Abstract:
A system and method for managing single instance storage. A computer system includes at least two backup servers, each backup server included in a single-instance storage pool. A first backup server conveys a first de-duplicated list identifying data segments from the first storage pool to a second backup server. The first backup server receives from the second backup server a second de-duplicated list identifying a subset of the data segments and conveys the subset of the data segments to the second backup server. In response to receiving the first list from the first backup server, the second backup server de-duplicates the first list against a second storage pool and conveys the second list to the first backup server. In response to receiving the subset of the data segments, the second backup server adds the received data segments to the second storage pool.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to computer systems and, more particularly, to replication and restoration of backup files within computer systems. 
   2. Description of the Related Art 
   There is an increasing need for organizations to protect data that resides on a variety of client devices via some type of backup mechanism. For example, numerous client devices may be coupled to a network to which one or more backup servers are also coupled. The backup servers may be further coupled to one or more tape drives or other backup media. A backup agent on each client device may convey data files to the backup server for storage on backup media according to a variety of schedules, policies, etc. For example, large backup datasets may be moved from a client device to a media server configured to store data for later retrieval, thereby protecting data from loss due to user error, system failure, outages, and disasters, and so on. Additionally, such backup procedures may be utilized for purposes of regulatory compliance, workflow tracking, etc. 
   In order to minimize the size of storage pools required to store backup data, Single Instance Storage (SIS) techniques are sometimes employed at each backup location. In some SIS techniques, data is stored in segments with each segment having a fingerprint that may be used to unambiguously identify the segment. For example, a data file may be segmented, and a fingerprint calculated for each segment. Duplicate copies of data segments are then replaced by a single instance of the segment and a set of references to the single instance. In order to retrieve a backup file, a set of fingerprints is sent to a backup server, where it is compared to the fingerprints of data stored in an associated storage pool. For each matching fingerprint, a data segment is retrieved. The resulting segments are re-assembled to produce the desired file. 
   In order to make data more readily available, it may be desirable to replicate portions of a storage pool. For example, the contents of a storage pool may be replicated and stored at a remote location from which they may be retrieved (e.g., to recover from a disastrous data loss). Alternatively, a multi-national enterprise may replicate a storage pool or a portion thereof during off hours to make data more easily retrievable from a variety of locations, perhaps on different continents, without the need to transmit large amounts of information on demand. In conventional systems, replication typically involves re-assembling the files to be replicated from their respective data segments stored in a source storage pool and sending them to a target storage pool where SIS techniques may be re-applied. Unfortunately, this process may lead to multiple re-assemblies of data for which there are multiple references. In addition, transmitting the resulting large datasets is costly in terms of time and bandwidth consumption. These issues also arise when data needs to be reverse replicated back to its original source storage pool, such as in the event of a server failure. In view of the above, an effective system and method for replicating single-instance storage pools that accounts for these issues is desired. 
   SUMMARY OF THE INVENTION 
   Various embodiments of a computer system are disclosed. In one embodiment, the computer system includes at least two single-instance storage pools, each storage pool including one or more backup servers. A first backup server is configured to convey a first de-duplicated list to a second backup server associated with a second storage pool that identifies one or more data segments of the first storage pool. In response to receiving the first list from the first backup server, the second backup server is configured to de-duplicate the first list against the second storage pool to create a second de-duplicated list that identifies at least a subset of the one or more data segments, and convey the second list to the first backup server. The first backup server is configured to receive the second list and convey the subset of one or more data segments to the second backup server. In response to receiving the subset of the one or more data segments, the second backup server is configured to add the subset to the second storage pool. 
   In a further embodiment, for each data segment stored in the first or the second single-instance storage pool, there is also stored (i) an associated fingerprint that identifies the data segment, and (ii) a reference to each of one or more clients from which a copy of the data segment was received for backup. In one embodiment, the first list includes each data segment&#39;s associated references and the second backup server is configured to de-duplicate the data segments and each data segment&#39;s associated references against the second single-instance storage pool. 
   These and other embodiments will become apparent upon consideration of the following description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates one embodiment of a system of computers. 
       FIG. 2  is a generalized block diagram of one embodiment of a pair of backup servers. 
       FIG. 3  illustrates one embodiment of a storage pool data entry. 
       FIG. 4  is a more detailed block diagram of one embodiment of backup server. 
