The following error situations can arise when working with a SESAM/SQL database:
computer failure (e.g. due to power outage)
device failure (e.g. a hard-disk failure due to a defective read/write head)
operating system software error that does not corrupt data
operating system software error that has falsified the data
database system software error that does not corrupt data
database system software error that has falsified the data
errors in the application software that cause incorrect or incomplete data to be stored in the database
SESAM/SQL takes care of error situations caused by computer failure, and error situations resulting from operating system software errors through its transaction logging features and the SESAM/SQL DBH's ability to perform a restart; these mechanisms work well, provided data has not been falsified and database system files have not been destroyed (see section “Transaction logging” and section “Restart”).
SESAM/SQL attempts to limit error situations resulting from a defective hard disk to the spaces located on the disk in question. In addition, the BS2000 function DRV (Dual Recording by Volume) makes it possible to handle error situations attributable to hard-disk errors and to render them invisible to SESAM/SQL.
When a new hard disk is added as a mirror disk, the data must be reconciled (i.e. rendered consistent) using means provided by BS2000. By contrast, after a system failure, areas of the database which have a differing status must not be reconciled by the database administrator, because SESAM/SQL's restart capabilities take care of this automatically.
Generally, all failure and error situations can be taken care of in principle with the aid of media recovery (see section “Media recovery”). However, the SESAM/SQL DBH's restart capabilities and BS2000's DRV function render the application available again much more quickly in such error situations.
In the following error situations, media recovery is the only means of restoring a defective SESAM/SQL database:
operating system software error that has falsified the data
database system software error that has falsified the data
errors in the application software that lead to incorrect or incomplete data in the database
destruction of database system files due to hardware faults