database mirroring

Medium fault is one of the most serious faults affecting the system , Seriously affect the availability of the database

Media failure recovery is time-consuming
To prevent media failure , The database administrator must dump the database periodically

Solutions to improve database availability

database mirroring （Mirror）

database mirroring

The database management system automatically copies the whole database or the key data in it to another disk
The database management system automatically ensures the consistency between the mirrored data and the master data whenever the master database is updated , The database management system automatically copies the updated data

In case of media failure

It can be used by the mirror disk
At the same time, the database management system automatically uses the mirrored disk data to restore the database
There is no need to shut down the system and reinstall the database copy

When there is no fault

Can be used for concurrent operations
A user adds an exclusive lock to the data and modifies the data , Other users can read the data on the mirror database , Instead of waiting for the user to release the lock

Copying data frequently will naturally reduce the system operation efficiency

In practical applications, users often only choose to mirror key data and log files
Instead of mirroring the entire database

Summary of database recovery technology

Concept and nature of transactions

A transaction is the logical unit of work of a database
The database management system ensures the atomicity of all transactions in the system 、 Uniformity 、 Isolation and continuity , This ensures that transactions are in a consistent state

Type of fault

Transaction failure
System failure
Media failure

The most frequently used technology in recovery

Database dump
Register log files

The fundamentals of recovery

Use the backup copy stored in 、 Log files and redundant data in the database image to rebuild the database

Business

It is not only the basic unit of recovery
It is also the basic unit of concurrency control
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