Information Technology Reference
In-Depth Information
Chapter 2
Operations on the Physical Database
The tuple collections of the logical database are stored in an underlying
physical
database
, which consists of fixed-size database pages stored in non volatile random-
access storage, usually on magnetic disk. For reading or updating tuples of the
logical database, the pages that contain the tuples must be fetched from disk to the
main-memory
buffer
of the database, from where the updated pages are later flushed
back onto disk, replacing the old versions. The buffer-management component of
the database management system takes care that frequently used database pages are
kept available in the buffer as long as possible so as to reduce the need of expensive
random reads and writes of the disk.
In this chapter we discuss issues of the physical database that are needed to
understand the interplay between transaction management at the logical database
level and the management of the underlying physical database structure under
operations triggered by the logical database actions. These issues include the
page-based organization of the database, the
fixing
of pages in the buffer for the
time of accessing the page, integrity constraints on the physical database, the
latching
of pages for protecting the accesses against interference by other concurrent
processes, and the need to implement
structure modifications
on the physical
database as atomic operation sequences that commit independently of the triggering
transactions.
2.1
Data Structures and Processes on the Server
Two
permanent (non volatile)
data collections are maintained on the transaction
server:
1.
Data disks
containing the relations of the database, their indexes, and the system
catalog or data dictionary.
2.
Log disks
containing the log records written for updates on the database.
