Microsoft Sql Server Managment studio backup size goes negative

Question

The problem is that I need to explain the different sizes of backups that are being made of a database in a plant. Sometimes the difference between the sizes is in negative, eve

Answer 1

The last step of the backup process is to append data from the log that reflects any changes made to the database during the backup process, this could account for the difference you are seeing.

Answer 2

SQL Server have 2 step process of storing your data. First, your data goes into log file, and it not only data that you inserted, but also the whole list of operations SQL performed on your data. So, if something wrong happens, SQL can 'replay' your transactions.

At some point CHECKPOINT happens, ans data gets written into data file. Log files have a tendency to grow and shrink.

During BACKUP, SQL will write data and log files EXACTLY as they look at the point of BACKUP. That's why you can see that difference in size.