       FIG. 5  illustrates one embodiment of a process for de-duplicating a dataset before storing it in a storage pool. 
       FIG. 6  illustrates one embodiment of a process for adding a backup dataset to a single-instance storage pool. 
       FIG. 7  illustrates one embodiment of a process for replicating a single-instance storage pool. 
       FIG. 8  illustrates one embodiment of a process for reverse replicating a single-instance storage pool. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates one embodiment of a computing system  100 . As shown, system  100  includes desktop clients  110  and  120  that are representative of any number of stationary client computers. System  100  also includes mobile clients  130  and  140  that are representative of any number of mobile client computing devices such as laptops, handheld computers, etc. System  100  further includes backup servers  150  and  160 , which may include backup media  155  and  165 , respectively. Backup media  155  and  165  may be removable media such as tape or disk as well as hard disk, memory, or other storage devices associated with backup servers  150  and  160 . In alternative embodiments, backup media  155  and  165  may be separate from backup servers  150  and  160 . Each of clients  110 ,  120 ,  130 , and  140  as well as servers  150  and  160  are coupled to a network  170 . Network  170  may include one or more local area networks (LANs) that may be connected to a wide area network (WAN)/Internet and or to the public switched telephone network (PSTN) via one or more modems. 
   In alternative embodiments, the number and type of clients is not limited to desktop clients  110  and  120  and mobile clients  130  and  140 . Almost any number and combination of desktop and mobile clients may be connected to network  170  via various combinations of modem banks, direct LAN connections, wireless connections, WAN links, etc. Also, at various times one or more clients may operate offline. In addition, during operation, individual client connection types may change as mobile users travel from place to place connecting, disconnecting, and reconnecting to network  170 . 
   Although system  100 , as shown, consists of clients and servers, in alternative embodiments each device that is connected to network  170  may, at various times, take on either a client or a server role. In a further alternative embodiment, system  100  may comprise a peer-to-peer network with or without centralized control services. 
   Within system  100 , it may be desired to protect data associated with any of clients  110 ,  120 ,  130 , and  140 . In order to protect client data, various backup operations are possible. For example, in one embodiment, backup server  150  and or backup medium  155  may store data from one or more clients in a first storage pool. A second copy of data from one or more clients may be stored in a second storage pool on backup server  160  and/or backup medium  165 . In operation, data protection software located on each of clients  110 ,  120 ,  130 , and  140  may execute in the background to perform data backups. Backup frequency and storage location may depend on a variety of factors including the urgency of data protection, availability of media storage space, network connection state, and enterprise policies. For example, in one embodiment, data from a storage pool on backup server  150  may be replicated on a storage pool on backup server  160 . Such replication may be done according to a schedule or at other times determined by administrative policy, security policy, or to meet other requirements of an enterprise. In addition, at various times, data that has been replicated may be reverse-replicated. For example, data that was replicated from a storage pool on backup server  150  to a storage pool on backup server  160  may be reverse replicated to the storage pool on backup server  150 . Reverse replication may be performed for a variety of reasons, such as to recover data lost due to inadvertent deletion, system failure, disasters, etc. 
   Turning now to  FIG. 2 , a generalized block diagram of backup servers  150  and  160  is shown. Backup server  150 , as shown, includes a single-instance storage (SIS) control  210 , a backup medium  220 , and a replication SIS control  240 . Backup server  150  is representative of one or more backup servers that together form a first storage pool. Backup medium  220  includes data entries  230 A- 230 N. Elements referred to herein by a reference numeral followed by a letter may be collectively referred to by the reference numeral alone. For example, data entries  230 A- 230 N may be referred to as data entries  230 . Each entry  230  includes data  234  and associated metadata  232 . Similarly, backup server  160  includes a single-instance storage control  250 , a backup medium  260 , and a replication SIS control  280 . Backup server  160  is representative of one or more backup servers that together form a second storage pool. Backup medium  260  includes data entries  270 A- 270 N. Each entry  270  includes data  274  and associated metadata  272 . 
