Monday, September 25, 2006
Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized.
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 varied types of data that are found in most environments. Each data type has its own compression ratio, and, therefore, there is no standard achievable for the total compression ratio that results. For example, database data, executables, and graphics files generally compress the least, and regular text and data files will compress the most.
If hardware data compression is used, then the compression is done by the tape drive. VERITAS 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.
To enable hardware compression, follow the instructions given below:
1. Right-click on a tape device
2. Select Properties
3. Select the Configuration tab
The method is similar for Backup Exec 9.0 for Windows Servers
If Enable compression appears grayed, 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.
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
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.
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.
If hardware data compression is used, then the compression is done by the tape drive. VERITAS 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.
To enable hardware compression, follow the instructions given below:
1. Right-click on a tape device
2. Select Properties
3. Select the Configuration tab
The method is similar for Backup Exec 9.0 for Windows Servers
If Enable compression appears grayed, 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.
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
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.
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.
# posted by Tech Mania @ 9:26 AM 
