volume/s data between the “production” side and the replica copy from scratch since one cannot guarantee consistency 
     As will be readily appreciated by one of ordinary skill in the art, it is desirable to detect such situations and regulate copy operations in order to prevent degradation of the customer environment. When regulated, the group copy is frozen in its state and no more control actions are performed on it. Thus, if a transfer is being performed, the transfer is continued. If a transfer is closed, it will remain closed. It is understood that closed refers to the pipe between the source and replica being closed, i.e., no data is passed between these copies. Similarly, a splitter state remains just as it was before the regulation took place, unless it changed ‘by itself’, i.e. the splitter moved to MOH. The term MOH refers to marking the changed places/data on the host/splitter in order not to perform a full/volume sweep when situation recovers, but just take the marked delta. In general, control does not issue actions for regulated copies. When a copy operations exits regulation, control returns to send actions. In addition, if a volume sweep is scheduled to occur, it occurs when the copy operation is no longer regulated. 
     Since a regulation mechanism in the control path is less sensitive than detecting jitters in frequencies of less then a second or even few seconds, for example, it is assumed that the latter kind of jitters are better handled in data-path, such as by immediate MOH. The regulation mechanism handles ongoing jitters that may happen every few minutes, tens of seconds, etc. 
     In an exemplary embodiment, the state of a copy operation is set to REGULATED automatically by the system if at least one of the control internal actions crosses the regulation boundaries, as described more fully below. The REGULATION state can exit in a number of ways, including system action and user action. System action to exit the REGULATION state can occur after expiration of a selected amount of time, e.g., twenty-four hours, for example. User action can include a settings change related to the group resulting in a change of the group state to UNREGULATED and/or running a CLI command to un-regulate all copy operations, i.e., change the state of all copies to UNREGULATE. 
     In an exemplary embodiment, the system determines whether to assign a copy operation to the REGULATED state by using a number of parameters. 
     EXAMPLE 
     Control internal Configuration commands (Internal control-path actions that are meant to trigger only other control-actions, and won&#39;t cause a direct implication on an external component—(n=10, T=10 min, Δt=45 s) 
     Splitter direct actions (Actions related to controlling the splitter side behavior. This 
     Actions are very sensitive as the may directly affect the customer&#39;s cluster)—(n=5, T=10 min, Δt=1 min) 
     Other Actions—(n=10, T=60 min, Δt=5 min) 
     It is understood that n=number of repetitions, T=time frame, and Δt=time window in which all (same) actions are counted as 1 (i.e. if 2 or more same actions occurred within the same Δt, ‘n’ will be increased only by one). 
     If action ‘A’ occurred at least ‘n’ times within time “T” (count respective to Δt) then the copy is regulated. 
     Table 1 below shows an exemplary situation in which the protection agent/splitter  144  ( FIG. 13 ) enters the REGULATED state due to repetitive transitions to a DELAY state in response to an excess number of write requests. In this example Δt is 45 seconds. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 State 
                 N 
                 Time 
               
               
                   
                   
               
             
            
               
                   
                 many writes → DELAY 
                 0 
                 00:00:00 
               
               
                   
                 (application slowed down) 
                   
                   
               
               
                   
                 SPLIT 
                 1 
                 00:00:10 
               
               
                   
                 DELAY (3 volumes) 
                   
                 00:00:13 
               
               
                   
                 SPLIT 
                 1 (in same timeframe) 
                 00:00:20 
               
               
                   
                 DELAY + highload 
                   
                 00:00:45 
               
               
                   
                 SPLIT 
                 2 
                 00:00:50 
               
               
                   
                 DELAY 
                   
                 00:01:40 
               
               
                   
                 SPLIT 
                 3 
                 00:02:37 
               
               
                   
                 DELAY 
                   
                 00:03:00 
               
               
                   
                 SPLIT 
                 4 
                 00:03:12 
               
               
                   
                 DELAY 
                   
                 00:03:30 
               
               
                   
                 SPLIT 
                 4 (in same time-frame) 
                 00:03:40 
               
               
                   
                 DELAY 
                   
                 00:04:00 
               
               
                   
                 SPLIT REGULATED 
                 5 (regulate) 
                 00:04:30 
               
               
                   
                 Still in delay. . . 
                   
                 00:04:30 
               
               
                   