   During operation, clients may backup data to backup server  150 . For example, a backup agent operating on a client may transmit data entities to backup server  150  via network  170 . A data entity, as used herein, may comprise one or more files and/or segments of files or other data structures. Within backup server  150 , SIS control  210  may receive data entities, perform de-duplication of the received data entities, and store the resulting data and metadata as one or more entries  230 . De-duplication, as used herein, refers to a process that includes finding multiple copies of data entities and replacing them with a single copy of the entity plus a reference to the entity for each copy. Copies of data entities may be identified by comparing a digital fingerprint of one entity to the fingerprint of another entity. If the fingerprints match, then the two entities may be deemed to be copies of one other. A digital fingerprint for a data entity may be created by applying some function, such as a hash function, to the data entity. In one embodiment, the digital fingerprints are encrypted. In one embodiment, a fingerprint generation function may comprise a Message-Digest algorithm 5 (MD5) hash function. Alternative hash functions include Secure Hash Algorithm (SHA), a checksum, signature data, and any other suitable function, cryptographic, or otherwise, for identifying a data entity. Each entry  230  within backup medium  220  may include a data entity  234  and associated metadata  232  that includes the references to data entity  234  produced during de-duplication. A more detailed description of an entry  230  is given below. 
   Data may be stored on backup server  160  in backup medium  260  in a similar manner. In addition, at various times, data may be replicated from one storage pool to another. More specifically, in one embodiment, replication SIS control  240  may assemble a set of data entities and transfer them from backup medium  220  to backup medium  260 . A reverse replication may also be performed in which replication SIS control  280  may assemble a set of data entities and transfer them from backup medium  260  to backup medium  220 . Further details of the operation of replication SIS controls  240  and  280  are given below. 
     FIG. 3  is a generalized block diagram of one embodiment of an entry  310  representing one of entries  230  or  270  of  FIG. 2 . Each entry  310  may include a metadata portion  320  and an associated data portion  330 . Metadata  320  may include one or more metadata references such as references  321 - 323  as shown. Each reference may include information describing the associated data  330 . For example, reference  321  includes a data entity name  341 , data size  351 , a fingerprint  361  of data  330 , a batch number  371 . Batch number  371  may be used to identify the particular backup operation in which the associated reference  321  to data  330  was created. Similar data is shown for references  322  and  323 . Each reference may include other information such as type, version number, ownership, permissions, modification time, error code, etc. Other forms of metadata and/or identifiers will be apparent to those of ordinary skill in the art. 
     FIG. 4  is a more detailed block diagram of one embodiment of backup server  150 . In addition to SIS control  210 , backup medium  220  and replication SIS control  240 , a backup history  245  is shown. During operation, replication SIS control  240  may subscribe to backup updates from SIS control  210  and store corresponding entries in backup history  245 . More specifically, during a backup operation, SIS control  210  may receive a batch of data segments labeled with a batch number. The batch number may be sent to the subscribing replication SIS control  240 . In one embodiment, backups may be incremental, i.e. when a data entity is modified, only the new data segments may be sent to backup server  150 . For each batch number received, replication SIS control  240  may create an entry in backup history  245 . For example, in the illustrated embodiment, replication SIS control  240  has created entries  290 - 292  in backup history  245 , each of which includes a respective batch number. Entries in backup history  245  correspond to the backup batches that have been received since the last replication operation. During a replication, the entries that have been stored in backup history  245  are used to determine which batches to replicate, rather than replicating all of backup medium  220 . Once a replication is completed, backup history  245  may be cleared. 
     FIG. 5  illustrates one embodiment of a process  500  for de-duplicating a dataset before storing it in a storage pool. Process  500  may begin with the reception of a list of fingerprints that are associated with a group of data segments, such as a backup dataset (block  510 ). Each fingerprint in the list may include one or more references, each of which indicates that a source has a copy of the data segment. For instance, if more than one backup agent has contributed a copy of a data segment to a backup dataset, the backup dataset may include a single instance of the segment with a reference to each source agent. Once the list of fingerprints is received, a fingerprint is selected (block  520 ). The selected fingerprint may be compared to fingerprints in the storage pool (block  530 ). If the selected fingerprint does not match the fingerprints of any data segment that is already present in the storage pool (decision block  540 ), then the fingerprint and its associated references may be maintained on the list (block  542 ). If the selected fingerprint does match the fingerprint of a data segment that is already present in the storage pool (decision block  540 ), then the references associated with the selected fingerprint may be compared to the references that are already associated with the matching fingerprint from the storage pool (decision block  550 ). If a reference does not match any existing reference to the matching fingerprint, the reference may be maintained on the list (block  552 ). If the reference matches any existing reference to the matching fingerprint the reference may be removed from the list. Once all of the references have been compared, the matching fingerprint may be removed from the list (block  560 ), retaining any references that did not match. Once the comparisons of the selected fingerprint and its references have been completed, if the selected fingerprint is the last fingerprint on the list (decision block  570 ), then de-duplication of the list is complete and the de-duplicated list may be returned (block  580 ). Otherwise, another fingerprint may be selected (block  575 ) and process  500  may return to block  530  such that fingerprint and reference comparisons are executed for each list entry. 
