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Title
Our Teradata database size is 20 TB, but when we take a backup in Netvault it shows the backup size as 77 TB. -
Description
Our Teradata database size is 20 TB, but when we take a backup in Netvault it shows the backup size as 77 TB. -
Cause
If the data is compressed using Block Level Compression (BLC), then Teradata ARC uncompresses the data when it is sent from the Database to the Backup facility, i.e. NetVault Backup. -
Resolution
Please confirm what type of compression is being applied on the Teradata database.
- Block Level Compression (BLC)
- Multivalue Compression (MVC)
- Algorithmic Compression (ALC)
BLC is compression at data block level. MVC and ALC are compression at column level. ARC uncompresses BLC during backup while it keeps MVC/ALC compressed during archive.
NetVault Backup Plug-in for Teradata backups are done trough the Teradata ARC utility (ARCmain). If the data is compressed using Block Level Compression (BLC), then ARC uncompresses the data when is sent from the Database to the Backup facility, i.e. NetVault Backup.
BLC compression ratios for standard data are about 3:1. However it can be higher depending on the data.
Teradata customer may refer to Teradata KCS002391 and KCS007426 for more information on compression ratio of BLC. There is no way to know the compression ratio for MVC/ALC. You need to compare tablesize with/without compression. The following SQL returns compressible columns
SELECT DATABASENAME,TABLENAME,COLUMNNAME,COMPRESSIBLEFROM DBC.COLUMNS WHERE COMPRESSIBLE IS NOT NULL ORDER BY 1,2,3;If it is confirmed that BLC is being used, the data that NetVault Backup is receiving is uncompressed. In this case, note that NetVault Backup compression could be used. If a Support case has been opened with Teradata, perhaps the End User could query if Teradata would recommend additionally applying Multivalue Compression (MVC).