The database log is a linear record of all queries and transactions that was committed to the database but not yet processed / saved to the main storage. It even logs aborted transactions. The log can be a major performance bottleneck, because the log needs to be durable, so that in case the database crashes, the log can be used as a trustworthy source for recovering unprocessed data. It is also helpful to have a layer of redundancy by archiving all logs.