Data and log files are initialized to overwrite any existing data left on the disk from previously deleted files. Data and log files are first initialized by filling the files with zeros when you perform one of the following operations:
- Create a database.
- Add files, log or data, to an existing database.
- Increase the size of an existing file (including autogrow operations).
- Restore a database or filegroup.
File initialization causes these operations to take longer. However, when data is written to the files for the first time, the operating system does not have to fill the files with zeros.
Instant File initialization was introduced from SQL Server 2005.
In SQL Server, data files can be initialized instantaneously. This allows for fast execution of the previously mentioned file operations. Instant file initialization reclaims used disk space without filling that space with zeros. Instead, disk content is overwritten as new data is written to the files. Log files cannot be initialized instantaneously.