Online restore to a selectable prior state for database engines

Online restore operations for a database engine can be performed. A request to restore a database to a previous state can be received. Previously stored content of the database, such as snapshot stored prior to the previous state, can be identified along with log records describing changes to be made to the content prior to the previous state. State information in a query engine can be updated based on the previously stored content and log records so that queries can be processed based on the state information at the database restore to the previous state.

BACKGROUND

Database systems provide restoration capabilities in order to allow users to undo or revert to a prior state of database. Restoring a database to a prior state may be useful in many different scenarios. For example, a database restoration may be performed to allow for errors introduced into the database as a result of changes to data or the schema of the data (e.g., column additions or deletions) to be removed or rolled-back. In another example, developers can use restore operations to as part of testing modifications to the schema of a database when developing applications or other tools that utilize the database. Techniques that improve the speed and flexibility of restore operations are thus highly desirable.

While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The words “include,” “including,” and “includes” indicate open-ended relationships and therefore mean including, but not limited to. Similarly, the words “have,” “having,” and “has” also indicate open-ended relationships, and thus mean having, but not limited to. The terms “first,” “second,” “third,” and so forth as used herein are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.) unless such an ordering is otherwise explicitly indicated.

DETAILED DESCRIPTION

Various embodiments of online restore for database engines are described herein. In various embodiments, a database may be utilized to support different systems, applications, or services. Restore operations to return a database to a previous state may be performed while the database or other query engine is online, in some embodiments, so that changes to a database can be reversed, examined or tested without offline restore and copy operations to generate the previous state of the database. For instance, instead of waiting for backup versions or snapshots of a database to be retrieved from archival or other remote storage systems to restore a database, online restore for database engines may be performed utilizing prior contents of the database co-located with data for the database, in some embodiments.FIG. 1is a logical block diagram illustrating online restore for a database engines, according to some embodiments.

Database130may be a set of data, collection of records, or other grouping of data objects, in one embodiment, which may be stored in a data store. A data store may be one or more directly or network attached storage devices, accessible to a database or other query engine110(e.g., block-based storage devices like hard disk drives or solid state drives) or may be a separate storage system, such as the storage service discussed below with regard toFIGS. 2-9, which may distribute the data amongst multiple different storage nodes in order to provide redundancy and availability for the data, in some embodiments. In at least some embodiments, data for the database130may be stored in one or more portions of data store, such as data pages. One or multiple data values, records, or objects may be stored in a data page. In at least some embodiments, data pages may include metadata and/or other information for providing access to database130. For example, data pages may store data dictionaries, transaction logs, undo and redo log records, in one embodiment. Query engine110may perform access requests (e.g., requests to read, obtain, query, write, update, modify, or otherwise access) to database130based on state information120. State information120may include data dictionaries, undo logs, transaction logs/tables, indexing structures, mapping information, data page caches or buffers, and the like, or any other information used to perform access requests with respect to the database, in some embodiments. For example, state information include mapping information (e.g., an index) to obtain data records that match certain search criteria (e.g., query predicates), in some embodiments.

In at least some embodiments, query engine110may receive a request102to change the database to a previous state. The request may be validated, in some embodiments, as discussed below with regard toFIG. 11, to determine whether the previous state is within an online restore range for the database, in some embodiments. Online restore may be facilitated by storage of snapshots, such as snapshots142a,142b, and so on, for different prior states of the database as part of the data store for database130(e.g., in storage locations or slots allocated to database130, as discussed below with regard toFIG. 7). Change log150may also be maintained describing the changes applied to the database (e.g., as log records), including changes to be made between the different snapshots. Online restore may be performed to a previous state within the range of states provided by snapshots142and change log150.

For example, previous state information corresponding to the previous state of the restore operation may be obtained112from the application of changes in change log150to a snapshot142(e.g., the snapshot prior and closest to the previous state of the database). The previous state information114may be used to replace, modify, or otherwise update state information120in query engine110. In some embodiments, an exclusion range may be identified to exclude log records that describe changes that are to be applied after a previous state. For example, exclusion range162may exclude log records from T1to T3, for a restore operation that occurs at T3(which returns the state of the database to the state at T1). Then query engine110can perform access requests, including changes described by log records in change log150from T3on.

Note that exclusion ranges do not trigger or identify log records for deletion, in various embodiments. Exclusion ranges162may not be permanent, in some embodiments. For example, a request to restore the database to a previous state (e.g., at T2) can be received even after the restore operation at T3. A new exclusion range can be identified so that log records from T2to T4are excluded in exclusion range164. In this way, restore operations can move forward or backward within the online restore range. Note that log records can change from being excluded to include (as the log records between T1and T2become included as a result of the new exclusion range164. Moreover, as the log records are not removed, even if excluded, then future restore operations can be performed to include changes made after a first restore operation but were later excluded as of a different restore operation. Continuing with the above examples, if another restore operation to be made at a later time (e.g., at T5—not illustrated), to return to state T4is performed, then multiple exclusion ranges from T1to T3and T4to T5may be identified and records from T3to T4which were changes made after the restore from T3to T1would be included for processing access requests. Additionally, because the state information is restored according the data physically in the data store, restore operations can be physical restore operations (e.g., which do not incur computational complexity to determine the logical state of the database but may rely upon change log150and snapshots142to determine the state of the database).

As discussed below with regard toFIGS. 7 to 9, the online restore range may change over time. Query engine110can proceed to process access requests based on the exclusion range, so that log records included in change log150are used to process access requests.

Please note,FIG. 1is provided as a logical illustration of online restore for database engines and is not intended to be limiting as to the physical arrangement, size, or number of components, modules, or devices implementing a query engine, database, state storage, state information, or change log.

The specification first describes an example of a database system as a network-based database service that stores data for a database managed by the database service in a separate data storage service, according to various embodiments. Included in the description of the example network-based database service are various aspects of the example network-based database service along with the various interactions between the database service, the storage service including the performance of online restore operations for a database engine. The specification then describes a flowchart of various embodiments of methods for online restore operations for database engines. Next, the specification describes an example system that may implement the disclosed techniques. Various examples are provided throughout the specification.

The systems described herein may, in some embodiments, implement a network-based service that enables clients (e.g., subscribers) to operate a data storage system in a cloud computing environment. In some embodiments, the data storage system may be an enterprise-class database system that is highly scalable and extensible. In some embodiments, queries may be directed to database storage that is distributed across multiple physical resources, and the database system may be scaled up or down on an as needed basis. The database system may work effectively with database schemas of various types and/or organizations, in different embodiments. In some embodiments, clients/subscribers may submit queries in a number of ways, e.g., interactively via an SQL interface to the database system. In other embodiments, external applications and programs may submit queries using Open Database Connectivity (ODBC) and/or Java Database Connectivity (JDBC) driver interfaces to the database system.

More specifically, the systems described herein may, in some embodiments, implement a service-oriented architecture in which various functional components of a single database system are intrinsically distributed. For example, rather than lashing together multiple complete and monolithic database instances (each of which may include extraneous functionality, such as an application server, search functionality, or other functionality beyond that required to provide the core functions of a database), these systems may organize the basic operations of a database (e.g., query processing, transaction management, caching and storage) into tiers that may be individually and independently scalable. For example, in some embodiments, each database instance in the systems described herein may include a database tier (which may include a single database engine head node and a client-side storage system driver), a separate, distributed storage system (which may include multiple storage nodes that collectively perform some of the operations traditionally performed in the database tier of existing systems), and a backup storage tier.

As described in more detail herein, in some embodiments, some of the lowest level operations of a database, (e.g., backup, restore, recovery, log record manipulation, and/or various space management operations) may be offloaded from the database engine to the storage layer (or tier), such as a distributed storage system, and distributed across multiple nodes and storage devices. For example, in some embodiments, rather than the database engine applying changes to a database (or data pages thereof) and then sending the modified data pages to the storage layer, the application of changes to the stored database (and data pages thereof) may be the responsibility of the storage layer itself. In such embodiments, redo log records, rather than modified data pages, may be sent to the storage layer, after which redo processing (e.g., the application of the redo log records) may be performed somewhat lazily and in a distributed manner (e.g., by a background process). Log sequence numbers may be assigned to the redo log records from a log sequence number space. In some embodiments, crash recovery (e.g., the rebuilding of data pages from stored redo log records) may also be performed by the storage layer and may also be performed by a distributed (and, in some cases, lazy) background process. In some embodiments, the storage layer may maintain backup versions of data volumes in a separate storage system (e.g., another storage service implemented as part of network-based services platform200) by leveraging peer-to-peer replication among storage nodes to identify and obtain new updates to data volumes for inclusion in backup versions.

