Why data backed up is so different from the source?

Posted on 2006-10-27
Medium Priority
Last Modified: 2010-04-03
I have SQL 7 running in NT 4 server with Backup Exec 7.5.
The backup job in Backup Exec is 3 user databases and 4 system databases.
Compression method: software if hardware unavailable.
The two user database files are 10.9/2.1GB and 1.6/4.5GB (mdf and ldf file respectively).
The other system databases are just a few MB, so neglectable here.

Here is the backup log:
Job Operation - Backup
Media operation - overwrite.
Software compression enabled.

Media Name: "Media created 10/26/2006 02:00:05 AM"
Backup of "ACCPAC_SQL "
Backup set #1 on storage media #1
Backup set description: "20061025-Accpac-DB-backup"
Backup Type: DATABASE - Back Up Entire Database
Backup started on 10/26/2006 at 2:00:34 AM.

Backup completed on 10/26/2006 at 2:25:24 AM.
Backed up 7 database(s)
Processed 10,917,190,301 bytes in  24 minutes and  50 seconds.
Throughput rate: 419.253 MB/min
Software compression ratio: 5:1

Job Operation - Verify

Verify of "ACCPAC_SQL "
Backup set #1 on storage media #1
Backup set description: "20061025-DB-backup"
Verify started on 10/26/2006 at 2:27:13 AM.

Verify completed on 10/26/2006 at 2:51:49 AM.
Verified 7 database(s)
Processed 2,159,818,245 bytes in  24 minutes and  36 seconds.
Throughput rate: 83.730 MB/min

Question#1: Why the backup job shows about 10GB data being backed up?
Question#2: If the raw data is about 19GB and software compression ratio is 5:1, then it comes about 4GB compressed. This number still does not match up '10GB' in the log.
Question#3: Why the data processed (2,159,818,245) during verification is different from the data (10,917,190,301) during backup stage?

Thank you for your help.
Question by:richtree
  • 2
LVL 81

Accepted Solution

arnold earned 800 total points
ID: 17821675
The log is likely reports the raw amount of data backed up.  The data reflected during verification reflects the 5:1 compression ration.  i.e. during the initial backup, the system accounted for the 10.9GB.  During the verification stage, my guess is that it only verifies that the archive is not corrupt and will only reflect the compressed size.  Are you accounting for the transaction logs in your space calculation for the database?

What are you trying to resolve.  As long as you can restore the backup and have the data functional, you should not be to concerned with the amount of space that is taken.  

Author Comment

ID: 17821764
I am trying to clear off the question that I cannot answer. Yes, I account for the size of .mdf and .ldf files. How to explain difference between the size of database files and the bytes backed up by Backup Exec?
LVL 81

Expert Comment

ID: 17822700
You can not.  The backup file might include additional information in the individual database backup file that is normally part of lets say the master/or msdb databases to provide the ability to restore just one database on a different system where other databases exist.  The backup exec while storing the item in a single file, might provide for some mechanism to maintain the integrity of the file by including additional information which might account for the added space.

Presumably, the backup set such that the transaction log (.ldf) is checkpointed/truncate following the backup.  Are you comparing and accounting for the size of the resulting backup to the mdf and ldf files prior to backup?  is part of your maintenance/backup plan to also shrink the database (mdf/ldf) is they have more then x% of free space?

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