Define Reliability and Availability. Write down reasons for fragmentation.
Reliability : Reliability can be defined as the probability that a system will produce correct outputs up to some given time t. Reliability is enhanced by feature that help to avoid, detect and repair hardware faults. A reliable system does not silently continue and deliver results that include uncorrected corrupted data.
Availability: Availability means the probability that a system is operational at a given time, i.e. the amount of time a device is actually operating as the percentage of total time it should be operating. High-availability system may report availability in terms of minutes or hours of downtime per year. Availability features allow the system to stay operational even when faults do occur. Availability is typically given as the percentage of the time a system is expected to be available , e.g. 99.999 percent.
Reasons for fragmentation:
The main reasons of fragmentation of the relations are to:
1. Increase locality of reference of the queries submitted to database.
2. Improve reliability and availability of data and performance of the system.
3. Balance storage capacities and minimize communication costs among sites.
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