In some embodiments, because only redo logs (and not modified data pages) are sent to the storage layer, there may be much less network traffic between the database tier and the storage layer than in existing database systems. In some embodiments, each redo log may be on the order of one-tenth the size of the corresponding data page for which it specifies a change. Note that requests sent from the database tier and the distributed storage system may be asynchronous and that multiple such requests may be in flight at a time.

In various embodiments, a database instance may include multiple functional components (or layers), each of which provides a portion of the functionality of the database instance. In one such example, a database instance may include a query parsing and query optimization layer, a query execution layer, a transactionality and consistency management layer, and a durability and space management layer. Rather than duplicating an entire database instance one or more times and adding glue logic to stitch them together to scale a database, the systems described herein may instead offload the functionality of durability and space management layer from the database tier to a separate storage layer, and may distribute that functionality across multiple storage nodes in the storage layer, in some embodiments.

In some embodiments, the database systems described herein may retain much of the structure of the upper half of the database instance, such as query parsing and query optimization layer, a query execution layer, and a transactionality and consistency management layer, but may redistribute responsibility for at least portions of the backup, restore, snapshot, recovery, and/or various space management operations to the storage tier. Redistributing functionality in this manner and tightly coupling log processing between the database tier and the storage tier may improve performance, increase availability and reduce costs, when compared to previous approaches to providing a scalable database, in some embodiments. For example, network and input/output bandwidth requirements may be reduced, since only redo log records (which are much smaller in size than the actual data pages) may be shipped across nodes or persisted within the latency path of write operations. In addition, the generation of data pages can be done independently in the background on each storage node (as foreground processing allows), without blocking incoming write operations. In some embodiments, the use of log-structured, non-overwrite storage may allow copy creation, backup, restore, snapshots, point-in-time recovery, and volume growth operations to be performed more efficiently, e.g., by using metadata manipulation rather than movement or copying of a data page. In some embodiments, the storage layer may also assume the responsibility for the replication of data stored on behalf of clients (and/or metadata associated with that data, such as redo log records) across multiple storage nodes. For example, data (and/or metadata) may be replicated locally (e.g., within a single “availability zone” in which a collection of storage nodes executes on its own physically distinct, independent infrastructure) and/or across availability zones in a single region or in different regions, in one embodiment.

In various embodiments, the database systems described herein may support a standard or custom application programming interface (API) for a variety of database operations. For example, the API may support operations for creating a database, creating a table, altering a table, creating a user, dropping a user, inserting one or more rows in a table, copying values, selecting data from within a table (e.g., querying a table), canceling or aborting a query, creating a snapshot, and/or other operations.

In some embodiments, the database tier of a database instance may include a database engine head node server that receives read and/or write requests from various client programs (e.g., applications) and/or subscribers (users), then parses them and develops an execution plan to carry out the associated database operation(s). For example, the database engine head node may develop the series of steps necessary to obtain results for complex queries and joins. In some embodiments, the database engine head node may manage communications between the database tier of the database system and clients/subscribers, as well as communications between the database tier and a separate distributed storage system.

In some embodiments, the database engine head node may be responsible for receiving SQL requests from end clients through a JDBC or ODBC interface and for performing SQL processing and transaction management (which may include locking) locally. However, rather than generating data pages locally, the database engine head node (or various components thereof) may generate redo log records and may ship them to the appropriate nodes of a separate distributed storage system. In some embodiments, a client-side driver for the distributed storage system may be hosted on the database engine head node and may be responsible for routing redo log records to the storage system node (or nodes) that store the segments (or data pages thereof) to which those redo log records are directed. For example, in some embodiments, each segment may be mirrored (or otherwise made durable) on multiple storage system nodes that form a protection group. In such embodiments, the client-side driver may keep track of the nodes on which each segment is stored and may route redo logs to all of the nodes on which a segment is stored (e.g., asynchronously and in parallel, at substantially the same time), when a client request is received. As soon as the client-side driver receives an acknowledgement back from a write quorum of the storage nodes in the protection group (which may indicate that the redo log record has been written to the storage node), it may send an acknowledgement of the requested change to the database tier (e.g., to the database engine head node). For example, in embodiments in which data is made durable through the use of protection groups, the database engine head node may not be able to commit a transaction until and unless the client-side driver receives a reply from enough storage node instances to constitute a write quorum, as may be defined in a protection group policy for the data.

In some embodiments, the database tier (or more specifically, the database engine head node) may include a cache in which recently accessed data pages are held temporarily. In such embodiments, if a write request is received that targets a data page held in such a cache, in addition to shipping a corresponding redo log record to the storage layer, the database engine may apply the change to the copy of the data page held in its cache. A data page held in this cache may not ever be flushed to the storage layer, and it may be discarded at any time (e.g., at any time after the redo log record for a write request that was most recently applied to the cached copy has been sent to the storage layer and acknowledged). The cache may implement any of various locking mechanisms to control access to the cache by at most one writer (or multiple readers) at a time, in different embodiments. Note, however, that in embodiments that include such a cache, the cache may not be distributed across multiple nodes, but may exist only on the database engine head node for a given database instance. Therefore, there may be no cache coherency or consistency issues to manage.

In some embodiments, the client-side driver running on the database engine head node may expose a private interface to the storage tier. In some embodiments, it may also expose a traditional iSCSI interface to one or more other components (e.g., other database engines or virtual computing services components). In some embodiments, storage for a database instance in the storage tier may be modeled as a single volume that can grow in size without limits, and that can have an unlimited number of IOPS associated with it. When a volume is created, it may be created with a specific size, with a specific availability/durability characteristic (e.g., specifying how it is replicated), and/or with an TOPS rate associated with it (e.g., both peak and sustained). For example, in some embodiments, a variety of different durability models may be supported, and users/subscribers may be able to specify, for their database, a number of replication copies, zones, or regions and/or whether replication is synchronous or asynchronous based upon their durability, performance and cost objectives.

In some embodiments, the client side driver may maintain metadata about the volume and may directly send asynchronous requests to each of the storage nodes necessary to fulfill read requests and write requests without requiring additional hops between storage nodes. The volume metadata may indicate which protection groups, and their respective storage nodes, maintain which partitions of the volume, in some embodiments. For example, in some embodiments, in response to a request to make a change to a database, the client-side driver may determine the protection group, and its one or more nodes that are implementing the storage for the targeted data page, and to route the redo log record(s) specifying that change to those storage nodes. The storage nodes may then be responsible for applying the change specified in the redo log record to the targeted data page at some point in the future, in some embodiments. As writes are acknowledged back to the client-side driver, the client-side driver may advance the point at which the volume is durable and may acknowledge commits back to the database tier, in some embodiments. As previously noted, in some embodiments, the client-side driver may not ever send data pages to the storage node servers. This may not only reduce network traffic, but may also remove the need for the checkpoint or background writer threads that constrain foreground-processing throughput in previous database systems.

In some embodiments, many read requests may be served by the database engine head node cache. However, write requests may require durability, since large-scale failure events may be too common to allow only in-memory replication. Therefore, the systems described herein may minimize the cost of the redo log record write operations that are in the foreground latency path by implementing data storage in the storage tier as two regions: a small append-only log-structured region into which redo log records are written when they are received from the database tier, and a larger region in which log records are coalesced together to create new versions of data pages in the background. In some embodiments, an in-memory structure may be maintained for each data page that points to the last redo log record for that page, backward chaining log records until an instantiated data block is referenced. This approach may provide good performance for mixed read-write workloads, including in applications in which reads are largely cached.

In some embodiments, copies of databases may be created in the storage tier that share data pages with the source of the copy. For example, a copy of a portion of a database (e.g., an extent of a database volume), may be stored on the same storage node as the source database and include pointers to data pages stored in the source database so that the resulting amount of storage consumed by the copy is limited to storing changes to data pages that differ from the original copy, providing a copy-on-write technique for creating and updating copies of a database.

FIG. 2is a logical block diagram illustrating a service system architecture for a network-based database service and a network-based storage service that utilize shared data pages in the storage service to create copies of a database managed in the database service, according to some embodiments. In the illustrated embodiment, a number of clients (shown as clients250a-250n) may interact with a network-based services platform200via a network260. Network-based services platform200may interface with one or more instances of a database service210, a distributed storage service220and/or one or more other virtual computing services230. Storage service may be implemented as log-structured storage using a single log sequence number space. It is noted that where one or more instances of a given component may exist, reference to that component herein may be made in either the singular or the plural. However, usage of either form is not intended to preclude the other.

