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prev gen time: 11/20/2009 00:26:26 Format LGVersion: (7.3704.16.1) Engine LGVersion: (7.3704.16.1) Signature: Create time:11/06/2009 00:06:00 Rand:466930790 Computer: EnvSystemPath: C:\Exchange\VIP\ EnvLogFilePath: C:\Exchange\VIP\ Env Log Sec size: 512 Env (CircLog,Session,Opentbl,VerPage,Cursors,LogBufs,LogFile,Buffers) ( off, 552, 27600, 15960, 27600, 2048, 2048, 24572) Using Reserved Log File: false Circular Logging Flag (current file): off Circular Logging Flag (past files): off Checkpoint at log creation time: : (0x733,8,0) 1 C:\Exchange\VIP\VIP.edb dbtime: 823366 (0-823366) objidLast: 4116 Signature: Create time:11/06/2010 00:06:00 Rand:466892842 Computer: MaxDbSize: 0 pages Last Attach: (0x4CF,9,86) Last Consistent: (0x4CE,8,1F) Last Lgpos: (0x736,34,0) Number of database page references: 304
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Integrity check passed for log file: e0100000736.log Operation completed successfully in 0.156 seconds.
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Transactions, buffers, and commitment
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After a client submits a message to the Store, an ESE session that is responsible for the transaction follows a well-defined order to apply the transaction to commit the new message to the Store. The same order is followed for other transactions such as deletes and moves. First, ESE obtains a timestamp using the internal time (called a db-time held in an 8-byte value) maintained in the database header. To modify a page, ESE must calculate a new db-time based on the current value in the header. Once it has calculated the new dbtime, ESE writes the records that make up the transaction into the current transaction log. After this operation completes, the session can go ahead and modify the appropriate pages in the database. Page modifications occur in an in-memory cache of dirty pages, so ESE might first have to fetch the necessary pages off the on-disk database. When a client performs an operation that modifies a database page, ESE follows this sequence:
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The database page is fetched from the ESE in-memory cache. If the page is not cached, ESE fetches it from disk.
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7 The Exchange 2010 Store
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A log record is generated to describe the page fetch and update the cache. The database page is modified and ESE marks the page as dirty. The page is not written immediately to disk as it might be modified by subsequent transactions. The version store tracks dirty pages to make sure that they are accounted for in case the database session is terminated abnormally. The version store is an internal component that keeps an in-memory list of modifications made to the database that the Store uses for purposes such as transaction rollback or to resolve attempts to apply multiple modifications to the same page. The database page is linked to the record in the cache to prevent ESE from flushing the page to disk before the transaction log record is written. ESE always commits pages to the database after it is sure that the pages have been successfully captured in a transaction log. Once the log buffer (1 MB) is full or a commit record is recorded for the transaction, ESE commits the changed page by recording it into the current transaction log. This operation might require a log roll (the creation of a new log generation). If a database is replicated, Exchange 2010 SP1 implements block replication at this point. See 8 for more information about how block replication works. Eventually, the dirty pages are flushed from memory and written into the database. The checkpoint is advanced.
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To ensure that data are always protected, it is a cardinal rule for ESE that database writes cannot occur before their transactions are first committed into a transaction log. If you look at the steps that make up a complete transaction, you see that the last step is to commit the transaction to disk. For example, the last step shown in Figure 7-14 is a commit command for the transaction identified as ESE session 8. Other prior steps in session 8 begin a transaction and insert some data to replace an existing page. The commit is a synchronous operation, so no other transaction can occur for that session until the write to disk is complete. Enabling write-back caching on the disk that holds the transaction logs improves performance by allowing the write to complete in the controller s memory and so release the synchronous wait. The controller is then responsible for writing the data to disk. Each log record represents an individual page modification. Transactions start with a Begin transaction record and proceed with the individual operations until the transaction is complete, a Commit transaction record is added, and the transaction is committed to the transaction log. The sequence of transaction records allows ESE to replay complete transactions into databases when it needs to recover data. If ESE reads in a transaction and cannot find a commit record, it considers the transaction to be incomplete and will not replay it into a database.
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