Page 189 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes The assumption that hardware errors and bugs in the software bring the system to a halt, but do
not corrupt the nonvolatile storage contents, is known as the fail-stop assumption. Well-designed
systems have numerous internal checks, at the hardware and the software level, that bring the
system to a halt when there is an error. Hence, the fail-stop assumption is a reasonable one.

Disk Failure

A disk block loses its content as a result of either a head crash or failure during a data transfer
operation. Copies of the data on other disks, or archival backups on tertiary media, such as
tapes, are used to recover from the failure.
To determine how the system should recover from failures, we need to identify the failure
modes of those devices used for storing data. Next, we must consider how these failure modes
affect the contents of the database. We can then propose algorithms to ensure database consistency
and transaction atomicity despite failures. These algorithms, known as recovery algorithms,
have two parts:

1. Actions taken during normal transaction processing to ensure that enough information
exists to allow recovery from failures.
2. Actions taken after a failure to recover the database contents to a state that ensures database
consistency, transaction atomicity, and durability.

Task Sometime your system not running properly its logical or system error.

11.3 Storage Structure

The various data items in the database may be stored and accessed in a number of different
storage media. To understand how to ensure the atomicity and durability properties of a
transaction, we must gain a better understanding of these storage media and their access methods.

Storage Types

Storage media can be distinguished by their relative speed, capacity, and resilience to failure,
and classified as volatile storage or nonvolatile storage. We review these terms, and introduce
another class of storage, called stable storage.

Stable Storage Implementation

To implement stable storage, we need to replicate the needed information in several nonvolatile
storage media (usually disk) with independent failure modes, and to update the information in
a controlled manner to ensure that failure during data transfer does not damage the needed
information.
RAID systems guarantee that the failure of a single disk (even during data transfer) will not
result in loss of data. The simplest and fastest form of RAID is the mirrored disk, which keeps
two copies of each block, on separate disks. Other forms of RAID offer lower costs, but at the
expense of lower performance.
RAID systems, however, cannot guard against data loss due to disasters such as fires or flooding.
Many systems store archival backups of tapes off-site to guard against such disasters. However,
since tapes cannot be carried off-site continually, updates since the most recent time that tapes

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