In various embodiments, the components illustrated inFIG. 2may be implemented directly within computer hardware, as instructions directly or indirectly executable by computer hardware (e.g., a microprocessor or computer system), or using a combination of these techniques. For example, the components ofFIG. 2may be implemented by a system that includes a number of computing nodes (or simply, nodes), each of which may be similar to the computer system embodiment illustrated inFIG. 13and described below. In various embodiments, the functionality of a given service system component (e.g., a component of the database service or a component of the storage service) may be implemented by a particular node or may be distributed across several nodes. In some embodiments, a given node may implement the functionality of more than one service system component (e.g., more than one database service system component).

Clients250may encompass any type of client configurable to submit network-based services requests to network-based services platform200via network260, including requests for database services (e.g., a request to create a copy of a database, etc.). For example, a given client250may include a suitable version of a web browser, or may include a plug-in module or other type of code module that can execute as an extension to or within an execution environment provided by a web browser. Alternatively, a client250(e.g., a database service client) may encompass an application such as a database application (or user interface thereof), a media application, an office application or any other application that may make use of persistent storage resources to store and/or access one or more databases. In some embodiments, such an application may include sufficient protocol support (e.g., for a suitable version of Hypertext Transfer Protocol (HTTP)) for generating and processing network-based services requests without necessarily implementing full browser support for all types of network-based data. That is, client250may be an application that can interact directly with network-based services platform200. In some embodiments, client250may generate network-based services requests according to a Representational State Transfer (REST)-style network-based services architecture, a document- or message-based network-based services architecture, or another suitable network-based services architecture.

In some embodiments, a client250(e.g., a database service client) may provide access to storage of databases to other applications in a manner that is transparent to those applications. For example, client250may integrate with an operating system or file system to provide storage in accordance with a suitable variant of the storage models described herein. However, the operating system or file system may present a different storage interface to applications, such as a conventional file system hierarchy of files, directories and/or folders. In such an embodiment, applications may not need to be modified to make use of the storage system service model. Instead, the details of interfacing to network-based services platform200may be coordinated by client250and the operating system or file system on behalf of applications executing within the operating system environment.

Clients250may convey network-based services requests (e.g., request to create a copy of a database, queries to a database, etc.) to and receive responses from network-based services platform200via network260. In various embodiments, network260may encompass any suitable combination of networking hardware and protocols necessary to establish network-based-based communications between clients250and platform200. For example, network260may generally encompass the various telecommunications networks and service providers that collectively implement the Internet. Network260may also include private networks such as local area networks (LANs) or wide area networks (WANs) as well as public or private wireless networks. For example, both a given client250and network-based services platform200may be respectively provisioned within enterprises having their own internal networks. In such an embodiment, network260may include the hardware (e.g., modems, routers, switches, load balancers, proxy servers, etc.) and software (e.g., protocol stacks, accounting software, firewall/security software, etc.) necessary to establish a networking link between given client250and the Internet as well as between the Internet and network-based services platform200. It is noted that in some embodiments, clients250may communicate with network-based services platform200using a private network rather than the public Internet. For example, clients250may be provisioned within the same enterprise as a database service system (e.g., a system that implements database service210and/or distributed storage service220). In such a case, clients250may communicate with platform200entirely through a private network260(e.g., a LAN or WAN that may use Internet-based communication protocols but which is not publicly accessible).

Generally speaking, network-based services platform200may implement one or more service endpoints that receive and process network-based services requests, such as requests to access data pages (or records thereof). For example, network-based services platform200may include hardware and/or software may implement a particular endpoint, such that an HTTP-based network-based services request directed to that endpoint is properly received and processed. In one embodiment, network-based services platform200may be implemented as a server system may receive network-based services requests from clients250and to forward them to components of a system that implements database service210, storage service220and/or another virtual computing service230for processing (e.g. another data storage service, such as an object data store which may store data objects that make up a backup version data volumes stored in the storage service220. In other embodiments, network-based services platform200may be implemented as a number of distinct systems (e.g., in a cluster topology) implementing load balancing and other request management features to dynamically manage large-scale network-based services request processing loads. In various embodiments, network-based services platform200may support REST-style or document-based (e.g., SOAP-based) types of network-based services requests.

In addition to functioning as an addressable endpoint for clients' network-based services requests, in some embodiments, network-based services platform200may implement various client management features, in some embodiments. For example, platform200may coordinate the metering and accounting of client usage of network-based services, including storage resources, such as by tracking the identities of requesting clients250, the number and/or frequency of client requests, the size of data tables (or records thereof) stored or retrieved on behalf of clients250, overall storage bandwidth used by clients250, class of storage requested by clients250, or any other measurable client usage parameter, in some embodiments. Platform200may also implement financial accounting and billing systems, or may maintain a database of usage data that may be queried and processed by external systems for reporting and billing of client usage activity. In certain embodiments, platform200may collect, monitor and/or aggregate a variety of storage service system operational metrics, such as metrics reflecting the rates and types of requests received from clients250, bandwidth utilized by such requests, system processing latency for such requests, system component utilization (e.g., network bandwidth and/or storage utilization within the storage service system), rates and types of errors resulting from requests, characteristics of stored and requested data pages or records thereof (e.g., size, data type, etc.), or any other suitable metrics, in some embodiments. In some embodiments such metrics may be used by system administrators to tune and maintain system components, while in other embodiments such metrics (or relevant portions of such metrics) may be exposed to clients250to enable such clients to monitor their usage of database service210, distributed storage service220and/or another virtual computing service230(or the underlying systems that implement those services).

In some embodiments, network-based services platform200may also implement user authentication and access control procedures. For example, for a given network-based services request to access a particular database, platform200may ascertain whether the client250associated with the request is authorized to access the particular database. Platform200may determine such authorization by, for example, evaluating an identity, password or other credential against credentials associated with the particular database, or evaluating the requested access to the particular database against an access control list for the particular database, in some embodiments. For example, if a client250does not have sufficient credentials to access the particular database, platform200may reject the corresponding network-based services request, for example by returning a response to the requesting client250indicating an error condition. Various access control policies may be stored as records or lists of access control information by database service210, storage service220and/or other virtual computing services230, in some embodiments.

It is noted that while network-based services platform200may represent the primary interface through which clients250may access the features of a database system that implements database service210, it need not represent the sole interface to such features. For example, an alternate API that may be distinct from a network-based services interface may be used to allow clients internal to the enterprise providing the database system to bypass network-based services platform200. Note that in many of the examples described herein, distributed storage service220may be internal to a computing system or an enterprise system that provides database services to clients250, and may not be exposed to external clients (e.g., users or client applications). In such embodiments, the internal “client” (e.g., database service210) may access distributed storage service220over a local or private network, (e.g., through an API directly between the systems that implement these services). In such embodiments, the use of distributed storage service220in storing databases on behalf of clients250may be transparent to those clients. In other embodiments, distributed storage service220may be exposed to clients250through network-based services platform200to provide storage of databases or other information for applications other than those that rely on database service210for database management. In such embodiments, clients of the distributed storage service220may access storage service220via network260(e.g., over the Internet). In some embodiments, a virtual computing service230receive storage services from storage service220(e.g., through an API directly between the virtual computing service230and storage service220) to store objects used in performing computing services230on behalf of a client250. In some cases, the accounting and/or credentialing services of platform200may be unnecessary for internal clients such as administrative clients or between service components within the same enterprise.

Although not illustrated, in various embodiments, storage service220may interface with backup data store, system, service, or device. Various data, such as data pages, log records, and/or any other data maintained by distributed storage service internal clients, such as database service210or other virtual computing services230, and/or external clients such as clients250athrough250n, may be sent to a backup data store.

Note that in various embodiments, different storage policies may be implemented by database service210and/or storage service220. Examples of such storage policies may include a durability policy (e.g., a policy indicating the number of instances of a database (or data page thereof) that will be stored and the number of different nodes on which they will be stored) and/or a load balancing policy (which may distribute databases, or data pages thereof, across different nodes, volumes and/or disks in an attempt to equalize request traffic). In addition, different storage policies may be applied to different types of stored items by various one of the services. For example, in some embodiments, storage service220may implement a higher durability for redo log records than for data pages.

