Specifies the algorithm that is used to compress and decompress data in file system data blocks.
Field Group
Compression
Valid Settings
| Setting | Description |
|---|---|
| ZLIB | ZLIB lossless data compression and decompression algorithms. |
| ELZS_H | Hardware-based Exar Lemple-Ziv-Stac lossless data compression and decompression algorithms. This type of compression requires that compression engine hardware be installed on every system node.
|
| ELZS_S | Software-based Exar Lemple-Ziv-Stac lossless data compression and decompression algorithms. This type of compression is used automatically if ELZS_H has been selected but problems are detected with the hardware compression engine board. Explicitly setting this algorithm in DBS Control is not recommended, except when recovering from a hardware compression engine failure.
|
Default
ZLIB
Changes Take Effect
After the DBS Control Record has been written. Changed setting applies to new data added to tables. Compression for existing data is not changed until that data is modified.
Usage Notes
BLC enables data compression at the data block (DB) level of the file system. Compression reduces the amount of storage required for a given amount of data. The BlockLevelCompression field of DBS Control enables and disables BLC.
After a data block has been compressed using a particular algorithm, the data retains that compression, even if the algorithm is subsequently changed. New data added to the table is compressed using the new algorithm. If previously compressed data is updated, which requires uncompressing and recompressing the data, the currently set algorithm is used for the recompression.
Consequently, a single subtable can exhibit a mixture of uncompressed data blocks and data blocks compressed using different compression algorithms, depending on the settings at the time the data was added to the table. To make the data consistent with respect to compression, use the Ferret COMPRESS command to compress all the data using the currently set algorithm, or use the Ferret UNCOMPRESS command to uncompress all the table data.
Related Information
| For more information on... | See... |
|---|---|
| BlockLevelCompression field | BlockLevelCompression. |
| ZLIB data compression algorithm | http://zlib.net. |
| Lempel-Ziv-Stac data compression algorithm | http://en.wikipedia.org/wiki/Lempel-Ziv-Stac. |
| COMPRESS and UNCOMPRESS commands | Ferret Utility (ferret). |
| BLOCKCOMPRESSION query band | Teradata Vantage™ - SQL Data Definition Language Syntax and Examples, B035-1144. |