DATABASE BACKUPS, RECOVER Y, AND MAINTENANCE in Font

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CHAPTER 7 DATABASE BACKUPS, RECOVER Y, AND MAINTENANCE
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3. Move to the Options tab, as shown in Figure 7-3, where we can define what options we want to happen as part of this backup.
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Figure 7-3. Database backup options The first section of this dialog box deals with what you want to happen on second and subsequent backups. The first time the backup is run, it creates the backup files, but when you run subsequent backups, do you want to append to the current data or overwrite it If this is a full backup, then you may overwrite, as you should be placing this full backup over an old nonrequired backup. However, if this is a differential backup, where it is perhaps the second or third of the week, then you will append to the existing backup set. This will be after the previous differential backups and means that if you need to do a restore, all the backups will be one after another and therefore will provide the fastest retrieval. The Check Media Set Name option forces the backup to check that where the data is going to be backed up to is still a valid name and, if appending, that the data set has not expired. You use the Back Up to a New Media Set, and Erase All Existing Backup Sets option when any previous backups are no longer required. This is ideal when moving the database from development to either user testing or even production, and you don t want to be able to restore from an incorrect backup. There is no point in wishing to restore a production server from a development backup, after all. The second section deals with the reliability of the backup. It is possible to simply back up the data and trust that everything worked perfectly, meaning no data transmission errors occurred between your SQL Server and the backup device, or that no errors occurred when writing the data. A situation such as this is unusual, but there will be times when it does happen. Do you trust that those times will occur when you will not need a backup I suggest this is something you cannot and should not rely on. Therefore, although it will increase the amount of time the backup takes, it is good to choose one
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CHAPTER 7 DATABASE BACKUPS, RECOVERY , AN D MAINTENANCE
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of the two options in this section. The first option allows a verification of the backup where SQL Server compares what has been backed up with what it expects to have been backed up, and the second option allows for checksum processing whereby SQL Server performs a mathematical calculation on the data to back up, which generates a checksum that can then be compared once the data has been transmitted from SQL Server to the backup device. If you select the second option, you can also specify whether to continue if you get a checksum error. If you are doing a transaction log backup, the next area of the dialog box will be enabled. You can logically shrink the transaction log by removing all entries that have just been backed up by selecting the first option, Truncate the Transaction Log. To save processing time, the physical transaction log is not shrunk. The second option, Back Up the Tail of the Log, is used when there has been some sort of database corruption. If you wish to back up transaction log records that have not been backed up prior to performing a restore to correct the corruption, then you would use this option. To clarify, a database becomes corrupt, and you need to be able to restore up to the last backup, then add all the transactions that have occurred since the last backup. By executing a backup of the tail of the log, you can restore the database and then use this tail log backup to add the missing transactions. The penultimate area of the dialog box is available if you are using tapes as your backup medium. You can eject the tape once the backup has finished. This is a useful option, as the computer operators would know to remove the tape for dispatch to the safe backup area. The second option, which specifies a rewind, is useful for full backups. On differential backups, however, SQL Server would be halted when running the next backup while the tape device got to the right place on the tape to continue the backup. Finally, it is possible to set the compression level for the backup. This is ideal to keep space taken for each backup to a minimum. Notice that by default, it takes the compression level set for the server, but it s possible to overwrite this with a new setting. If you do use compressed backups, then you will save on I/O and the backup operation will complete in less time, but completing the compression will increase the amount of processing SQL Server is using. It is possible to reduce the priority of completing a compressed backup so that it doesn t affect SQL Server by using the resource governor. This is an advanced topic and not covered in this book. Clicking OK starts the backup. Once the backup is finished, you should see the dialog box shown in Figure 7-4.
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Figure 7-4. A successful backup The first backup of the ApressFinancial database has now taken place and should have been successful. If we now move to the directory on the hard drive where the backup took place, then we will see the ApressFinancial file. Recall that it was mentioned earlier that a company lost a week s worth of data. It had set up the option to append to media, the tape had become full, and the administrator had not set up the proper scenario to alert someone when a problem occurred. So there was not just one failure in the system, but two; however, it still highlights that if you are using the option to append to media, you must check that enough room is available on the medium that you are appending to for the backup to succeed. Creating a backup of your database and the data is the most crucial action in building a database solution. Get it wrong, and you may as well go home. Well, not quite, but if (or when) things go wrong, and you don t have a valid or recent enough backup that is acceptable to the users of your database, it will take a long time for you as a developer to recover from that situation and get back to the excellent working relationship you had beforehand. The backup taken in the preceding example is the simplest backup to perform. It is a complete backup of our particular SQL Server database, and it happens while we are watching. If it goes wrong, we will instantly see and be able to deal with it. However, most backups do not happen when you are there and instead happen throughout the night. In the next section,
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