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Compression (or lack of it) backing up to Sony AIT3

Created: 03 Feb 2005 • Updated: 22 May 2010 | 4 comments

I cannot seem to get any compression on backups using Backup Exec 9.1 to Sony AIT3 drives (I have 2 of them).

Both drives give me 100Gb per tape and Backup Exec shows the compression ratio is 1:1.

The Properies of the devices show that compression is enabled and I have tried backup jobs with hardware and software compression but no joy.

One drive is mostly backing up Word documents and Excel files and I would expect reasonable compression on these.

Does anyone have any other tips or experiences with Compression?



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Ameet Thakkar's picture

When using compression it is important to remember that the 2:1 compression ratio frequently assumed or stated in technical documentation is an ideal and seldom reached as a practical average.

Some files compress better than others. Text files are probably the most compressible files in general while high density graphics files like files with .jpg, .gif, .zip, or .cab extensions are not likely to compress at all. Also, many programs, including some database and mail programs, already compress the files they control.

The incorrect media capacity problem is usually observed on tape devices which do not have Media Capacity Checker functionality. Generally, for most of the tape devices, this functionality is initiated from the firmware of the tape device. Even though the tape capacity is reported incorrectly, during the backup operation, the entire media capacity should be consumed.

Total capacity of the media that is currently loaded in the device. If a tape device supports reading of the total capacity amount, then Total capacity is derived from the total capacity amount. Otherwise, Total capacity is estimated based on past usage of the media. Perform a full backup until the tape is filled, for example if the tape capacity is 50 GB then perform a backup of 50GB with overwrite option. Once the backup is completed and next time when the tape is inserted for backup, check the media capacity displayed.

Media capacity is calculated based upon past experience, once a tape has been read from beginning to end via backups to the tape and then when the tape is overwritten afterwards; or from a long erase or full catalog job.

Hence we request you to perform an erase operation on the tape and check. Also perform a full backup until the tape is filled for example if the tape capacity is 160GB then perform a backup of 160GB with overwrite option. Once the backup is completed and next time when the tape is inserted for backup, check the media capacity displayed.

Uninstall the existing device drivers and reinstall the latest Veritas drivers. Run tapeinst.exe to install Veritas driver.
"TAPEINST.EXE" is located in the directory x:\Program Files\VERITAS\Backup Exec\NT (TAPEINST.EXE" is an automated installation application for installing tape device drivers )

VERITAS Backup Exec (tm) 9.1 - Device Driver Installer (20050115) - Includes Tape Driver Release (20041231) and Robotic Library Release (45) (Intel Only)

Please update the drive firmware to the latest.

To diagnose the cause of issue please cross-check the compression ratio from Windows's native backup utility (ntbackup) with hardware compression(if supported by tape device)enabled. Select the same type/amount of data to backup and observe the result.

The following can also affect hardware compression:

1. The tape drive may be trying to compress data that is already compressed. 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.

For further information, please refer the technote below:



We would like to inform you that Backup Exec does support STAC compression, which is used in software compression. When you select software compression option, Veritas uses STAC software, which is a third party application software (A data compression technique), which runs in background and carries out the compression tasks.

When the hardware compression is used, the compression is done by the backup device not by the Backup Exec software. The Backup Exec issues an instruction to tape drive to compress the data. In software compression, the compression is performed by the Backup Exec. If your device does not support hardware compression, you can select software compression.

CPU utilization is more incase of software compression, where as in case of hardware compression (usually faster than software), does not involve more of CPU cycles to carry out the compression task.


Also refer to the following points.
1) Stop the Removable Storage Service and then try to perform backup.
2) Avoid using SCSI ID 0,1 and 6 for tape devices Try to use another fresh tape to test if its being used as per its mentioned size which is 100/200GB. Compare and verify the result.
Try to use another fresh tape to test if its being used as per its mentioned size which is 100/200GB. Compare and verify the result.

Also try using the windows native NT Backup utility. Compare the results with the results from Backup Exec and revert back to us with the same.
Kindly refer to the following technotes for relevant information

-Title: How to figure out if COMPRESSION is working. How much data can be written to a 12/24 GB tape?

(Note: This technote contains information related to Netware, however we would suggest you please refer to the explanation mentioned)

We hope this helps.

Sheetal Risbood's picture


In reference to our previous reply to your post, we would request you to update us on the progress.

However, if we do not receive your intimation within two business days, this post would be 'assumed answered' and archived.