An area of interest I have built up over the last few months is around corruption and the use of DBCC CHECKDB to detect this. Within any production environment checking for corruption is extremely important and as a guideline should be run at least once a week depending upon database sizes and maintenance windows.If this can be run more frequently then great, as Data has a great importance to businesses and detecting corruption early on is vital. Anyone who wishes to learn more about DBCC CHECKDB and corruption I would recommend checking out the SQLskills pluralsight course on Detecting and Correcting Database Corruption by Paul Randall
This leads me onto how the DBCC CHECKDB operation is carried out online. It is possible for this operation to be carried out online as the DBCC CHECKDB command uses a snapshot of the database upon which you are running the command against. This in effect is a transactionally consistent point in view time of the database. The operation needs to ensure that there are no changes or anything with an open transaction in the view of the database to be able to run the consistency checks. The database snapshot that is created is hidden and it is not possible to see the snapshot. This is due to the snapshot being created as an NTFS alternate streams of the existing data files.
It is important to note that due to a snapshot being used problems can occur. One example of this is due to the snapshot running out of space. The reason for this is that the DB snapshot work by any pages that are changed in the source DB after the snapshot has been created are pushed into the DB Snapshot using the copy-on-write process, resulting in the snapshot growing in size. If this then runs out of space the DBCC CHECKDB command shall fail.
For anyone that has an interest in DBCC CHECKDB I would recommend the following blog posts from Paul Randall(B|T).
