Purpose
Determines the number of Depot cylinders the file system allocates per pdisk (storage device) to contain small slots (1 MB). A small slot can hold a single data block during depot operations.
The actual number of small-depot cylinders used per AMP is this value multiplied by the number of pdisks per AMP.
Field Group
File System
Valid Range
1 through 10 cylinders
Default
2
Changes Take Effect
After the next Teradata Database restart.
Usage Notes
The Depot is a set of transitional storage locations (a number of cylinders) used by the file system for performing in-place writes of DBs or WAL DBs (WDBs). An in-place write means that the changed DB is written back to exactly the same place on disk from which it was originally read. In-place writes are only performed for modifications to DBs that do not change the size of table rows, and therefore do not require any reallocation of space.
Writing the changed DB directly back to its original disk location could leave the data vulnerable to various hardware and system problems that can occur during system resets, such as a disk controller malfunctions or power failures. If such a problem occurred during the write operation, the data could be irretrievably lost.
The Depot protects against such data loss by allowing the file system to perform disk writes in two stages. First the changed DB (or WDB) is written to the Depot. After the data has been completely written to the Depot, it is written to its permanent location on the disk. If there is a problem while the data is being written to the Depot, the original data is still safe in its permanent disk location. If there is a problem while the data is being written to its permanent location, the changed data is still safe in the Depot.
During database startup, the Depot is examined to determine if any of the DBs or WDBs should be rewritten from the Depot to their permanent disk locations.