     FIG. 6  illustrates one embodiment of process  600  for adding a backup dataset to a single-instance storage pool. Process  600  may begin with the reception of a deduplicated backup data batch (block  610 ). For example, a backup server in a client-server computer network may receive a data batch to be stored in a backup storage pool from a backup agent executing on a client in the network. Further, the batch may be filtered according to a process such as the one illustrated in  FIG. 5 . In an alternative embodiment, if the backup data batch is not deduplicated by the sending client, it may be deduplicated upon reception by the receiving backup server. Once the data batch is received, each item in the batch may be added to a subscription list (block  630 ). The items on the subscription list may be forwarded to a replication control (block  640 ), where they may be stored in a backup history (block  650 ). In addition, new data segments from the de-duplicated batch may be added to the single-instance storage pool (block  660 ) and new references from the de-duplicated batch to existing data segments may be added to their associated data segments in the single-instance storage pool (block  670 ). After all of the new data segments and references have been added to the single-instance storage pool, process  600  is complete (block  680 ). 
     FIG. 7  illustrates one embodiment of process  700  for replicating a single-instance storage pool. In process  700 , a storage pool from a replication source (left side of  FIG. 7 ) may be replicated to a target pool on a replication target (right side of  FIG. 7 ). The replication source and target may be, for example, two backup servers. Process  700  may begin with the reception of a replication request at a replication source (block  710 ). In response to the replication request, the replication source may send a history consisting of a list of data segments and references to the replication target (block  720 ). In one embodiment, the history may comprise a list of data segments and references that have changed in the source single-instance storage pool since the last replication. In response to receiving the history, the replication target may de-duplicate the history against the target single-instance storage pool (block  730 ) and return the de-duplicated history to the replication source (block  740 ). In response to receiving the de-duplicated history, the replication source may assemble a corresponding de-duplicated data batch (block  750 ) and forward the de-duplicated data batch to the replication target (block  760 ). In response to receiving the de-duplicated data batch, the replication target may add the data segments and references from the de-duplicated data batch to the target single-instance storage pool (block  770 ). After all of the batched data segments and references have been added to the target single-instance storage pool, process  700  is complete (block  780 ). 
     FIG. 8  illustrates one embodiment of process  800  for reverse replicating a single-instance storage pool. In process  800  a storage pool from a reverse replication source (left side of  FIG. 8 ) may be reverse replicated to a target pool on a reverse replication target (right side of  FIG. 8 ). The reverse replication source and target may be, for example, two backup servers. Process  800  may begin with the reception of a reverse replication request at a reverse replication source (block  810 ). In response to the reverse replication request, the reverse replication source may assemble a de-duplicated list of local storage pool entries (block  820 ) and send the list to the reverse replication target (block  830 ). In one embodiment, the list may include entries corresponding to data segments and references that have changed in the source single-instance storage pool since the last reverse replication. In response to receiving the list, the reverse replication target may de-duplicate the list against the target single-instance storage pool (block  840 ) and return the de-duplicated list to the reverse replication source (block  850 ). In response to receiving the de-duplicated list, the reverse replication source may assemble a corresponding de-duplicated data batch (block  860 ) and forward the de-duplicated data batch to the reverse replication target (block  870 ). In response to receiving the de-duplicated data batch, the reverse replication target may add the data segments and references from the de-duplicated data batch to the target single-instance storage pool (block  880 ). After all of the batched data segments and references have been added to the target single-instance storage pool, process  800  is complete (block  890 ). 
   It is noted that the above-described embodiments may comprise software. In such an embodiment, the program instructions that implement the methods and/or mechanisms may be conveyed or stored on a computer readable medium. Numerous types of media which are configured to store program instructions are available and include hard disks, floppy disks, CD-ROM, DVD, flash memory, Programmable ROMs (PROM), random access memory (RAM), and various other forms of volatile or non-volatile storage. 
   Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.