To avoid the full read HUT lock that Teradata ARC normally places, specify USE GROUP READ LOCK for a rolling HUT lock. It is also possible to avoid the full read HUT lock by initiating an online archive using the ONLINE option during an ALL AMPS archive, or by using LOGGING ONLINE ARCHIVE ON.
If the GROUP keyword is specified, the Teradata Database does the following:
- Applies an access HUT lock on the whole table.
- Applies a read HUT lock sequentially on each group of rows in the table as it archives them, then:
- Releases that read HUT lock.
- Applies another read HUT lock on the next group of rows to be archived.
Using the GROUP keyword locks out updates to a table for a much shorter time than an archive operation without the option. The size of the locked group is approximately 64,000 bytes.
A transaction can update a table while the table is being archived under a group read HUT lock. If an update occurs, the archive might contain some data rows changed by the transaction and other data rows that were not changed. In this case, the archived copy of the table is complete only when used in conjunction with the after image journal created by the transaction.
Use a group read HUT lock only for tables with an after image journal. This means that online backups of database-level objects also require every table in the database have after-journaling defined. If a table is excluded as part of a database-level GROUP READ LOCK archive, it must still have an after-journal table defined so ARC can accept the GROUP READ LOCK option.
If a group read HUT lock is specified for tables without an after image journal, the Teradata Database does not archive the table, and it prints an error message.
If the GROUP keyword is not specified and the archive operation references a database, the Teradata Database locks the database. This HUT lock is released after all data tables for the database have been archived.
If ARCHIVE specifies the archive of individual data tables, the Teradata Database locks each table before it is archived and releases the HUT lock only when the archive of the table is complete. HUT locks are released only if the RELEASE LOCK keywords are specified.