FIG. 3is a block diagram illustrating various components of a database system that includes a database engine and a separate storage service, according to one embodiment. In this example, database system300includes a respective database engine head node320for each of several databases and a distributed storage service310(which may or may not be visible to the clients of the database system, shown as database clients350a-350n). As illustrated in this example, one or more of database clients350a-350nmay access a database head node320(e.g., head node320a, head node320b, or head node320c, each of which is a component of a respective database instance) via network360(e.g., these components may be network-addressable and accessible to the database clients350a-350n). However, storage service310, which may be employed by the database system to store a database volume (such as data pages of one or more databases, as well as redo log records and/or other metadata associated therewith) and/or copies of a database volume on behalf of database clients350a-350n, and to perform other functions of the database system as described herein, may or may not be network-addressable and accessible to the storage clients350a-350n, in different embodiments. For example, in some embodiments, storage service310may perform various storage, access, change logging, recovery, log record manipulation, and/or space management operations in a manner that is invisible to storage clients350a-350n.

As previously noted, each database instance may include a single database engine head node320that receives requests (e.g., queries to read or write data, etc.) from various client programs (e.g., applications) and/or subscribers (users), then parses them, optimizes them, and develops an execution plan to carry out the associated database operation(s). In the example illustrated inFIG. 3, a query parsing, optimization, and execution component305of database engine head node320amay perform these functions for queries that are received from database client350aand that target the database instance of which database engine head node320ais a component. In some embodiments, query parsing, optimization, and execution component305may return query responses to database client350a, which may include write acknowledgements, requested data pages (or portions thereof), error messages, and or other responses, as appropriate. As illustrated in this example, database engine head node320amay also include a client-side storage service driver325, which may route read requests and/or redo log records to various storage nodes within storage service310, receive write acknowledgements from storage service310, receive requested data pages from storage service310, and/or return data pages, error messages, or other responses to query parsing, optimization, and execution component305(which may, in turn, return them to database client350a). Client-side storage device may maintain mapping information about the database volume stored in storage service310, such that a particular protection group maintaining a partition of the database volume may be determined. Read requests and redo log records may then be routed to storage nodes that are members of the protection group according to the partition of user data to which the read request is directed or to which the redo log record pertains.

In this example, database engine head node320aincludes a data page cache335, in which data pages that were recently accessed may be temporarily held. As discussed below with regard toFIG. 12, data pages in the cache may be retained in the event of an online restore operation, in some embodiments. As illustrated inFIG. 3, database engine head node320amay also include a transaction and consistency management component330, which may be responsible for providing transactionality and consistency in the database instance of which database engine head node320ais a component. For example, this component may be responsible for ensuring the Atomicity, Consistency, and Isolation properties of the database instance and the transactions that are directed that the database instance. As illustrated inFIG. 3, database engine head node320amay also include a transaction log340and an undo log345, which may be employed by transaction and consistency management component330to track the status of various transactions and roll back any locally cached results of transactions that do not commit.

Note that each of the other database engine head nodes320illustrated inFIG. 3(e.g.,320band320c) may include similar components and may perform similar functions for queries received by one or more of database clients350a-350nand directed to the respective database instances of which it is a component.

In some embodiments, the storage systems described herein may organize data in various logical data volumes, extents (which may include partitions of the database (e.g., user data space) in the volume and a segmentation of the log for the volume) made durable among a protection group of storage nodes, segments (which may be data stored on an individual storage node of a protection group) and pages for storage on one or more storage nodes. For example, in some embodiments, each database is represented by a logical volume, and each logical volume is partitioned over a collection of storage nodes into extents. A protection group may be composed of different storage nodes in the storage service that together make an extent durable. Multiple segments, each of which lives on a particular one of the storage nodes in a protection group, are used to make the extent durable.

In some embodiments, each data page may be stored in a segment, such that each segment stores a collection of one or more data pages and a change log (also referred to as a redo log) (e.g., a log of redo log records) for each data page that it stores. Thus, change logs may be log records segmented to the protection group of which the segment is a member. As described in detail herein, the storage nodes may receive redo log records (which may also be referred to herein as ULRs) and to coalesce them to create new versions of the corresponding data pages (e.g., if a data page of a copy of a database is shared with the database and the new version is created to create a different version included in the copy and not visible to the database) and/or additional or replacement log records (e.g., lazily and/or in response to a request for a data page or a database crash). In some embodiments, data pages and/or change logs may be mirrored across multiple storage nodes, according to a variable configuration, such as in a protection group (which may be specified by the client on whose behalf the databases are being maintained in the database system). For example, in different embodiments, one, two, or three copies of the data or change logs may be stored in each of one, two, or three different availability zones or regions, according to a default configuration, an application-specific durability preference, or a client-specified durability preference.

One embodiment of a storage service is illustrated by the block diagram inFIG. 4. In at least some embodiments, storage nodes430-450may store data for different clients as part of a multi-tenant storage service. For example, the various segments discussed above and below with regard toFIG. 6, may correspond to different protection groups and volumes for different clients.

In some embodiments, a client, such as a database engine head node, may communicate with storage system server nodes that store data as part of a database managed by a client-side storage service driver at the client. In this example, storage service includes multiple storage system server nodes (including those shown as430,440, and450), each of which includes storage for data pages and redo logs for the segment(s) it stores, and hardware and/or software may perform various segment management functions437. For example, each storage system server node may include hardware and/or software may perform at least a portion of any or all of the following operations: replication (locally, e.g., within the storage node), coalescing of redo logs to generate data pages, log management (e.g., manipulating log records), crash recovery (e.g., determining candidate log records for volume recovery), creating snapshots of segments stored at the storage node (e.g., as discussed below with regard toFIG. 7) and/or space management (e.g., for a segment or state storage). Each storage system server node may also have multiple attached storage devices (e.g., SSDs, HDDs, or other persistent storage devices) on which data blocks may be stored on behalf of clients (e.g., users, client applications, and/or database service subscribers).

In the example illustrated inFIG. 4, storage system server node430includes data page(s)433, segment redo log(s)435(as discussed in more detail below with regard toFIG. 7) segment management functions437, and attached storage devices471-478. Similarly, storage system server node440includes data page(s)443, segment redo log(s)445, segment management functions447, and attached storage devices481-488; and storage system server node450includes data page(s)453, segment redo log(s)455, segment management functions457, and attached storage devices491-498.

In some embodiments, each of the storage system server nodes in the storage system may implement a set of processes running on the node server's operating system that manage communication with the database engine head node, e.g., to receive redo logs, send back data pages, etc. In some embodiments, all data blocks written to the storage system may be backed up to long-term and/or archival storage (e.g., in a remote key-value durable backup storage system).

In some embodiments, storage service310may also implement a storage service control plane412. Storage service control plane412may be one or more compute nodes that may perform a variety of different storage system management functions. For example, storage control plane may implement a volume manager (not illustrated), which may maintain mapping information or other metadata for a volume, such as current volume state, current writer, truncation tables or other truncation information, or any other information for a volume as it is persisted in varying different, extents, segments, and protection groups. The volume manager may communicate with a client of storage system410, such as client-side driver in order to “mount” or “open” the volume for the client, providing the client-side driver with mapping information, protection group policies, and various other information necessary to send write and read requests to storage nodes430-450. The volume manager may provide the maintained information to storage clients, such as a database engine head node or client-side driver or to other system components such as backup agents418. For example, the volume manager may provide a current volume state (e.g., clean, dirty or recovery), current epoch or other version indicator for the volume and/or any other information about the data volume.

In at least some embodiments, storage service control plane412may implement backup management414. Backup management414may implement or direct multiple backup agents418which may backup data volumes stored at storage nodes. For example, in some embodiments task queue(s) may be implemented that identify backup operations to be performed with respect to data volumes (e.g., describing the range of LSNs of redo log records being included in a chunk or portion of data that is to be uploaded to the backup data store). Volume backup metadata may be included as part of the backup performed by backup agent(s)418, including the volume geometry or configuration. As discussed above with regard toFIG. 1, changes made to a database after a restore operation may be included in a log. Backups of the log records, whether or not the log records are within an exclusion range may be performed, in some embodiments by backup agent(s)418.

FIG. 5is a block diagram illustrating the use of a separate storage system in a database system, according to one embodiment. In this example, one or more client processes510may store data to one or more databases maintained by a database system that includes a database engine520and a storage system530. In the example illustrated inFIG. 5, database engine520includes database tier components560and client-side driver540(which serves as the interface between storage system530and database tier components560). In some embodiments, database tier components560may perform functions such as those performed by query parsing, optimization and execution component305and transaction and consistency management component330ofFIG. 3, and/or may store data pages, transaction logs and/or undo logs (such as those stored by data page cache335, transaction log340and undo log345ofFIG. 3). In various embodiments, database engine520may have obtained a volume epoch indicator or other identifier from storage system530granting access writes to a particular data volume, such as by sending a request to open the data volume to storage system530.

