The file system automatically performs a mini cylinder pack (MiniCylPack) background operation if the number of free cylinders equals or falls below the threshold value specified for MiniCylPackLowCylProd in the DBS Control record. For additional information, see MiniCylPackLowCylProd.
MiniCylPack begins by attempting to honor the FSP that was specified for a table. either when the table was created or modified using CREATE TABLE or ALTER TABLE, or the last time PACKDISK was run on the table. If the previously defined FSP does not allow enough space to be reclaimed, MiniCylPack decreases the FSP by five percent and tries again. This continues until either the necessary cylinders are freed or MiniCylPack reaches zero FSP and still no space can be freed.
When a table affected by a MiniCylPack requires the availability of free space to permit growth without requiring the addition of new cylinders, this operation can result in non-optimal system performance. For example, when many MiniCylPacks occur, they can produce a state known as thrashing. This happens because storage is packed too tightly to permit growth, so frequent allocation of new cylinders to the table space is required. Because this action removes cylinders from the available cylinder pool, more MiniCylPacks are required and an ongoing cycle of thrashing results.
AutoCylPack, a background task the periodically runs to maintain an appropriate level of free space on cylinders, helps alleviate the need for PACKDISK and MiniCylPacks, however, because all MiniCylPacks have a negative effect on system performance, you should monitor system cylinder usage.
|To monitor …||Do the following …|
|cylinder utilization||Use the Ferret SHOWSPACE command.|
|occurrences of MiniCylPacks||Check the software event log (DBC.SW_Event_Log), or the Linux event log, /var/log/messages.|
Schedule PACKDISK operations as often as is necessary to prevent MiniCylPacks from occurring.
For more information about MiniCylPacks, see Teradata Vantage™ - Database Administration, B035-1093.