SQL Server : Understanding Differential Change Map (DCM) Page and Differential Backups

In our earlier posts, we have discussed about different types of pages.Data Pages, GAM and SGAM,PFS page and IAM page. In this post, we will try to understand the Differential Change Map (DCM) pages and differential backups.

Differential Change Map(DCM): SQL Server uses Differential Changes Map (DCM) page  to track extent modified after the last full backup operation. DCM page is the 6th page in the data file.DCM page track the extents modified after the full backup.DCM has a bit for every extent it tracks. If the bit is 1, the corresponding extent is modified after the last full backup, if the bit is 0, there is no change in the corresponding extent after the last full backup.A DCM page can hold information of around 64000 extents. Precisely DCM pages repeat after every 511232 pages .One DCM page can track the changes of exactly 63904 extents.The second DCM page will be 511238th page.

Differential backups read the DCM pages to determine which extents have been modified after the last full backup. This greatly reduces the number of pages that a differential backup must scan. The length of time that a differential backup runs is proportional to the number of extents modified since the last full backup and not the overall size of the database.

Let us see an example:

CREATE DATABASE Mydb
GO
BACKUP DATABASE Mydb TO DISK='D:\mydb.bak'
GO
DBCC TRACEON(3604)
DBCC PAGE('MYDB',1,6,3)

FIG 1

From the output it is clear that, four extents which starts from 0th page to 32th page have changed after the last full backup. It is not necessary to have changes in all these 32 pages. But we can clearly say that , there were changes in at least four  pages , one page from each extents.This might happen due to the changes triggered by full backup command in the internal tables

Now let us try to add some data to this database and again see the DCM page

SELECT * INTO mydb..SalesOrderDetail FROM AdventureWorks2008.sales.SalesOrderDetail
DBCC PAGE('MYDB',1,6,3)

FIG 2

As part of select we have create a table and inserted data into that. It will trigger changes in many of the system tables. As a result we can see many extents are marked as modified after the last full backup.Till the page number 1:176 , the changes are due to the internal objects modification. Pages from 1:176 to 1:1671 are got modified due to data inserted into the SalesOrderDetail table.When we take differential backup, SQL server just read the DCM pages and include the pages marked as changed in the differential backup. For example as we mentioned earlier,there might be only changes to four pages in the first entry (1:0 - 1:24) but differential backup include all 32 pages in the backup. Let us take a differential backup.

BACKUP DATABASE Mydb TO DISK='D:\Mydbdifferential.bak' WITH DIFFERENTIAL

DBCC PAGE('MYDB',1,6,3)

FIG 3


















After the differential backup , there is no change in the DCM pages except two more extents are marked as changed.Before the differential backups, the extents 72-80 and 136-144 were marked as not changed. After the differential backup, these two extents are marked as changed. Refer Fig 2 and Fig 3.This is happened due the changes triggered by differential backup in the internal tables.From this it is clear that, differential backup will not clear the DCM page. In Short differential backup always include all the changes happened from the last full backup. Not the changes from the last differential backup.There is a misconception that, the differential backup contain the changes happened from the last differential backup which is completely wrong.  

If we look into the FIG 3, we can see that the size of the differential backup file is 13452 KB.Let us try ti calculate it using the DCM page information.From the Fig 3 , we can see that DCM page has marked 1616 pages as changed. The calculation is given below

(32-0)+(64-48)+(88-72)+(168-112)+(1672-176)=1616

The size of the page is 8 KB. So the 1616 page account to 12928 KB (1616X8). Which is very close to the size of the differential backup file. The difference is overhead of the file header etc.

Now Let us take a full backup and see what will happen.

BACKUP DATABASE Mydb TO DISK='D:\MydbFull.bak' 

DBCC PAGE('MYDB',1,6,3)

FIG 4

Now SQL server cleared DCM page.Everything is marked as Not Changed except the four extents 1:0 -1:32. No this backup will work as base for all subsequent differential backups.There is an option to take full backup with out disturbing the existing backup chain. 




BACKUP DATABASE Mydb TO DISK='D:\MydbFull_Copy.bak' WITH COPY_ONLY

In this scenario SQL server will not reset the DCM pages. Still MydbFull.bak will be the base full backup for subsequent differential backups. This will be helpful in some scenario where you need to take full backup but do not want disturb the backup chain.

To summaries, SQL server tracks the changes to the extents through the differential change map(DCM) page.While taking differential backup, SQL server just backup the extents marked as changed in the DCM pages. It helps SQL server to speed up the differential backup process with out scanning through all pages to check it is modified or not after the last full backup.Differential backup will not clear the DCM page and differential backup contain all the changes happened from the last full backup. While taking full backup, SQL server reset the bits in DCM page to mark it is not changed.

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