In this example, one or more client processes510may send database query requests515(which may include read and/or write requests targeting data stored on one or more of the storage nodes535a-535n) to database tier components560, and may receive database query responses517from database tier components560(e.g., responses that include write acknowledgements and/or requested data). Each database query request515that includes a request to write to a data page may be parsed and optimized to generate one or more write record requests541, which may be sent to client-side driver540for subsequent routing to storage system530. In this example, client-side driver540may generate one or more redo log records531corresponding to each write record request541, and may send them to specific ones of the storage nodes535of specific protection groups storing the partition user data of user data space to which the write record request pertains in storage system530. Storage nodes535may perform various peer-to-peer communications to replicate redo log records531received at a storage node to other storage nodes that may have not received the redo log records431. For instance, not every storage node may receive a redo log record in order to satisfy a write quorum (e.g., 3 out of 5 storage nodes may be sufficient). The remaining storage nodes that do not receive or acknowledge the redo log record may receive an indication of it from a peer storage node that did acknowledge or receive the redo log record. Client-side driver540may generate metadata for each of the redo log records that includes an indication of a previous log sequence number of a log record maintained at the specific protection group. storage system530may return a corresponding write acknowledgement(s)523for each redo log record531to database engine520(specifically to client-side driver540). Client-side driver540may pass these write acknowledgements to database tier components560(as write responses542), which may then send corresponding responses (e.g., write acknowledgements) to one or more client processes510as one of database query responses517.

In this example, each database query request515that includes a request to read a data page may be parsed and optimized to generate one or more read record requests543, which may be sent to client-side driver540for subsequent routing to storage system530. In this example, client-side driver540may send these requests to specific ones of the storage nodes535of storage system530, and storage system530may return the requested data pages533to database engine520(specifically to client-side driver540). Client-side driver540may send the returned data pages to the database tier components560as return data records544, and database tier components560may then send the data pages to one or more client processes510as database query responses517.

In some embodiments, various error and/or data loss messages534may be sent from storage system530to database engine520(specifically to client-side driver540). These messages may be passed from client-side driver540to database tier components560as error and/or loss reporting messages545, and then to one or more client processes510along with (or instead of) a database query response517.

In some embodiments, backup nodes537may receive peer-to-peer indications from storage nodes535. By evaluating these indications backup nodes537may identify additional redo log records received at storage nodes535that have not been backed up. Backup node(s)537may send chunks or objects containing a set of redo log records551to backup storage system570to be stored as part of a backup version of the data volume. In some embodiments, data pages553to create a full backup of the data volume (as opposed to log records describing the changes to the data volume) or copy of the data volume that may reference data pages stored in another data volume in backup storage system570may be requested from storage nodes and sent to backup storage system570.

In some embodiments, the APIs531-534of storage system530and the APIs541-545of client-side driver540may expose the functionality of the storage system530to database engine520as if database engine520were a client of storage system530. For example, database engine520(through client-side driver540) may write redo log records or request data pages through these APIs to perform (or facilitate the performance of) various operations of the database system implemented by the combination of database engine520and storage system530(e.g., storage, access, change logging, recovery, and/or space management operations). As illustrated inFIG. 5, storage system530may store data blocks on storage nodes535a-535n, each of which may have multiple attached SSDs. In some embodiments, storage system530may provide high durability for stored data block through the application of various types of redundancy schemes.

Note that in various embodiments, the API calls and responses between database engine520and storage system530(e.g., APIs531-534) and/or the API calls and responses between client-side driver540and database tier components560(e.g., APIs541-545), and between storage system430and backup data store570inFIG. 5may be performed over a secure proxy connection (e.g., one managed by a gateway control plane), or may be performed over the public network or, alternatively, over a private channel such as a virtual private network (VPN) connection. These and other APIs to and/or between components of the database systems described herein may be implemented according to different technologies, including, but not limited to, Simple Object Access Protocol (SOAP) technology and Representational state transfer (REST) technology. For example, these APIs may be, but are not necessarily, implemented as SOAP APIs or RESTful APIs. SOAP is a protocol for exchanging information in the context of network-based services. REST is an architectural style for hypermedia systems. A RESTful API (which may also be referred to as a RESTful network-based service) is a network-based service API implemented using HTTP and REST technology. The APIs described herein may in some embodiments be wrapped with client libraries in various languages, including, but not limited to, C, C++, Java, C # and Perl to support integration with system components.

In the storage systems described herein, an extent may be a logical concept representing a highly durable unit of storage that can be combined with other extents (either concatenated or striped) to represent a volume. Each extent may be made durable by membership in a single protection group. An extent may provide an LSN-type read/write interface for a contiguous byte sub-range having a fixed size that is defined at creation. Read/write operations to an extent may be mapped into one or more appropriate segment read/write operations by the containing protection group. As used herein, the term “volume extent” may refer to an extent that is used to represent a specific sub-range of bytes within a volume.

As noted above, a data volume may consist of multiple extents, each represented by a protection group consisting of one or more segments. In some embodiments, log records directed to different extents may have interleaved LSNs. For changes to the volume to be durable up to a particular LSN it may be necessary for all log records up to that LSN to be durable, regardless of the extent to which they belong. In some embodiments, the client may keep track of outstanding log records that have not yet been made durable, and once all ULRs up to a specific LSN are made durable, it may send a Volume Durable LSN (VDL) message to one of the protection groups in the volume. The VDL may be written to all synchronous mirror segments (i.e. group members) for the protection group. This is sometimes referred to as an “Unconditional VDL” and it may be periodically persisted to various segments (or more specifically, to various protection groups) along with write activity happening on the segments. In some embodiments, the Unconditional VDL may be stored in log sector headers.

FIG. 6is a block diagram illustrating an example configuration of a database volume610, according to one embodiment. Volume610may be a logical concept representing a highly durable unit of storage that a user/client/application of the storage system understands, in some embodiments. A volume may be stored or maintained in a distributed store that appears to the user/client/application as a single consistent ordered log of write operations to various user pages of a database. Each write operation may be encoded in a User Log Record (ULR), which represents a logical, ordered mutation to the contents of a single user page within the volume. As noted above, a ULR may also be referred to herein as a redo log record. Each ULR may include a unique identifier (e.g., a Logical Sequence Number (LSN)) assigned from a log sequence number space. Each ULR may be persisted to one or more synchronous segments in the log-structured store that form a Protection Group (PG) maintaining the partition of user data space (i.e. extent) to which the update indicate by the log record pertains in order to provide high durability and availability for the ULR. A volume may provide an LSN-type read/write interface for a variable-size contiguous range of bytes. In some embodiments, a volume may consist of multiple extents, each made durable through a protection group. In such embodiments, a volume may represent a unit of storage composed of a mutable contiguous sequence of Volume Extents. Reads and writes that are directed to a volume may be mapped into corresponding reads and writes to the constituent volume extents. In some embodiments, the size of a volume may be changed by adding or removing volume extents from the end of the volume.

In this example, data corresponding to each of various address ranges615(shown as address ranges615a-615e) is stored as different segments645(shown as segments645a-645n). A segment maybe a limited-durability unit of storage assigned to a single storage node, in some embodiments. Multiple segments may be implemented in a protection group to persist an extent. More specifically, a segment may provide limited best-effort durability (e.g., a persistent, but non-redundant single point of failure that is a storage node) for a specific fixed-size byte range of data. This data may in some cases be a mirror of user-addressable data, or it may be other data, such as volume metadata or erasure coded bits, in various embodiments. A given segment may live on exactly one storage node, in one embodiment. Within a storage node, multiple segments may live on each storage device, and each segment may be restricted to one storage device (e.g., a segment may not span across multiple storage devices). In some embodiments, a segment may not be required to occupy a contiguous region on an storage device; rather there may be an allocation map in each storage device describing the areas that are owned by each of the segments. As noted above, a protection group may consist of multiple segments spread across multiple storage nodes. In some embodiments, a segment may provide an LSN-type read/write interface for a fixed-size contiguous range of bytes (where the size is defined at creation). In some embodiments, each segment may be identified by a Segment UUID (e.g., a universally unique identifier of the segment). More specifically, data corresponding to each of various address ranges615may be organized into different extents (shown as extents625a-625b, and extents635a-635h), and various ones of these extents may be included in different protection groups630(shown as630a-630f), with or without striping (such as that shown as stripe set620aand stripe set620b). In this example, protection group1illustrates the use of erasure coding. In this example, protection groups2and3and protection groups6and6represent mirrored data sets of each other, while protection group4represents a single-instance (non-redundant) data set. In this example, protection group8represents a multi-tier protection group that combines other protection groups (e.g., this may represent a multi-region protection group). In this example, stripe set1(620a) and stripe set2(620b) illustrates how extents (e.g., extents625aand625b) may be striped into a volume, in some embodiments.

