Transaction Log Space

The transaction log contains sufficient information about each transaction to ensure that it can be recovered. This transaction information can be interleaved within the log.

This transaction log shows the space allocation of the transaction log.


If a total allocated log segment is considered a contiguous logical space, then additional log records are appended to the end of the log. When there is no more space available at the end of the log, portions of the log that have been freed in the beginning of allocated space will be used to add records. This use of the freed portion of log can introduce a circular or wrapped around form within the allocated space.


The span of a completed transaction in the log is the portion of a log between begin log record of the transaction in the log and end log record of the transaction. The actual logging done by a transaction could be very minimal, however due to intermediate interleaved log records by other transaction, this span in terms of transaction log space could be considerable.


As transaction log space allocation is done in terms of pages, the span of an active transaction or incomplete transaction is also defined in terms of the number of log pages. There can be many active transactions at any point in the log. The portion of the transaction log occupied by span of an active transaction cannot be truncated with dump transaction. The total portion of the log which cannot be truncated with dump transaction is equal to span of the oldest active transaction