c# itextsharp datamatrix barcode DATABASE BACKUPS, RECOVERY, AND MAINTENANCE in Visual C#

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CHAPTER 7 DATABASE BACKUPS, RECOVERY, AND MAINTENANCE
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The first item to note is that every database within a SQL Server instance has its own transaction log. Every time SQL Server is requested to do any data modifications whether these are additions, deletions, or modifications a record is kept of the action. These recorded actions are kept in a place called a transaction log. There are several reasons for this. First of all, a piece of code could in fact do several different updates at once, either to different rows of data or rows of data in different tables or even databases. If one of the updates fails, for example, you are attempting to place ASCII characters into a column that only allows numerics, then you may wish to return the values in all the updated fields to their original value. This is called rolling back a transaction. SQL Server achieves this, in part, by looking at the data held in the transaction log. However, any successful action where all the updates are valid could be permanently stored on file, a process called committing a transaction. As more and more actions are placed in the transaction log, it will become full. Some of these actions will still be within a transaction, and others may form part of a completed transaction ready to be committed to the database. At certain points, SQL Server will want to remove all the actions it can to relieve some space in the transaction log for further actions. One point could be when the transaction log reached 70 percent full. SQL Server would then issue a checkpoint. The use of a checkpoint ensures that any data modifications are actually committed to the database and not held in any buffers, so that if a problem occurs, such as a power failure, there is a specific point that you can start from. Therefore, at the end of a checkpoint transaction, you know the database is in a consistent and valid state. As SQL Server knows that at a checkpoint all is well within the database, there is no need to keep the completed transactions recorded in the transaction log stored up to the checkpoint. SQL Server will therefore issue a truncation of the transaction log to remove these records, minimizing the size of the log on the computer. This is known as truncating the transaction log. It is thus necessary to ensure that you have a large enough transaction log defined to hold the largest valid uncommitted transaction, as these transactions obviously will not be truncated. A transaction log can become full with a rogue query as well, one that is incorrectly coded and just keeps adding more and more uncommitted transactions. When the transaction log reaches 70 percent, there is nothing to checkpoint and eventually the transaction log will fill up and your SQL Server will stop. This is where you will need the help of an experienced database administrator. If you had a power failure, you might have to replay all the work completed since the last backup, and the transaction log could also be used to do this, in certain scenarios. When a data modification is completed via a T-SQL command, the details are recorded first of all in the transaction log. These changes are flushed to disk, and therefore no longer in memory, before SQL Server starts to make changes to the tables that you are affecting. SQL Server doesn t write data immediately to disk. It is kept in a buffer cache until this cache is full or SQL Server issues a checkpoint, and then the data is written out. If a power failure occurs while the cache is still filling up, then that data is lost. Once the power comes back, though, SQL Server would start from its last checkpoint state, and from the transaction log, any updates after the last checkpoint that were logged as successful transactions will be performed.
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Note A disk cache is a space in the system where changes to the tables within the database are held. By
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doing so, a whole block of data can be written at once, saving on the slow process of disk head movement.
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