More specifically, in this example, protection group1(630a) includes extents a-c (635a-635c), which include data from ranges1-3(615a-615c), respectively, and these extents are mapped to segments1-4(645a-645d). Protection group2(630b) includes extent d (635d), which includes data striped from range4(615d), and this extent is mapped to segments5-7(645e-945g). Similarly, protection group3(630c) includes extent e (635e), which includes data striped from range4(615d), and is mapped to segments8-6(645h-645i); and protection group4(630d) includes extent f (635f), which includes data striped from range4(615d), and is mapped to segment10(645j). In this example, protection group6(630e) includes extent g (635g), which includes data striped from range5(615e), and is mapped to segments11-12(645k-645l); and protection group7(630f) includes extent h (635h), which also includes data striped from range5(615e), and is mapped to segments13-14(645m-645n).

Please note that the striping, erasure coding, and other storage schemes for the database volume apply to the user data space of the database volume, not the log records pertaining to the volume. Log records are segmented across protection groups according to the partition of the volume maintained at the protection group. For example, log records indicating updates to the user data striped from range5maintained in PG6, pertain to the user data in PG6.

FIG. 7is a logical block diagram illustrating an example segment with state storage for segment snapshots, according to some embodiments. In some embodiments, a segment, such as segment780, implement a hot log zone720to accept new writes from the client as they are received by the storage node. For example, writes may be received from a client as Delta User Log Records (DULRs), which specify a change to a user/data page in the form of a delta from the previous version of the page, and Absolute User Log Records (AULRs), which specify the contents of a complete user/data page, may be written completely into the log. Log records may be added to this zone in approximately the order they are received (e.g., they are not sorted by LSN) and they can span across log pages. The log records may be self-describing, e.g., they may contain an indication of their own size.

In some embodiments, the storage systems described herein may maintain various data structures, such as page mapping770, in memory. For example, for each user page present in a segment, a user page table may store a bit indicating whether or not this user page is “cleared” (i.e., whether it includes all zeroes), the LSN of the latest log record from the cold log zone for the page, and an array/list of locations of all log records from the hot log zone for page. For each log record, the user page table may store the sector number, the offset of the log record within that sector, the number of sectors to read within that log page, the sector number of a second log page (if the log record spans log pages), and the number of sectors to read within that log page. In some embodiments, the user page table may also store the LSNs of every log record from the cold log zone and/or an array of sector numbers for the payload of the latest AULR if it is in the cold log zone.

In some embodiments of the storage systems described herein, an LSN index may be stored in memory. An LSN index may map LSNs to log pages within the cold log zone. Given that log records in cold log zone are sorted, it may be to include one entry per log page. However, in some embodiments, every non-obsolete LSN may be stored in the index and mapped to the corresponding sector numbers, offsets, and numbers of sectors for each log record.

In some embodiments of the storage systems described herein, a log page table may be stored in memory, and the log page table may be used during garbage collection of the cold log zone. For example, the log page table may identify which log records are obsolete (e.g., which log records can be garbage collected) and how much free space is available on each log page).

Segment780may be a segment stored for a database. As noted above log records received at a storage node may be stored710in a hot log zone720. Log records may be received out of order, appended to the hot log zone720as they are received. For example, inFIG. 7the ordering of log records proceeds from record702r, then702p,702q,702o,702n,702s, and finally702m(contrary to a sequential ordering which might start with702mto702s). Log records sent to a storage system, such as described above inFIG. 5, may be sent asynchronously, leading to log records received out of order at hot log720.

As discussed above, log records may be moved from the hot log720to store the log records730in the cold log740. The cold log zone may be populated by copying log records from the hot log zone. In some embodiments, only log records whose LSN is less than or equal to some threshold LSN value may be eligible to be copied to the cold log zone. When moving log records from the hot log zone to the cold log zone, some log records (such as many change log records) may not need to be copied because they are no longer necessary. In addition, some additional coalescing of log records to generate new versions750of user pages762may be performed at this point (e.g., either to overwrite or store a new version of a user page in a separate location). In some embodiments, log records stored in data blocks grouped together in log pages. In some embodiments, once a given hot zone page or data block has been completely written to cold log740and is no longer the newest hot zone data block, and all log records on the hot zone data block have been successfully copied to the cold log zone, the hot zone data block may be freed and reused.

Cold log zone740may, in various embodiments, maintain log records for a log-structured data store, such as log records702a,702b,702c,702d,702e,702f,702g,702h,702i,702j,702k, and7021respectively. The log records, of which many various descriptions presented above, may be AULRs, DULRs, or any other type of log record for the example storage system described above, or any other log-structured data store. These log records may be linked to or associated with a user page762. For example, a log record may describe an update/change/modification for some portion, or all, of the user page, such as change relative to a previous record or version of the data page (e.g., a DULR). In some embodiments, log records may be stored sequentially in data blocks or pages. Thus, the latest LSN in the ordering of log records maintained in a data block may indicate that all log records in the log page are prior to the latest LSN.

Base page storage760, may maintain entries or versions of user pages762a,762b,762cthrough762n. For example, each entry in base page storage760may maintain a replica or copy of the respective user page. In some embodiments, each entry may be compressed, encrypted, or otherwise modified. Other data, such as other log records linked to the data page, may also be stored with the data page in the entry for the data page in backstop760. Page mapping770may identify the locations of user pages, so that when a request to access a user page762is received (e.g., in order to read a user page762), page mapping770can be used to access the page.

State storage770may store snapshots of the segment (e.g., of base page storage760) at the same storage node, and in at least some embodiments, on the same storage device as base page storage760. For example, state storage may include a number of base page storage slots for storing snapshots of base page storage, such as snapshots764,772, and782. As discussed below with regard toFIG. 8, each snapshot need not store an individual copy of each data page but may, in some embodiments, store user pages that are different between snapshots, such as user page766b,766c,774b,774n,784a,784b,784c, and784n, and keep as available space for a user page as a result of sharing that user page copy with a prior snapshot (e.g., available user pages766a, and774a, share user page784a, available user page774cshares user page784c, and available user page766nshares user page774n). In this way, the online restore range for a segment can be increased by efficiently storing snapshots. However, in other embodiments, each snapshot may be a complete snapshot with no sharing of user data pages.

FIG. 8is a logical block diagram illustrating the relationships between snapshots of prior database states in state storage, according to some embodiments. As illustrated inFIG. 8, snapshot810of a database may be the source or parent snapshot (e.g., the oldest state) from which various other snapshots may be created. Snapshot820may be made (e.g., to add changes that occurred since snapshot810) from snapshot810. Likewise snapshot830may be created based on the differences between snapshot820and snapshot830. Together these snapshots may provide snapshots of the database at different states, in some embodiments. As each snapshot may be made by sharing database pages, as discussed above, the same data page may be shared amongst multiple snapshots. For example, snapshot820may share a data page with snapshot810. If that data page is not changed for snapshot820when snapshot830is made, then snapshot830share the same data page stored as part of snapshot810.

For example, page mapping850for snapshot830may store page mappings for the pages of snapshot830, such as mappings for pages852a,852b,852n, and so on. Each mapping852may include indication of the version of the data page, such as an LSN (e.g., LSNs854a,854b, and854n). The LSN854may indicate what changes are included in the stored data page (by indicating the point in the log record sequence for which the current version includes changes described in log records with lesser LSN values), in one embodiment. Page mappings852may also include a reference count for a page, such as ref counts856a,856b, and856n. Reference counts854may indicate whether a data page is relied upon or referenced by another snapshot, in some embodiments. In one embodiment, separate reference counts may be maintained for each snapshot. For example, snapshot810may maintain a reference count of one for data pages that it stores, snapshot820page mapping may maintain a reference count of two for a data page pointed to by snapshot820(as both snapshot820and snapshot810point to the shared page), and snapshot830page mapping may maintain a reference count of three for a data page pointed to by snapshot830in snapshot810(as snapshot830, snapshot820and snapshot810) point to the page. When determining whether a data page in a snapshot needs to be retained for a snapshot that is no longer within the online restore range (e.g., has an LSN prior to the garbage collection LSN as discussed below with regard toFIG. 8), the sum of all reference count values for the snapshots (e.g., snapshot810, snapshot820, snapshot830, and snapshot840) may be evaluated. If equal to 1, then only the snapshot outside the online restore range refers to that page and the page can be overwritten, deleted, or reclaimed for storing a new snapshot. As new snapshots are stored, the ref count values856may be updated correspondingly.

