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Garbage Collection in SAP HANA Environments

16 Jun 2015 1:36 pm || 0

SAP HANAWhat is garbage collection in SAP HANA environments?


In the SAP HANA environments, The term 'garbage collection' is quite unclear as it can be related to various activities.

In this tutorial to make it easy for users to understand we have listed below the following naming conventions while discussing the various types of garbage collection:




Row store version consolidation

Garbage collection in the row store basically refers to read versions of data of the which is no longer required. For avoiding misunderstandings, it is always good to name these as "version consolidation".

Column store version consolidation

Garbage collection in column store relates to the cleanup of versions of data which is no longer required. It is good to name this activity "version consolidation". In the column store both the current as well as the older versions of records are recognized via the unique document ID vectors (UDIV). Incase the number of UDIV values is knowingly larger than the number of (current) records in a table, this indicates the requirement of version consolidation.

Merges and reclaims of delta storage and compression optimizations occur when significant changes are executed against the tables and indirectly take care of the cleanup of the version wehch are not required anymore. Therefore, version consolidation is not really an issue in the column store.

Memory garbage collection

The memory areas which are allocated are cleaned for making more memory available for other tasks.

Persistence garbage collection

These don’t require persistence areas anymore (e.g. undo data) are cleaned for ensuring disk space can be re-used. User should be aware that this is no actual defragmentation, so the allocated disk size will not be reduced.

LOB garbage collection

Garbage collection relates to the hybrid LOB columns

liveCache garbage collection

Garbage collection refers to the SAP HANA integrated liveCache

Calculation engine garbage collection

Model garbage collection of the calculation engine

Metadata cache garbage collection

Garbage collection of the metadata cache