Hi There! If you are preparing for SQL DBA Interview, you are in right place. This article describes how to answer to the question Why Instant File Initialization (IFI) is not enabled by default in SQL Server.
I have already explained What is IFI and few possible interview questions in What is Instant File Initialization? SQL DBA Interview Q & A . Please read and come back to this for better understanding.
Now you may think that since IFI significantly improves performance, the system should enable it by default. Let’s understand why it is not so.
One Liner Answer:
By default, SQL Server does not enable IFI mainly to prevent security issues and the risk of exposing deleted/wrong data.
Next obvious question would be “Could you please explain the risk?”
Answer with Explanation:
IFI is not enabled by default because when users delete data, the Free Space Management system in the operating system does not physically delete the data. Rather it marks the space(blocks) holding the data as reusable.
Technically marking the storage blocks as “unallocated” in a file system table or bitmap. The actual data remains until overwritten, but the space is immediately available for new files, often using techniques like linked list or bitmaps.
When IFI is not enabled, the system zeros out the page before writing any data. However, when IFI is enabled, the system skips the zero-initialization step and overwrites the deleted disk content only when it writes new data to the files.
As a result, the newly allocated pages may contain previously deleted data, and a slight risk exists that a third-party tool could access the deleted content until another process writes new data to that specific area of the data file.
Additional Information:
This section is only for your knowledge and generally you don’t have to explain in Interview. But helps in case the interviewer asks further details.
While the database file remains attached to the SQL Server instance, the file’s discretionary access control list (DACL) reduces the risk of information disclosure. The DACL allows access only to the SQL Server service account, its service SID, and the local administrator. However, when someone detaches the file, a user or service without the SE_MANAGE_VOLUME_NAME privilege may still be able to access it.
Hope this helps in better understanding and you’ll be able to explain to the Interviewer Why IFI is not enabled by default. You may also like to go through the following for your SQL DBA Interview preparation.
Junior SQL Server DBA Interview Questions and Answers
Junior SQL DBA Interview Question Answers -Part2
How To Identify SQL Server Service Account: SQL DBA Interview Q&A
Junior SQL DBA Interview Questions Answers -Part3
Junior SQL DBA Interview Question Answers: Part 4
What is Instant File Initialization? SQL DBA Interview Q & A
