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SQL Server differential backup file size smaller than previous one?


SQL Server differential backup file size smaller than previous one?

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sani
sani
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Hi All,

I use Production Database Backup Strategy as follows:

- Full Backup: every 2 weeks

- Differential Backup: every day

- Transactional Backup: every hour

Sometimes differential backup file size become smaller than previous one. Could anyone give me some more detail regarding this issue.

Is it normal or differential backup file size should increase until the next full backup?

Is this differential backup reliable? As the database size is very large (around 2.5 TB) and according to hardware resource limitation, It's not possible to restore database to verify backup accuracy.

Thanks in advance,
Sanaz.



GilaMonster
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Someone took an ad-hoc full backup?
Lots of data was deleted?

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
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HanShi
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GilaMonster (6/11/2013)
Someone took an ad-hoc full backup?

You can check this by restoring the header of the backups and look at (the sequence of) the LSN numbers.
Do you use compression on the backup? That could explain some differences in the size of the files.

sani (6/11/2013)
As the database size is very large (around 2.5 TB) and according to hardware resource limitation, It's not possible to restore database to verify backup accuracy.
You should get some place where you can verify your backup by using a complete restore. Without a proper restore you will never know for sure if your backup is valid.

** Don't mistake the ‘stupidity of the crowd’ for the ‘wisdom of the group’! **
GilaMonster
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HanShi (6/11/2013)
GilaMonster (6/11/2013)
Someone took an ad-hoc full backup?

You can check this by restoring the header of the backups and look at (the sequence of) the LSN numbers.


Or, much easier, query the msdb backup tables and see if there's an other full backup listed.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass


sani
sani
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Thanks for the reply.

As a DBA, I have fully control on Backup activities and I am sure no full backup job has been run.

As a business process point of view, It is possible large amount of data have been deleted. Now I want to know does the deletion consequent to have smaller differential backup size?

Many Thanks,
Sanaz.



GilaMonster
GilaMonster
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sani (6/11/2013)
Now I want to know does the deletion consequent to have smaller differential backup size?


It can do, depending on what's deleted and how much. Backups only back up allocated extents.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass


sani
sani
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What do you mean by "what's deleted".

Could you please give me more detail information or any MSDN link regarding this issue.

Thanks in advance,
Sanaz.



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