Over the past years, I have seen multiple DBAs execute DBCC CHECKDB with REPAIR ALLOW DATA LOSS to repair the SQL Server database corruption in their database without understanding repercussions. Even when you are working with CSS on such issues, CSS engineers actually explain what the command does and also send across a disclaimer stating why this command should NOT be executed on a database and what the repercussions are. Alas due to a lack of a sound database backup strategy, a lot of DBAs are forced to resort to losing data. In a nutshell, Repair Allow Data Loss tries to first repair the corruption in the page but if it cannot, then it checks if it can drop the page and repair the error and hence the name Allow Data Loss! I had a question for all the DBAs who advocate the use of this command:
How would you like your Payroll DBA to run the command? (Good enough food for thought!!)
The only way to compare how much data you lost would be to perform a CHECKDB before running the CHECKDB with REPAIR ALLOW DATA LOSS and after the CHECKDB has completed. The row counts returned by the two CHECKDB outputs would have to be compared. If CHECKDB is failing due to system catalog corruption, then you are plain out-of-luck.
What most people fail to understand that CHECKDB can verify integrity of the database based on the SQL database rules imposed on it like referential integrity, data length, record structures, page structures, header correctness etc. But these are database integrity rules that we are aware of. What about the business logic rules that you imposed while designing the database.
Eg: Let’s say you had a database table called Employee which depended on a Department ID valued to be present in the Department table. No foreign key relationship exists between the two tables. You experienced corruption in the database and used Repair Allow Data Loss command along with CHECKDB which dropped a few pages from the Department table. However, CHECKDB cannot know that a Department ID is missing in the Department table because you didn’t create a Foreign Key between the two tables. So when your application now tries to retrieve the Department ID of the Employee, it will BARF on you! This is the kind of scenario we specifically want to avoid which is why we advise against the use of data loss commands.
SQL Server wouldn’t know what data is supposed to present in the table rows. The only thing that SQL can verify while you are retrieving data or checking integrity of the database is that the column data and column datatype match. I have heard questions like “Why can’t SQL Server figure out if the value is correct?”. Well, would you know why a person’s name is Uncle Sam and not Uncle Bill? :) So, how would SQL Server know that the FirstName column (an example) was supposed to store Sam and not Bam. As per SQL Server, both strings match the column datatype, hence from a structural and logical integrity, the data is correct.