In this article we take a close look at several processes and phases that are typically encountered while performing a SQL Server recovery.
Sometimes, you may require to recover or restore data from a failure, and this has to be done in a sequential and logically correct form. Through the SQL Server recovery, you can recover an entire database, a data file, or just one date page. However, no matter in what way you restore it, you cannot recover it unless you roll it forward enough so that it becomes consistent and usable with your database. You have to copy the data from backup and roll it ahead to the point of recovery. This process is called as ‘Recovery’.
The various phases that you will witness in the sql recovery process are discussed as follows:
When you recover a data, you have to apply certain changes in the database to make it consistent with the time and the current database. This phase is called Rolling Forward. When you restore an entire set of all the data combined, it is called a Roll Forward Set.
Each instance of restoration is carried out by undertaking certain steps or operations. They are collectively termed as the Restoration Sequence.
Stages of Restoration
The process of restoration consists of many phases. The 3 phases are called– copying the data, rolling forward the data and undoing the data. These are discussed as follows:
- Copying the data
In this stage, you copy the data from the backup and then re-adjust the data back to the time when they were first seized by that backup.
- Roll Forward or Redo
In this stage, you bring the data forward in the database, i.e. bring it back to the original state. However, if the data that was initially ‘read only’ and still remains so, you don’t have to perform this step. In the Point of Recovery, you bring back the data to its initial stage. At the Recovery point, you particularly mention which data you wish to recover. At the Redo stage, all the data is rolled forward to the state where you can undo it.
- Undo or Roll Back
At the Recovery point, once the changes have occurred, you might come across some data that is uncommitted. This will render the data that has been redone, inconsistent with the transaction. So you undo these uncommitted transactions by rolling back. However, if the data is consistent, you need not take this step as it will not be required.
The process of Recovery will usually consist of both the Roll Forward and Roll Back stages, but only once the backups are restored.
Path of Recovery
A path of recovery is the specific set of modifications that have changed or altered the database, but at the same time, have preserved their consistency.
These are some of the quick pointers or overview of the typical processes and phases you may see during the SQL Server recovery process. Hopefully, they will throw some light on them and your familiarity with the process will increase.
Victor Simon is a data recovery expert in DataNumen, Inc., which is the world leader in data recovery technologies, including fix Access and sql recovery software products. For more information visit www.datanumen.com