## Converter

### Use

The converter assigns logical data pages to physical storage locations (block addresses consisting of volume ID and position).

Converter entry

271397 P 1/0000271404

The logical page number 271397 is a permanent (P) data page. It is stored in the first data volume at position 271404.

Types of Data Pages

Type | Description
P | Permanent data page
S | Data page in a SAP liveCache database instance
T | Temporary data page

The database system imports the complete converter into the converter cache when the database instance starts.

If the database system fails to find a page number in the data cache, it searches for the page number in the converter cache and uses this to calculate the physical position of the data page in the data volumes.

At each savepoint, the database system writes the contents of the converter cache to the data area and gives the stored converter a version number. The converter saved at the next savepoint is then given the next higher version number. Each converter version corresponds to a consistent state of the database instance.

The converter uses the shadow storage concept. The database system stores the database state most recently written into the data area (most recent converter version) until the next savepoint. If the database user wants to change data pages, the database system first imports the data pages into the working memory and makes the changes there. The database system then writes the changed data pages to a new location in the data volumes at the next savepoint. Only at the end of the savepoint does the database system release the old data pages in the data volumes for overwriting.

The same logical page can be stored at different physical locations at different times.

See also:

Data Storage

Caches (Working Memory)

Volumes (Permanent Storage)

Savepoint

Freezing the Data Area (Snapshot)