Note that the discussion and examples given above with regard toFIGS. 7-8are not intended to be limiting. Different state storage, for example, can have different numbers of snapshots, or different dependencies between snapshots. Similarly the numbers or location of shared pages can be different or can change.

FIG. 9is a sequence diagram illustrating garbage collection for a database that implements online restore operations, according to some embodiments. Database engine920may implement a client-side driver, such as client-side driver940, to store redo log record(s) describing changes to data pages made to portions of a database stored in a protection groups935a,935b, and935c, which may replicate data amongst segments according to peer-to-peer synchronization (e.g., a gossip-based redo log record replication technique) stored at storage nodes938a,938b, and938crespectively. As discussed above with regard toFIG. 7, storage nodes938may include as part storage for the database, state storage that stores snapshots of base page storage as of different prior states of the database. In order to reclaim log storage and state storage, a garbage collection point for the log may be determined, in various embodiments.

Client-side driver940may send a request950to each protection group935(each set of storage nodes938) to get a candidate log sequence number from that storage node for garbage collection, in some embodiments. In one embodiment, each storage node938may send960a candidate log sequence number corresponding to the state of the second oldest snapshot of the database (in order to progress garbage collection and the online restore range forward from the oldest snapshot). Client-side driver940may then select the oldest of the candidate LSNs (e.g., the lowest LSN value) as the new garbage collection LSN and send870the garbage collection LSN to the storage nodes938so that segment management at the storage node (e.g., segment management functions437inFIG. 4) may begin garbage collection of the log records up to the garbage collection LSN. Removal of snapshots in state storage may occur on a page by page basis as the reference count for a page in the snapshot becomes zero (as discussed above with regard toFIG. 8). In at least some embodiments, client-side driver may also update the online restore range at database engine920so that requests for online restore prior to the garbage collection LSN are denied, as discussed with regard toFIG. 11below.

The storage service and database service discussed inFIGS. 2 through 9provide examples of a data store that may implement online restore for database engines. However, various other types of storage systems and database systems may implement shared pages for database copies.FIG. 10is a high-level flowchart illustrating methods and techniques to implement online restore for database engines, according to some embodiments. Various components, systems, or devices, described above may perform the techniques described below with respect toFIGS. 10-12as well as different database systems and data stores.

As indicated at1010, a request to restore a database to a previous state may be received, in various embodiments. For example, a request may be received via a network-based interface (e.g., formatted according to an API), that identifies a database and state selection or indication, in one embodiment. State selection or indications of a previous state for a request to restore a database may include a point in time (e.g., date and time), a sequence number, or, in some embodiments, an event (e.g., a previous request that performs an update to the database, such as a Data Definition Language (DDL) type of request or a Data Manipulation Language (DML) type of request). In at least some embodiments, the state selection or indication may be an identifier of a state of the database that was previously provided in response to a request for recommend restore states for the database (e.g., that correspond to events such as DDL or DML events, power or other failure events, etc.). In at least some embodiments, the request may be validated. If, for instance, the request indicates a previous state of the database for which online restore operations are not available (e.g., outside of an online restore window), then request may be rejected.

As indicated at1020, state information of a query engine may be updated based, at least in part, on contents of the database stored prior to the previous state and one or more log records describing changes to be made to the contents of the database stored prior to the previous state, in some embodiments. State information may include data dictionaries, undo logs, transaction logs/tables, indexing structures, mapping information, data page caches or buffers, or any other information used to perform access requests with respect to the database, in some embodiments. At least some of the state information may be obtained from the data store by applying log records to prior contents (e.g., a snapshot of the database or portion thereof). For instance, a snapshot describing a state of the database that is before the previous state and closest to the previous state may be identified (e.g., by comparing a restore state sequence point determined based on the previous state (e.g., converting a timestamp to an sequence number) and the log record(s) describing changes to be made up to the previous state may be identified (by identifying log records with sequence values less than or equal to the restore point sequence number). The snapshots may include data pages, blocks, structures, or other state information that may be included as part of the update to the state information of query engine. For example, data dictionary information (e.g., pages), describing the structure or schema of the database (e.g., number of columns, column names, column data types, etc.) may be stored in a snapshot. The log records to be applied may include (or may not include) changes to the data dictionary information which can be applied to generate the data dictionary information of the database as of the previous state to which the database is being restored. Thus, the generated data dictionary from the snapshot may be included in the update to the state information of the query engine. Other state information changes, such as changes to the data page cache discussed below with regard toFIG. 12, may be performed separate from contents of the data store. Once updated, the query engine may be available for processing access requests to the database as of the restored state.

FIG. 11is a high-level flowchart illustrating methods and techniques to store snapshots of a database as part of a data store for a database to perform online restore for database engines, according to some embodiments. As indicated at1110, snapshots (or other data/contents that can represent the state of a database at a point in time) of different states of a database may be stored as part of a data store that stores data for the database and a log of changes to the database. For example, as discussed above with regard toFIG. 7, state storage may be allocated as part of database storage (e.g., different slots) which may allow for a number of snapshots to be stored for the different states. In some embodiments, the stored snapshots may determine the online restore window for the database. For example, an earliest snapshot may be the lower bound of the online restore window for the database (e.g., as of the sequence number associated with the earliest snapshot).

As indicated at1120, a request to restore the database to a previous state may be received, in various embodiments. As discussed above with regard toFIG. 10, a request may be received via a network-based interface (e.g., formatted according to an API), that identifies a database and state selection or indication, in one embodiment. State selection or indications of a previous state for a request to restore a database may include a point in time (e.g., date and time), a sequence number, or, in some embodiments, an event (e.g., a previous request that performs an update to the database, such as a Data Definition Language (DDL) type of request or a Data Manipulation Language (DML) type of request). In at least some embodiments, the state selection or indication may be an identifier of a state of the database that was previously provided in response to a request for recommend restore states for the database (e.g., that correspond to events such as DDL or DML events, power or other failure events, etc.). Note that the request may be a request to change the state of the database to an earlier state in the restore window or a later state in the restore window, in some embodiments.

In at least some embodiments, the request may be validated. If, for instance, the request indicates a previous state of the database for which online restore operations are not available (e.g., outside of an online restore window or range), as indicated by the negative exit from1130, then the request may be denied, as indicated at1132, in some embodiments. In some embodiments, the denial may provide indications of the online restore range and/or recommendations of restore states for the database. As the online restore range may change over time (e.g., due to garbage collection as discussed above with regard toFIG. 8), a request for a state that would have previously been within the online restore range may no longer be within the online restore range at a later time, in some embodiments.

If the request is valid, as indicated by the positive exit from1130, then, as indicated at1140, one or more log records that describe changes to be made to one of the stored snapshots of the database stored prior to the previous state indicated in the request may be applied to the one snapshot to generate new state information for a query engine, in various embodiments. For example, a snapshot describing a state of the database that is before the previous state and closest to the previous state may be identified (e.g., by comparing a restore state sequence point determined based on the previous state (e.g., converting a timestamp to an sequence number) and the log record(s) describing changes to be made up to the previous state may be identified (by identifying log records with sequence values less than or equal to the restore point sequence number). The log records may generate new state information that includes data dictionaries, undo logs, transaction logs/tables, indexing structures, mapping information, data page caches or buffers, or any other information used to perform access requests with respect to the database, in some embodiments. As indicated at1150, current state information in the query engine may be replaced with the new state information, in some embodiments. For example, memory locations that store the state information may be overwritten with the new state information.

In at least some embodiments, an exclusion range in the log may be identified as part of the online restore operation that excludes log records describing changes to be made after the previous state, as indicated at1160. For example, truncation mapping or other state information may be updated to identify a range of sequence number values that should not be viewed, applied, or otherwise used to process access requests to the database. Note that these log records may not be deleted from the log. As discussed above with regard toFIG. 1, the requests to restore the database may be performed multiple times both forward and backward within the online restore range. Thus, the identification and update of exclusion ranges may include changing a previously identified exclusion range to include log records that were previously identified as excluded prior to the request received at1120. As exclusion range values may change so that some log records are included or excluded from the log, exclusion range values may track an order or history or restore operations in some embodiments so as to determine which sequence numbers occurred after each restore operation.

