Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized, or the total byte count in the job log is higher than the maximum possible compression rate of the tape media.

Article:TECH6076  |  Created: 2009-01-27  |  Updated: 2014-05-13  |  Article URL
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Technical Solution



Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized, or the total byte count in the job log is higher than the maximum possible compression rate of the tape media.


Twice the native capacity of a media is usually given as the maximum possible amount of data that could be written to a tape. It should be noted that this figure is rarely achieved. This is due to the various types of data that are found in most environments. Each data type has its own compression ratio, therefore, there is no standard achievable for the total compression ratio that results. For example, database data (see NOTE at the end of this document for exceptions), encrypted files, executables, and graphics files will generally achieve a small amount of compression (or none at all).  Regular text, log files, and data files will have a very high compression rate.
If hardware data compression is used, then the compression is done by the tape drive. Backup Exec (tm) only sends instruction to the tape drive to do compression. If data is not compressing (either because hardware compression on the tape drive is disabled or because the tape drive does not support it), software compression may be used. Software compression, however, consumes additional system resources and media can only be cataloged and restored from with the Backup Exec software.Even the Sgmon log will show if the compression is on or off.


To enable hardware compression, follow the instructions given below:

1. Right-click on a tape device

2. Select Properties

3. Select the Configuration tab below 
Figure 1

The method is similar for Backup Exec 9.x and 10.x  for Windows Servers (Figure 2).
    Figure 2

If Enable compression appears greyed, the tape drive may not support compression, or the drivers/firmware for the device may need to be updated. Contact the manufacturer of the device for more information.
Note: Backup Exec offers the choice of either hardware or software compression. Software compression may be a viable alternative if poor results are achieved with the use of hardware compression. The use of hardware or software compression can be configured through the Advanced tab within the Backup Job Properties dialog box (Figure 3).
Figure 3
Figure 4 displays the Compression type option in Backup Exec 9.0 for Windows Servers.
Figure 4

The following can also affect hardware compression:

1. The tape drive may be trying to compress data that is already compressed or encrypted. If the data cannot be compressed any further than it already is, the attempt may cause the data to expand. Run a test backup with no compression to compare how much data can be written to the tape media without compression. When using hardware compression, software compression should be turned off, and vice-versa.

2. The system may not be able to keep up with the tape drive. If data is sent to the tape drive at a rate that is either slower or faster than the rate at which the tape drive can write the data to the tape, then the tape device must stop and wait for the computer. Each time the tape drive stops, it writes tracks of undefined data (gap tracks), repositioning the read/write heads for the time when more data becomes available. This causes the tape drive to stop and restart frequently, affecting tape capacity.

3. The tape media may be ready for retirement. When tapes are written to for longer than what the manufacturer recommends, an excessive number of rewrites can occur, causing a reduction in performance and tape capacity. Use a new tape to test compression and confirm that the media is the correct type for the tape drive.

4. The tape drive may need to be cleaned. A buildup of oxidation and debris on the tape heads can cause soft/hard write errors and eventually could cause damage to the tape media and tape drive. Clean the device as per the manufacturer's recommendation and replace the cleaning tape when necessary.

If in doubt about whether the software or the environment is the cause of a reduced media capacity with hardware compression enabled, it is suggested that the services for Backup Exec be stopped in the control panel, and a backup should be performed with the native Windows NT Backup. In most cases, the results should be identical between the two applications.

NOTE:  Database Applications, such as Oracle and SQL, can show a very high byte count/compression ratio (as high as 9:1 with hardware compression) when the volumes that these database files reside on are backed up.  A very high compression ratio can sometimes be achieved because of the structure/contents of the data files and log files that are associated with each database and because of this, the amount of data that the job log reports as backed up can exceed what the rated compression capacity is for the tape media.  When extremely high compression is observed, the throughput rate can also be reported as a value that is higher than what is theoretically possible for the make/model of the tape drive.  Symantec supports backing up and restoring open/online Oracle/SQL Databases via the Oracle/SQL Agents and does not support or recommend backing up open/online database applications at the file level because of possible database corruption that can occur.  Backups of closed/offline database data files, via a file system backup, is a valid/recommended method, per what is documented in the Backup Exec Administrator Guide under "Backing up a Closed Database" in the Oracle Agent Chapter of the manual or per the information in the Related Document below "How to perform a Flat-File Backup of Microsoft SQL 2000 Server".

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