tailieunhanh - SQL Server Tacklebox- P42

SQL Server Tacklebox- P42: This book, as with almost all books, started out as an idea. I wanted to accumulate together those scripts and tools that I have built over the years so that DBAs could sort through them and perhaps adapt them for their own circumstances. | 8 Finding data corruption to the operating system and subsequently the disk controller driver and disk itself. For example I have seen this sort of data corruption caused by a power outage in the middle of a transaction. However it is not just disk subsystem failures that cause data corruption. If you upgrade a database from SQL Server 2000 to SQL Server 2005 or 2008 and then interrogate it using the corruption-seeking script provided in this chapter you may be surprised to find that you will receive what can be construed as errors in the database files. However fortunately these are just warnings regarding space usage between versions and there are recommended steps to address the issue such as running DBCC UPDATEUSAGE. Whatever the cause the DBA does not want to live in ignorant bliss of possible corruption for any length of time. Unfortunately the corruption monster is often adept at hiding and will not rear its head until you interact with the corrupt data. By this time the corruption may have worked its way into your backup files and when falling through to your last resort of restoring the database you may simply restore the same corruption. The importance of a solid regular backup strategy cannot be overstated so I will state it quite often . On top of that you need a script or tool that will regularly check and report on any corruption issues before it s too late. I ll provide just such a script in this chapter. Consequences of corruption As noted in the previous section most of the time corruption occurs due to failure in an external hardware source like a hard disk controller or power supply. SQL Server 2005 and later uses a feature called Page Checksum to detect potential problems that might arise from this. This feature creates a checksum value during writes of pages to and subsequent reads from disk. Essentially if the checksum value read for a page does not match what was originally written then SQL Server knows that the data was modified outside of .

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