SQL Server Backup and Restore - Simple Talk

Figure 5-1: A transaction log with 8 VLFs.
Chapter 5: Log Backups
The concept of the active log is an important one. A VLF can either be "active," if it
contains any part of what is termed the active log, or "inactive," if it doesn't. Any log
record relating to an open transaction is required for possible rollback and so must
be part of the active log. In addition, there are various other activities in the database,
including replication, mirroring and CDC (Change Data Capture) that use the transaction
log and need transaction log records to remain in the log until they have been processed.
These records will also be part of the active log.
The log record with the MinLSN, shown in Figure 5-1, is defined as the "oldest log record
that is required for a successful database-wide rollback or by another activity or operation in
the database." This record marks the start of the active log and is sometimes referred to as
the "head" of the log. Any more recent log record, regardless of whether it is still open or
required, is also part of the active log; this is an important point as it explains why it's a
misconception to think of the active portion of the log as containing only records relating
to uncommitted transactions. The log record with the highest LSN (i.e. the most recent
record added) marks the end of the active log.
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