What is the Tar PM, and why does it use the old tar file format?
The Tar PM is a transactional storage engine that is specially made for JCR content. Like relational databases, it supports ACID (atomicity, consistency, isolation, durability), but it doesn’t know SQL, or relational integrity, and stores the data in another way. Append Only The biggest difference to a regular database is: the Tar PM is append-only. It doesn’t do any in-place updates. Whenever there is a change, the Tar PM appends an entry to the newest file. Even when deleting content, an (empty) entry is appended. While this seems wasteful, it is actually faster than a regular database, because each change results in only one write operation. A database first stores a change in the log file (in many cases the old data and then the new data), and then writes the data again, this time in the main area. The Tar PM only writes the data once. Unused, old data is removed in a separate optimize process during off-peak hours, for example at night. A regular database appends changes in a log