                 SPLIT 
                 0 (system unregulated) 
                 06:04:30 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen, the splitter changes states from SPLIT to DELAY and back. Each time the splitter goes to the split state in a different time frame, the count N is incremented. When the count N is equal to a selected value, the splitter enters a REGULATED state. In table 1 above, the state SPLIT REGULATED state is entered when N equals five. 
     As can be seen, the state changes due to the action, which increments the count. For example, one DELAY is done only on three volumes, which is a different state than a delay on all volumes. In another DELAY, there are also highloads regarding these volumes which reflect a different state. In addition, issuing a split_action command may not result in a state change (for some reason/problem in the splitter it might refuses to do so). However, this still results in an increase of count even though the state did not change. N is incremented based on split_action command by the control process, regardless of the current state. 
     It is understood that upon entering into the regulated state a timer is activated. This amount of time to wait can be changed through a system configuration file, for example. In this example, the timer is set to six hours. If after six hours, the copy did not get out of the regulated state in one of the user ways described above (CLI command or some settings change related to this copy) the system unregulated command will activate and get the copy out of regulation. 
     The splitter  144  is performing write operations, which require a wait to receive acknowledgment from the data protection appliance before writing the data to storage. When the splitter receives more writes than can be processed, the splitter enters a DELAY state and slows down the applications requesting the write operations. While applications are slowed down, the splitter handles its “backlog” so the replication module  800  on the data protection appliance  112 ′ moves it back to SPLIT state and applications are no longer slowed down. 
     Since the same amount of writes keeps arriving, the splitter soon again enters the DELAY state, slowing down applications. After a time, the splitter moves back to the SPLIT state. This process continues until the application crashes or it becomes non-responsive since the application is stuck waiting for the splitter. 
     During this time, the state of the system can change. In response to heavy loads, the replication module  800  in the data protection appliance can enter various “high-load” states, both the DELAY and the high-loads can be on different volumes each time and many other changes can be present in the complex state of the system. These changes within the same repetitive situation demonstrate why conventional state-based repetition detection techniques may perform poorly. 
     In contrast, using the exemplary inventive action-based repetition-detection allows regulating parts of the control module when a “storm” of writes occur, for example. In one embodiment, the splitter is made to pass its delay-expiry timeout and (according to policy) write to the storage without waiting for the data protection appliance acknowledgment. This is much faster and keeps the applications up and responding. 
     After a period of time, the system gets out by itself from this regulation state (assuming the “storm” has passed), perform some initialization and keeps working as usual from there. 
       FIG. 14  shows an exemplary state diagram  900  showing state transitions for a system without action regulation for a repetitive internal problem. In state  902 , the system is in an idle state. In response to an internal problem, e.g., a splitter falsely gets out of the SPLIT state, the system transitions to a state  904  in which the system reacts, e.g., returns the splitter to SPLIT state, to the problem. Once the internal problem is resolved, the system transitions to a state  906  in which the system has recovered from the problem. If the problem does not repeat, the system transitions to the idle state  902 . However, if the internal problem repeats, the system returns to state  904  to react to the problem creating a potential loop between states  904  and  906 . 
       FIG. 15  shows an exemplary state diagram  1000  for a system having an action regulation mechanism in accordance with exemplary embodiments of the invention to handle a repetitive internal problem. From idle state  1002 , the system transitions to a state  1004  to react to a problem in response to detecting an internal problem. If the problem disappears, the system transitions to a state  1006  in which the system has recovered from the internal problem. If the internal problem does not repeat, the system transitions back to the idle state  1002 . 
     If the problem does repeat, the system transitions to a state  1008  in which the action regulation module  802  ( FIG. 13 ) examines given parameters to determine whether the system should enter the regulation state  1010 . Examining exemplary parameters is described above in conjunction with Table 1, and also described below. 
       FIG. 16  shows an exemplary state diagram  1100  for a system having an action regulation mechanism for a repetitive external problem. The system transitions from an idle state  1102  in response to an external problem to a state  1104  in which the system reacts to the detected external problem. Upon finishing reacting to the problem, the system transitions to a state  1106  in which the system is stable on new disaster state. That is, the is system finished applying all its control actions as part of reacting to the problem and has stabilized on a new “working state”, given the current “disaster state”. When the external problem disappears, the system enters a state  1108  in which the system reacts to a recovery of environment. During this state, the system performs “control actions”, which are intended to improve working state considering the new “disaster state”, which includes a fix of a disaster that occurred. Once finished, the system transitions to a state  1110  in which the system is stable on a new ‘good’ state. This ‘good’ state, represents the “working state” in good path, meaning the working state after recovering from disaster and stabling the working state. If the external problem does not repeat, the idle state  1102  is entered. If the problem does repeat, a state  1112  is entered for the regulation module to determine whether a regulation state should be entered. If not, state  1104  is entered. If so, a state  1114  is entered in which the system is in a regulation state. 
       FIG. 17  shows an exemplary sequence of steps for implementing action regulation in accordance with exemplary embodiments of the invention. In step  1200 , a control action is received and in step  1202  the system checks the user action threshold. In practice, user actions (settings changes or commands invocations) result in a small “storm” of different system actions, so whenever a user action, which is related to this copy, is performed the system records the time of that action, and allows “user_action_threshold” (currently 30 seconds) in which the system does not count the actions performed on this copy for regulation purposes. In step  1204 , it is determined whether the action was a user action. If so, processing ends. If not, the regulation module begins processing in step  1206  and in step  1208  determines if the current time minus the last action time is less than a given timer interval, as described above in conjunction with Table 1. If so processing ends. If not, in step  1210  the action time is inserted to the end of a queue. 
     The queue is per action per copy and holds the times of the previously performed actions (which are still relevant for regulation purposes). Actions are entered into the queue only after passing previous thresholds: user_action_threshold, delta-t, and are removed from the queue when they are no longer in the T (examined time-frame for regulation), after entering a new action-time to the queue. If after entering a new action-time to the queue and after removing old actions from the queue the system is left with a queue which is bigger than N. In step  1212 , old actions are removed from the queue, i.e., those for which the action time is less than the current time. 
     In step  1214 , the system determines whether the queue size has reached a selected value. If so, in step  1216 , the system regulates the action. For example, in Table 1 above, The splitter moves to state SPLIT REGULATED when N=5, i.e., when splitter enters the SPLIT state for the fifth time over a given time. 
     The exemplary methods and apparatus of this invention may take the form, at least partially, of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, random access or read only-memory, or any other machine-readable storage medium, including transmission medium. When the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. The methods and apparatus of the present invention may be embodied in the form of program code such that when the program code is received and loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code combines with the processor to provide a unique apparatus that operates analogously to specific logic circuits. The program code (software-based logic) for carrying out the method is embodied as part of the system described below. In other embodiments, functionality is performed in part or in whole on logic circuits in combination with program instructions executed by a processor. It is understood that the partition between hardware and software can vary to meet the needs of a particular application. 
     In reading the above description, persons skilled in the art will realize that there are many apparent variations that can be applied to the methods and systems described. Thus it may be appreciated that, in addition to data replication systems, the optimal journaling policy of the present invention has widespread application to journaling systems including database systems and version control systems. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific exemplary embodiments without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.