What is VLF in SQL?

VLF  stand for virtual log file. Its the internal structure of log file.

Virtual log file means while running (DBCC loginfo)it will shows the status 0 indicates Recoverable 1 indicates Active 2 indicates unused 3.indicates reused.

The Tlog file internally divided into virtual log files. You can see how many vlfs you have in your log file can be find out by running dbcc loginfo. Too many vlfs will might slow down the db recovery. Each vlf will be having status 0 to 9. These status number indicates the current vlf's status i.e 0 reusable 1 waiting for log backup 2 Active and so on.