Database Reference
In-Depth Information
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Monitor space utilization of all production files regularly.
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Expand disk space for files reaching the end of the allocated space.
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Archive database records not needed for regular, current use.
Maintain
archived tape files in a safe place.
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Whenever needed, bring back archived tape files, restore them on a special
instance of the database, and allow users to use the data.
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Drop one-time, obsolete tables and recover disk space.
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Allocate space quotas to authorized users for creating one-time temporary
tables. Revise or remove quotas as and when necessary.
Concurrency Control
Chapter 15 covered concurrent processing of transactions and methods for con-
trolling concurrency from producing adverse effects. You have noted the need for
concurrency and also studied the locking techniques needed to prevent problems
associated with concurrent transactions. Although database management systems
provide adequate lock management, the DBA must still keep monitoring locks and
potential deadlock situations. Routinely, the DBA has options to manage and adjust
concurrency control techniques.
Usually, a DBMS provides utility scripts to check the current state of a database.
When a database system is running slowly and two or more transactions are waiting
in deadlock situations, the DBA can run the script and find out what locks are held
and which transactions are waiting. This facility usually provides the following infor-
mation that can be used to resolve deadlocks:
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Transactions that have another transaction waiting on a lock and are not them-
selves in a wait state
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DDL locks held and requested (DDL locks prevent changes to database defi-
nitions while a transaction is executing.)
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DML locks held and requested (DML locks relate to read/write locks in shared
and exclusive modes.)
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All locks held or requested
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Transactions that are waiting for database locks and transactions that are cur-
rently holding locks
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Locked database objects and the transactions that are currently accessing them
The locking mechanisms of most DBMSs work well. However, if some transac-
tions do not lock at the lowest level of database objects needed by them, deadlock
situations could result. Manual intervention by the DBA becomes routinely neces-
sary. After finding out the transactions that are caught in deadlock situations, the
DBA has the option of terminating one or more transactions in deadly embrace,
thus enabling the waiting transactions to proceed.
Problem Resolution
The DBA is involved in detection and resolution of certain problems on an ongoing
basis. Here are a few major problem resolution tasks.
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