Chapter 19, Problem 27E

Explanation of Solution

a.

Degree of durability in a remote backup system when data loss must be avoided but some loss of availability may be tolerated:

• The degree of durability is defined by when a transaction commits at the stable storage.
• In a system using two-very safe scheme, a transaction is committed after log records are written to stable storage at both the backup and the primary site.
• Consider a remote backup system which must avoid data loss at any cost. However, it can tolerate some loss of data availability...

Explanation of Solution

b.

Degree of durability in a remote backup system when transaction commit must be accomplished quickly:

• When a system uses one safe scheme a transaction is committed after log records are written to stable storage at the primary site only.
• Consider a remote backup system where transactions commit needs to be quick, even if some committed transaction is lost in a disaster...

Explanation of Solution

c.

Degree of durability in a remote backup system when a high degree of availability and durability is required:

• The two safe scheme works as two-very safe scheme if both the backup and primary sites are active.
• In case just the primary site is active, a transaction can be committed after log records are written to stable storage at the primary site only...