SQL server

Viewing VLFs in the SQL log file.

The SQL log file is split internally into VLFs (Virtual Log Files). There is an undocumented command that can be used to view VLFs: DBCC loginfo. This command returns detailed information about the SQL log file and has one row per VLF. The status column shows which of the VLFs are active, and the parity can be 0,64, or 128. When the log is first created, the parity will be 0 (hence instant file initialization cannot be used like in the case of data files). If the parity is 0, and then it is written to, the parity will change to 64. This will switch between 64 and 128 to help with the cyclic reading of the log file by the SQL OS.

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