SAP HANAWhat is garbage collection in SAP HANA environments?
Solution
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:
|
Area |
Details |
|
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 |