Plan Additional Space on Destination System - Teradata Database

Sometimes more permspace is required on the destination system than was required for the same database/user on the source system. As a rule of thumb, 20% free permspace on the destination system database is recommended, but still may not be sufficient in some cases.

If the migration involves a hash function change, there must be enough free space in each database to hold an extra temporary copy of the largest table in that database. A hash function change may result in many rows being located on different AMPs on the destination system. When a row is redistributed to another AMP, the row gets copied into a buffer to be sent to the new AMP. The space for the original copy of the row(s) is not freed until the entire table has been restored and redistributed.

The most common reasons that pre-existing database space is insufficient for a newer system include the following:

• New features and functionality add fields to existing system tables or may require new system tables.
• Table headers increase in size.
• The new system may be configured with more AMPs. Each AMP will has a copy of every table header.
• Hash function changes may result in data skewing related to NOPI tables or PPI tables.
• The source and destination systems have different DBS Control settings. Some DBS Control settings, especially those related to block-level compression, affect space usage.

The following query can be used to show both the used and available space for each database

SELECT DatabaseName,SUM(CurrentPerm), SUM(MaxPerm)
FROM DiskSpace GROUP BY 1
WITH SUM(CurrentPerm),SUM(maxperm) ORDER BY 1;

If SUM(CurrentPerm)/SUM(MaxPerm)>80%, the available free space on the system is already below the recommended threshold for migration. If the migration involves a hash function change, the available free space required depends on the size of the tables being restored and could require more than 20% available free space. It is not just large databases that can run out of space.

SELECT SUM(DataBaseSpace.CurrentPermSpace)
(NAMED CurrentPerm, FORMAT '-,---,---,---,---,---,---,--9')                                                                               
FROM  DBC.Dbase, DBC.DataBaseSpace, DBC.TVM
WHERE DataBaseSpace.TableID <> '000000000000'XB
 AND  DataBaseSpace.TableID = TVM.tvmid
 AND  TVM.DatabaseId = Dbase.DatabaseId
 AND  TVM.TableKind NOT IN ('G','M','V')
 AND  TVM.ProtectionType = 'N';