As indicated at1170, one or more access requests may be performed with respect to the database restored to the previous state based, at least in part, on the exclusion range in the log, in some embodiments. For example, requests to write or update the database may be described in log records with sequence numbers that are assigned to values outside of the truncation log range (e.g., by inserting a gap in sequence number after the last log record in the exclusion range so as to prevent in-flight log records in architectures similar to those described above with regard toFIGS. 2-9from being included in the restored state of the database). Read requests, for which data pages are generated, as discussed above with regard toFIG. 5, may not apply changes described by log records in the exclusion range, in some embodiments.

As discussed above, state information for a query engine may include data that is used to process access requests, such as data page cache (or buffer page cache), which stores versions of data pages in memory so that the data pages do not have to be retrieved from memory when processing an access request, in some embodiments. In various embodiments, data pages in the data page cache can store various types of information, including user data (e.g., data values, records, fields, etc.) or metadata or other state information for performing access requests (e.g., data dictionary pages). Because data page caches can save significant I/O costs when processing access requests techniques that can leave a data page cache “warm” (e.g., storing relevant data pages) when performing an online restore can increase the speed of processing access requests upon completion of the online restore operation.FIG. 12is a high-level flowchart illustrating methods and techniques to update a data page cache as part of an online database restore, according to some embodiments.

The data pages currently stored in a data page cache during an online restore operation may be evaluated in order determine which data pages in the cache can remain for processing access requests. For example, as indicated at1210, a sequence number (or other identifier indicating the state of the database corresponding to the data page) of a data page in a data page cache can be compared with a restore point sequence number, in various embodiments. A restore point sequence number may be a sequence number associated with the previous state of the database to which the restore operation is directed. The comparison may determine whether the sequence number for the data page is greater than the restore point sequence number, as indicated at1220, and thus describes a data page corresponding to a state of the database that occurs after the state to which the database is being restored, in some embodiments.

If, as indicated by the positive exit from1220, the sequence number for the data page is greater than the restore point sequence number, then the data page may be removed from the data page cache, as indicated at1222, in some embodiments. For instance, the data page may be marked, overwritten, deleted, invalidated, or otherwise removed from consideration when processing subsequent access requests, in various embodiments. New data pages retrieved from a data store for the database may be written to the location or slot within the data page cache, in one embodiment. If, as indicated by the negative exit from1220, the sequence number of the data page is not greater than the response point sequence number, then the data page may remain in the data page cache (as the data page would be consistent the state of the database to which the restore operation is directed, in various embodiments. If, as indicated by the positive exit from1230, other data pages remain the data page cache to evaluate, then the same evaluation may continue until all remaining data pages have been evaluated, in various embodiments.

FIG. 13is a block diagram illustrating a computer system according to various embodiments, as well as various other systems, components, services or devices described above. For example, computer system2000may implement a database engine head node of a database tier, or one of a plurality of storage nodes of a separate distributed storage system that stores databases and associated metadata on behalf of clients of the database tier, in different embodiments. Computer system2000may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, handheld computer, workstation, network computer, a consumer device, application server, storage device, telephone, mobile telephone, or in general any type of computing device.

Computer system2000includes one or more processors2010(any of which may include multiple cores, which may be single or multi-threaded) coupled to a system memory2020via an input/output (I/O) interface2030. Computer system2000further includes a network interface2040coupled to I/O interface2030. In various embodiments, computer system2000may be a uniprocessor system including one processor2010, or a multiprocessor system including several processors2010(e.g., two, four, eight, or another suitable number). Processors2010may be any suitable processors capable of executing instructions. For example, in various embodiments, processors2010may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of processors2010may commonly, but not necessarily, implement the same ISA. The computer system2000also includes one or more network communication devices (e.g., network interface2040) for communicating with other systems and/or components over a communications network (e.g. Internet, LAN, etc.). For example, a client application executing on system2000may use network interface2040to communicate with a server application executing on a single server or on a cluster of servers that implement one or more of the components of the database systems described herein. In another example, an instance of a server application executing on computer system2000may use network interface2040to communicate with other instances of the server application (or another server application) that may be implemented on other computer systems (e.g., computer systems2090).

In the illustrated embodiment, computer system2000also includes one or more persistent storage devices2060and/or one or more I/O devices2080. In various embodiments, persistent storage devices2060may correspond to disk drives, tape drives, solid state memory, other mass storage devices, or any other persistent storage device. Computer system2000(or a distributed application or operating system operating thereon) may store instructions and/or data in persistent storage devices2060, as desired, and may retrieve the stored instruction and/or data as needed. For example, in some embodiments, computer system2000may host a storage system server node, and persistent storage2060may include the SSDs attached to that server node.

Computer system2000includes one or more system memories2020that can store instructions and data accessible by processor(s)2010. In various embodiments, system memories2020may be implemented using any suitable memory technology, (e.g., one or more of cache, static random access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM, synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM, non-volatile/Flash-type memory, or any other type of memory). System memory2020may contain program instructions2025that are executable by processor(s)2010to implement the methods and techniques described herein. In various embodiments, program instructions2025may be encoded in platform native binary, any interpreted language such as Java™ byte-code, or in any other language such as C/C++, Java™, etc., or in any combination thereof. For example, in the illustrated embodiment, program instructions2025include program instructions executable to implement the functionality of a database engine head node of a database tier, or one of a plurality of storage nodes, backup nodes, or restore nodes of a separate distributed storage system that stores databases and associated metadata on behalf of clients of the database tier, in different embodiments. In some embodiments, program instructions2025may implement multiple separate clients, server nodes, and/or other components.

In some embodiments, system memory2020may include data store2045, which may be implemented as described herein. For example, the information described herein as being stored by the database tier (e.g., on a database engine head node), such as a transaction log, an undo log, cached page data, or other information used in performing the functions of the database tiers described herein may be stored in data store2045or in another portion of system memory2020on one or more nodes, in persistent storage2060, and/or on one or more remote storage devices2070, at different times and in various embodiments. Similarly, the information described herein as being stored by the storage tier (e.g., redo log records, coalesced data pages, and/or other information used in performing the functions of the distributed storage systems described herein) may be stored in data store2045or in another portion of system memory2020on one or more nodes, in persistent storage2060, and/or on one or more remote storage devices2070, at different times and in various embodiments. In general, system memory2020(e.g., data store2045within system memory2020), persistent storage2060, and/or remote storage2070may store data blocks, replicas of data blocks, metadata associated with data blocks and/or their state, database configuration information, and/or any other information usable in implementing the methods and techniques described herein.

In one embodiment, I/O interface2030may coordinate I/O traffic between processor2010, system memory2020and any peripheral devices in the system, including through network interface2040or other peripheral interfaces. In some embodiments, I/O interface2030may perform any necessary protocol, timing or other data transformations to convert data signals from one component (e.g., system memory2020) into a format suitable for use by another component (e.g., processor2010). In some embodiments, I/O interface2030may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some embodiments, the function of I/O interface2030may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some embodiments, some or all of the functionality of I/O interface2030, such as an interface to system memory2020, may be incorporated directly into processor2010.

Network interface2040may allow data to be exchanged between computer system2000and other devices attached to a network, such as other computer systems2090(which may implement one or more storage system server nodes, database engine head nodes, and/or clients of the database systems described herein), for example. In addition, network interface2040may allow communication between computer system2000and various I/O devices2050and/or remote storage2070. Input/output devices2050may, in some embodiments, include one or more display terminals, keyboards, keypads, touchpads, scanning devices, voice or optical recognition devices, or any other devices suitable for entering or retrieving data by one or more computer systems2000. Multiple input/output devices2050may be present in computer system2000or may be distributed on various nodes of a distributed system that includes computer system2000. In some embodiments, similar input/output devices may be separate from computer system2000and may interact with one or more nodes of a distributed system that includes computer system2000through a wired or wireless connection, such as over network interface2040. Network interface2040may commonly support one or more wireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or another wireless networking standard). However, in various embodiments, network interface2040may support communication via any suitable wired or wireless general data networks, such as other types of Ethernet networks, for example. Additionally, network interface2040may support communication via telecommunications/telephony networks such as analog voice networks or digital fiber communications networks, via storage area networks such as Fibre Channel SANs, or via any other suitable type of network and/or protocol. In various embodiments, computer system2000may include more, fewer, or different components than those illustrated inFIG. 13(e.g., displays, video cards, audio cards, peripheral devices, other network interfaces such as an ATM interface, an Ethernet interface, a Frame Relay interface, etc.)

The various methods as illustrated in the figures and described herein represent example embodiments of methods. The methods may be implemented manually, in software, in hardware, or in a combination thereof. The order of any method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc.