TempBLCRescanPeriod - Advanced SQL Engine - Teradata Database

Purpose

The number of days between two scan intervals of the temperature-based block-level compression (TBBLC) task, where TBBLC data is validated to determine if data needs to be compressed or uncompressed. This determination is based on data temperature changes. When the new scan cycle starts, TBBLC either starts a completely new scan or continues where it was. See Usage Notes below.

This field is effective only when TBBLC is enabled.

Field Group

Compression

Valid Range

1 through 90 days

Default

7 days

Changes Take Effect

The first time the background compression task runs after the DBS Control Record has been written.

Usage Notes

Each AMP has a TBBLC task that starts scanning through the Master Index after a database restart, starting at the table that occupies the most cylinders.

Following that initial scan, the task always starts a new scan after the number of days defined in TempBLCRescanPeriod.

Depending on the progress of TBBLC in the previous scan cycle, one of the following will occur:

• If all cylinders of the AMP were processed by TBBLC during the previous scan cycle, a new TBBLC scan starts from scratch, starting at the table that occupies the most cylinders.
• If the previous TBBLC scan cycle had not completed processing all cylinders of the AMP:
  • The TBBLC task continues with the same table that it was going to process in the previous cycle, but the cylinders that were processed in the previous cycle are marked to be scanned again.
  • Eventually the TBBLC task will need to scan all cylinders again, including cylinders that were already scanned in the previous cycle.
  • A TBBLC scan cycle is not considered complete until all cylinders are processed during a given TempBLCRescanPeriod.

    Assuming stable temperatures for AUTOTEMP tables, cylinders that were done previously should be quickly processed because on most cylinders compression/uncompression should not be necessary.

Data temperatures refers to frequency of access:

• Hot data is frequently accessed.
• Cold data is infrequently accessed.
• Warm data is accessed moderately frequently.

Data temperature is measured separately for each AMP at a cylinder level and is a relative measure because the temperature of cylinders can change even if the data it is not directly accessed. This is because it depends on how often other cylinders are accessed in comparison. TBBLC automatically compresses cold data to save storage space and decompresses Warm or Hot data if it was previously Cold (and thus compressed). At any time, different portions of a table, which are spread among many cylinders, may exist in different states of compression depending on their temperatures.

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.

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