Verifying the Hardware Compression Setting in Backup Exec
Backup Exec typically uses the hardware compression mode on a tape drive to maximize the amount of data that can be placed on a tape. There are many times when end users are concerned that even though Backup Exec has been configured to use compression mode, that the drive is for some reason still not compressing the data. This document shows how to use the SCSI Tracer utility available in Backup Exec for Windows Serves 11d and later to assist in definitively verifying the compression mode of the drive.
Please note that the SCSI log is the lowest level interface to the tape drive. This is information that can be used by both Symantec technicians and the Hardware Manufacturer's technicians to see what is being requested of the tape drive. The SCSI trace is definitive in terms of the drive's compression mode.
Getting a SCSI Trace:
The first step is to get Backup Exec ready to perform a backup operation. Create a small backup operation (this keeps down the size of the tracer log) to the device in question. Submit this job on hold.
Then use the Tracer utility to get a SCSI trace of the commands that are being issued to the tape drive.
- Open a Windows Explorer window
- Navigate to the C:\Program Files\Symantec\Backup Exec directory.
- Locate the Tracer.exe file and double click on it.
- Click on the green button to begin recording (or choose the Trace/Start menu option).
Go back to the Backup Exec administration console and run the test backup job.
When the job completes, go back to the Tracer utility and press the red button to stop recording (or choose the Trace/Stop menu option).
Note: If desired, the original job that is being questioned can be used for gathering the SCSI trace. However, the trace should be stopped once the backup job starts writing data to the tape to minimize the size of the trace log.
The File/Save As menu command can be used to save a trace log for future analysis, or to be sent to a Symantec technician. The File/Export As Text menu command should be used when sending the trace results to a device manufacturer technician who may not have access to the Tracer utility.
Interpreting the Log:
The first step is to find the relevant section of the log. To do this, scroll down using the scroll bar of the top section of the log until a series of Write operations are found. This is the beginning of the backup operation.
Now, scroll up between 10 and 20 commands until Mode Select and Mode Sense commands are found. The purpose of this process is to find the very last set of these commands that are issued to the drive before the backup begins.
Select the Mode Select command in the upper pane as in Figure 2.
The details for this Mode Select command are shown in the lower pane of the Tracer utility. The Mode Select command is used to modify various "pages" of settings in the device. It is important to find the Mode Select command that has the fifth byte of the Data field (highlighted in the red box in Figure 2) of 0F This indicates that the Data Compression Page is being modified.
The seventh byte of the data (also highlighted in a red box in Figure 2) contains the Data Compression Enabled (DCE) bit. The DCE bit of this field is the high order bit. A value of 80 or C0 in this field indicates that Data Compression is being turned on. A value of 00 or 40 indicates that Data Compression is being turned off.
Note: It is normal for Backup Exec to turn off Data Compression when the engine first starts up. The important thing to be looking for is to be sure that Data Compression is enabled at the time of the backup job.
Right after setting the Data Compression Enabled bit in the Data Compression page for the device, Backup Exec reads the Data Compression page back in to verify that Data Compression is enabled. This is done with the Mode Sense command.
Select the Mode Sense command just below the Mode Select command as in Figure 3.
The data coming back from the Mode Sense command shows the device settings for the Data Compression Page. The fifth byte returning from the Mode Sense command should be 0F to indicate the Data Compression Page ( highlighted in Figure 3). If this field is not a 0F, then the wrong Mode Sense command has been chosen. Search for the last Mode Sense command before the backup that has this field set to 0F.
The seventh byte of the data (also highlighted in a red box in Figure 2) contains the Data Compression Enabled (DCE) bit. The DCE bit of this field is the high order bit. A value of 80 or C0 in this field indicates that Data Compression is turned on. A value of 00 or 40 indicates that Data Compression is turned off.
If the SCSI Trace indicates that Backup Exec has turned on the Data Compression Enable bit on the drive, and that is then verified by reading the Data Compression setting back from the drive, then the drive is in hardware compression mode. There is nothing more that Backup Exec can do in order to further compress the data.
If for some reason the hardware compression is not performing properly on the drive, Backup Exec does have a software compression feature which can be used in its place.