SQL server Recovery Models and Backup types

In this post I am going to explain the importance of recovery models and backup types. 

Why I am writing this post ?

Because, when I was teaching about the topic to my PHP friend .He got doubts on bulk logged model and differential backups. I have given some examples and cleared his doubts. I am going to share this with you guys too.

I hope this will help for fresher's and DBA managers because they don't know about the recovery models and backups types. (RPO/RTO data’s at all). This will help for newbies especially an interview time.

Recovery models and backup types are important to plan the recovery point objective (RPO)and recovery time objective (RTO). We can’t differentiate both the recovery models and backup types. Simply it’s a mixture.

Let's see the discussion.

Manager:

We need minimum data loss and down time.

DBA:

Existing system has low disk space and the transaction log backups are failed often. We need more disk space to do this, Sir!

Manager:

I asked about this to management they will give soon until, you will do something with your DBA knowledge.

DBA:

Changing the recovery models full to simple and back to full and shrinking the log file daily.The DBA send an email to his manager. The shrinking is not good practices, Sir!

Manager:

???

(Story ended)


Let’s come to the point.

Recovery Models

SQL server has three types of recovery models.

1. Full

2. Bulk Logged

3. Simple

Full

•  In full recovery model all the database operations are fully logged. This means all the modifications have written fully to the log file.

• The full recovery model is always best one for production servers.

• In full recovery model we can take all kind of backups like full, differential, transaction Log and point in time (PTR) recovery too.

Bulk Logged

• The name itself you can understand the answer. Bulk operations have minimally logged in this mode.This means it has written only minimal roll backing information to the log file not whole (i.e not fully logged).The BCM page contains all the required information. See an example you can understand.

• Bulk logged recovery model is best model for server performance. Because all the bulk changes have been written minimally (Not fully) to the transaction log file.

• In bulk logged recovery model we can take all kind of backups like full, differential and transaction Log but, the drawback is point in time (PTR) recovery is not possible, when there is a bulk operations  have done to the transaction log file.

Keep in mind,in full and bulk logged recovery model the log files are grow big until the BACKUP LOG has done.

For more clarity. See an example:

Just assume 5,000 bulk operations have written in 8000 pages.

In full recovery model each 5,000 bulk operations have written to 8000 pages, because all the changes have written fully in the full recovery model.

But when we used bulk logged recovery model, it has written 1000 pages only ,Because SQL server has written the bulk changes to an extent wise not the page wise i.e. minimally logged operation (8 pages = 1 extent).It will helps to improve the overall server performance.

Now you know, Writing 8000 pages are good or 1000 pages.

Note: This bulk logged model is good only for the bulk operations.

Simple

• The name itself you can understand the answer. In simple recovery model is just simple this means SQL server will run the checkpoint every minute and truncate the transaction log file often.

• Most of the time the simple recovery model is good choose for non production servers.

• In simple recovery model the transaction log backup is not possible. We can take full and differential backups only.

 Backup Types

SQL server has number of backup types. Here I am going to explain the main three types.

1. Full

2. Differential

3. Transaction log

I have recommended to reading Paul S Randals's backup survey post.

 

http://www.sqlskills.com/BLOGS/PAUL/post/Importance-of-having-the-right-backups.aspx

Full backup

• A full backup backs up the full/whole database. That is backs up all the committed data.

• The full database backup has been done in all the recovery models.

Differential backup

• A differential backup only backs up the changed data/extents that were modified after the full database backup has done.The DCM has tracked all the changed extents.

• The differential backup has been done in all the recovery models.

For more clarity. See an example:

Assume we are scheduled every Sunday full backup @12 am and daily differential backup 10 PM.

Note: The data changes daily 500 MB.

Question:

How much data got back up on Thursday?

The student answers 500 MB.

As I already told, differential database backup "backs up all the data that has changed since the last full database backup".

The answer is,

Monday 500MB + Tuesday 500 MB + Wednesday 500MB + Thursday 500MB= 2000MB

Edit: I added these lines after i had replied the forum question SSC.

So the differential backup on Thursday has all the data. That is (Monday+Tuesday+Wednesday+Thursday). It helps to bring the database online as quickly as possible.

Transaction log backup

• A transaction log backup backs up the transaction log file. That is all the modifications/changes.

• The transaction log backup only possible in full and bulk logged recovery model.

• The transaction log backup is important to minimize data loss and log files size too.

It's  very important to keep the transaction log backup as much as safe,because the restoration needs the sequence of the transaction log backup if, you deleted/missed any one of the backup then you can't recover the whole data.

I have suggested running the log backups often. It will reduce the log file size and helps to minimize your data loss.

 Question:
How to restore the database after a disaster? (OR) What is the sequence to restore the database?


Answer:
Restore a full backup and followed by recent differential backup and all the log backup one by one in a sequence.

For more clarity. See an example:

Assume we are scheduled every Sunday full backup @12 am and daily differential backup 10 PM and an hourly log backup.

The database crashed on Wednesday at 1:30 PM. Then the restoration will be on Sunday full backup+ Wednesday differential backup (Most recent)+ log backup @11+@12+@1.

Believe me guys, Test your backups often and keep at least two copy of the full backup safely, Because the full backup is important to restore the all other backup. This means without a full backup you can't restore the differential and log backups.

sql servers basics

In this article, I am going to discuss about the basics of SQL server which is helpful for newbies.

 

SQL-Structured Query Language (pronounced as SEQUEL) is a Database, computer language designed for managing data in relational database management system (RDBMS).

Most of the people have to know SQL server query has designed for set based (declarative) .

The SQL commands are

DQL (Select)

DML (Insert, Update, Delete)

DDL (Create, Alter, Drop,Truncate)

DCL (Grant, Revoke, Deny)

TCL (Commit,Rollback)

Database

A Database stores all the data information (like objects) and has default data file and log file. Data file is called mdf, ndf…. (One-mdf,more than one ndf). Log file is called ldf(More than one ldf)

SQL Server System Databases

1. Master

2. Resource (2005 onwards)

3. Tempdb

4. Model

5. Msdb

1. Master

• The Master database manages the SQL Server instance.

• In SQL Server 2005 onwards Master database stores only the logical information in the system objects residing in the sys schema.
• In the previous editions of SQL Server, the Master database physically stores all the system objects information.

• One of the main thing is master and resource DB must be located in same directory.Otherwise the server goes down :-)

2. Resource

• Introduced in SQL Server 2005 to help in managing the upgrade and rollback of the system objects.
• Prior to SQL Server 2005 the system related data was stored in the master database.
• Its a read-only database that is not accessible via SSMS.
  
  

3. TempDB

• TempDB is a temporary database to store temporary tables like #local, ##global, table variables, cursors, work tables sorted in TempDB, etc...
• When the SQL Server instance is rebooted, the TempDB database is recreated every time.

4. Model

• Model is template database for all user defined databases.
• So we could use the model as template to other user databases (such as the recovery model, DB size,SPs etc…)

5. MSDB

• Msdb is the main database to manage the SQL Server Agent configurations.
• It is provides some of the configurations for the SQL Server Agent service (such as jobs alerts ect..)

Important: Keep takes the system database backup regularly except tepmDB.

For More:

http://msdn.microsoft.com/en-us/library/ms178028.aspx