Using Backup Exec's Discover tool to determine the maximum allowable block size for a tape drive.

Problem

Using Backup Exec's Discover tool to determine the maximum allowable block size for a tape drive.

Solution

Beginning with Backup Exec 12.5, the Backup Exec Discover tool was enhanced to output the necessary information needed to determine the max supported block size for a tape drive. Increasing the block size has been known to increase performance in tape drive performance in certain environments.
The largest supported block size can be found by reviewing tracer output to determine the following values:

1.      Max drive bytes
2.      Max controller bytes
3.      The total of Max Controller Pages multiplied by the Page Size

The lowest of the above three values would be the limiting factor on the block size, and that the lowest of these values would therefore become the maximum block size allowed for the tape drive. The values of these settings are outputted when running discover.

NOTE: Veritas recommends that all backup job have the verify option enabled to verify that data is being properly written to tape, regardless of whether or not the block size setting is adjusted. Users who modify the block settings on their tape drives are advised to not only verify their data, but to perform a test restore after making changes to the block size to verify that the hardware is correctly able to handle the larger blocks of data.

Discover.exe is a command-line utility and is located in the install directory of Backup Exec (by default C:\ Program Files\veritas\Backup Exec).  This tool must be run with the Backup Exec services stopped. After stopping the Backup Exec services, run discover from a command prompt and locate the information regarding the tape drive.

For more information for running the Discover tool and reading it's values, please see the related documents at the bottom of this article.  

Example Discover Output:

System Page Size: 4096

 03:00:01:00 01 -- \\.\Scsi3:
             INQ 00 "HP      Ultrium 2-SCSI  F5CD"
             INQ cc "HP      ULTRIUM460 DRV  F5CD"
             INQ dc ""
              INQ 83 "HP      Ultrium 2-SCSI  F330001209"
             INQ 80 "F330001209"
             TUR 00 00000
             RES 00 00 00000
             REL 00 00 00000
             MAX drive bytes 0xffffff       (16777215)
             MIN drive bytes 0x1
             BUS SCSI(1)
             MAX controller bytes 0xffffffff pages 0x11

The information highlighted above are the values that are used to determine the max block size. The system page file is only value used that is not outputted in hexadecimal (HEX), and that value is 4096 bytes.

For the remaining values, we will need to convert them from HEX to decimal (DEC).

Converting the Hexadecimal numbers into Decimal:

One simple way to do this would be to use the built-in Windows calculator. Using the calculator tool (Start > All Programs > Accessories > Calculator), enable the Scientific mode going to View > Scientific.

Click the 'HEX' radio button, and input the HEX the value you wish to determine. Using the above discover output, we'll determine the Max Drive Byte, which is ffffff (Figure 1):

Figure 1


Click on the 'DEC' radio button, and that will convert the number into a decimal value of 16,777,215 bytes, or 16MB (Figure 2):

Figure 2


 This tells us that the Max Drive Bytes is 16MB.

For the remaining values:

Use the above steps to determine the values for Max Controller Bytes and Max Controller Pages:

Max Controller Bytes = ffffffff (HEX) = 4,294,967,295 bytes
Max Controller Pages = 11 (HEX) = 17 bytes

After determining the Max Controller Pages, multiply that number by the System Page File (recall that the System Page File = 4096 bytes):

Max controller pages (17) x System Page File (4096) = 69,632 bytes

Final Values:

Max Drive Bytes = 16,777,215 bytes
Max Controller Bytes = 4,294,967,295 bytes
Max Controller Pages (x) System Page File = 69,632 bytes

Since the value of the Max Controller Pages * the System Page File is the lowest value of these three values, it would be the limiting factor for the block size, which in this case would be 69,632 bytes (or 68k). The tape drive is capable of handling blocks of data up to 16MB, but the controller is limited to only 68k. An updated controller which allows larger Max Controller Pages would be able to process bigger blocks of data.

The above example is from a tape drive that is connected to an Adaptec 29320 controller.