Sql server Blog Forum Helping SQL server DBAs and Developers

7Jun/1131

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 recovery 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 Freshers and DBA managers.

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

Let's see the discussion.

Manager:

We need minimum data loss and downtime.

DBA:

The existing system has low disk space and the transaction log backups are often failing. 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 best practices, Sir!

Manager:

???

 

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 like all redo information. This means all the modifications have written fully to the log file.
  • The full recovery model is always the best one for production servers.
  • In the full recovery model we can take all kinds of backups like full, differential, transaction Log and point in time recovery (PTR) too.

Full recovery mode, the transaction log file will grow like anything(big), until we do a log backup. In other words, The log file will not be reusable/clear until the log backup taken.

Bulk Logged

  • The name itself you can understand the answer. The bulk operations (bulk operations) are minimally logged in this mode. This means it does not have sufficient information in the log to replay the transaction. The BCM page contains all the required information. See an example you can understand.
  • Bulk logged recovery model is the 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 kinds of backups like full, differential and transaction Log but, the drawback is the point in time recovery (PTR) is not possible, when there is a bulk operations  have done with the transaction log file.

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

For more clarity. See an example: (This is not a concept just an example)

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

In full recovery model each 5,000 bulk operations have written in 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 help to improve the overall server performance.

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

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

 

Simple

  • The name itself you can understand the answer all operations are fully logged except bulk operation, Since bulk are minimally logged. Simple recovery model is just simple this means SQL server will run the checkpoint every time and truncate the transaction log file and marked the space for reuse . Mostly the log file will not grow larger.
  • Most of the time the simple recovery model is a good choice for non production servers. Because the log file will not grow larger. Also we would not take log backups. (If you’re planning to take the log backup then don’t put in simple)
  • In the simple recovery model the transaction log backup is not possible. We can take full and differential backups only.

Now you could know the answer for the following questions.

What is the use of Full, Bulk logged and Simple recovery model.

Which one is best for your case?

 

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 recommend to read 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 backs up all the 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 file 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/missing any one of the backups then you can't recover the whole data.

It's good to take a log backup 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 recent full backup and followed by a recent differential backup and all the log backup one by one, that has done after a recent differential backup.

For more clarity. See an example:

Assume we have scheduled every Sunday full backup @12 am and daily differential backup 8 PM and an hourly log backup.

The database crashed on Wednesday at 11PM. Then the restoration will be on Sunday full backup (@12AM)+ Wednesday differential backup (Most recent @8PM) + log backup @9+@10+@11 PM.

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

Note: The example has given is just for more understanding. Take a look at MSDN for more information.

 

 

Muthukkumaran Kaliyamoorthy

I’m currently working as a SQL server DBA in one of the top MNC. I’m passionate about SQL Server And I’m specialized in Administration and Performance tuning. I’m an active member of SQL server Central and MSDN forum. I also write articles in SQL server Central. For more Click here

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12Nov/102

sql server internal architecture

 

Database Physical Structure & Internals:

SQL server internals Allocation Map is important for DBAs. It will help to understand the database internals.

Database:

A Database is a collection of data that is stored (or) organized. We can manage and access data easily.
When we create a new database, by default two files will be created physically.
We can create more than one log files and data files (NDF).

  • Data file (MDF)
  • Log file (LDF)

Data File:
All the data will be stored in a data file. Internally the data’s are stored in data pages.

A database file has unique FileID and PageID.

Log files do not have pages. They have series of log records. I will cover the Log file section in my next article.

Data Page:

The data page is a fundamental unit of data storage. Each data page has 8KB size (128 pages per MB). Each page has 96 byte header system information.

 

All the data rows are stored in data page. Expect BLOB (text, ntext, image, xml, varchar (max), nvarchar & varbinary)

Extents:

Extents are fundamental units in which space is allocated to table (or) an index. Extents are 8 contiguous pages 64KB size. (16 extents per MB).

Two Types of extents

  • Uniform/Dedicated extents
  • Mixed extents

 

Uniform Extents:

Uniform extents are owned by a single object. All the 8-pages can be used to owning object.

 

Mixed Extents:

When we create a new table (or) an index of default pages are allocated to mixed extents first.

When the data grow and its reach 8 pages then it switched to uniform extents.

 

How the Data Storage Size Internally For Page & Extents:


Storage Size Page Extents
Size per KB 8KB(1-Page) 64KB (8-Pages)
Size per MB 128 Pages 16 Extents


IAM: (Index Allocation Map)
IAM page tracks with which extents within the specified GAM interval belongs to a single entity (or) file. A Database file has multiple files some of the DB size more than 4 GB

 

IAM interval size is 64000 extents i.e. 4GB

 

GAM: (Global Allocation Map)

Data files split up into GAM intervals (Conceptually)

Bit -->1 Free

An extent is available for allocation.

Bit --> 0 Allocated

An extent is already allocated.

 

GAM interval size is 64000 extents i.e. 4GB

 

SGAM: (Shared Global Allocation Map)

SGAM bitmaps are exactly same as GAM.

Bit --> 1 mixed extent one (or) more free page available

An extent is mixed one (or) more unallocated page available.

Bit --> 0 Either Uniform/Mixed extent No unallocated pages

An extent is either uniform (or) mixed all are already allocated.

 

SGAM interval size is 64000 extents i.e. 4GB

PFS: (Page Free Space)
PFS has tracks with which pages have free space.

 

A PFS page is the first page after the file header page in a data file (with page number 1).
Next comes a GAM (with page number 2) followed by an SGAM (page 3).

PFS interval size is 8088 pages i.e. 64MB

PFS does not have bitmap only have byte map.
DCM: (Differential Changed Map)

DCM tracks extents that have changed since the last BACKUP DATABASE statement.

Bit --> 1 extent modified

Extent has been modified since the last BACKUP DATABASE.

Bit --> 0 extents is not modified

Extent has not been modified since the last BACKUP DATABASE.

 

BCM: (Bulk Changed Map)

BCM tracks extents that have been modified by bulk operations since the last BACKUP LOG statement.

Bit --> 1 extents modified

Extent has been modified by a bulk logged operation after the last BACKUP LOG statement.

Bit --> 0 extent is not modified

Extent has not been modified by bulk logged operations .Although the BCM pages appear by default in all databases.

 

How the Data Stored/Allocated Internally for the entity:

(Allocation Maps)


Type of storage

(Bit Type)

GAM
SGAM
DCM
BCM
Bit 1 Extent Free Mixed Extent at least one free pages available Extent modified Extent modified
Bit 0 Extent Allocated Either Uniform (or) Mixed No free space Extent not modified
Extent not modified






Referred links :

Muthukkumaran Kaliyamoorthy

I’m currently working as a SQL server DBA in one of the top MNC. I’m passionate about SQL Server And I’m specialized in Administration and Performance tuning. I’m an active member of SQL server Central and MSDN forum. I also write articles in SQL server Central. For more Click here

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