Sql checksum
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Repair_allow_data_loss is the minimum repair level for the errors found by DBCC CHECKDB (SomeTestDatabase).Īs we know the modification was made in the non-clustered index the resolution is fairly straightforward as we just need to drop and recreate the index. Index node page (0:0), slot 0 refers to child page (1:166) and previous child (0:0), but they were not encountered.ĬHECKDB found 0 allocation errors and 3 consistency errors in table ‘tblTestingTable’ (object ID 245575913).ĬHECKDB found 0 allocation errors and 3 consistency errors in database ‘SomeTestDatabase’.
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Test (IS_OFF (BUF_IOERR, pBUF->bstat)) failed. In addition to the CHECKSUM error being reported from a query we can run a database level consistency check and see the error being returned as well: This is highly recommended as these particular errors could indicate potential errors in the IO subsystem that require urgent attention. Here’s an example of creating an alert to capture Error 824 and we can then add a response to the alert to send an email to a group address warning us of any potential errors. I need the relevant index ID so I will use the following query to return the specific ID for my non-clustered index: I have a very small table consisting of three rows and a clustered index on the ID field and a non-clustered index on the LastName field.Īs I have already written the data to disk I am going to use the undocumented DBCC WRITEPAGE command to force an alteration to one of the pages making it “inconsistent” and hopefully picked up by our CHECKSUM operation.īefore I can do that I need to use another undocumented command DBCC IND to return the relevant Page ID’s for my non-clustered index as I want to ensure I am forcing an edit on the correct type of object. So using a test database let’s have a look at the CHECKSUM option in action. Set the PAGE_VERIFY database option to CHECKSUM.
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In fact Books Online makes its recommendation very clear:
Sql checksum verification#
The CHECKSUM option however bases its verification on a value calculated by using the entire page making the comparison between operations a much more thorough and effective option for page verification. When a page is written the first 2 bytes of each 512 byte sector is stored in the page header and when the page is read back again SQL Server makes a comparison between the stored information and the sector bytes to detect any discrepancy and return an error if the comparison fails. The TORN PAGE DETECTION option operates in a similar way to CHECKSUM. Having no page verification is a risky idea the whole purpose of the Page Verify option is to provide a crucial check between when a page is written to disk and when it is read again to ensure its consistency, this gives an indicator to possible corruption issues at the IO subsystem so it is clearly a vital option to check and be aware of. For Pre-SQL 2005 versions this was not the case, so occasionally when looking into database instances that have been migrated from earlier versions we might see one of the other Page Verify options selected they are TORN PAGE DETECTION or NONE. In most cases we’ll notice that the Page Verify option is set to CHECKSUM and that’s because nowadays this is the default option for all databases. We can also view the database options under its properties to see what option is enabled: