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Changing Block Size on Windows Media Agent
Appendix A A-51
Changing Block Size on Windows Media Agent
In order to increase the maximum block size on a Windows Media Agent
client, you have to modify its Registry. After modifying the Registry,
restart the computer. Drivers read MaximumSGList at boot time. The
actual formula that a Windows class driver uses to determine the
maximum transfer size is:
maximum size = ((number of supported scatter/gather
elements - 1)* 4096)
For the typical aic78xx case, it renders the following:
((17-1) * 4096) = 64k (which corresponds to 56k usable
data for Data Protector)
Windows provides a mechanism to support more scatter/gather elements
via the Registry. Start the regedit32 and add a DWORD value in the
following Registry key:
\\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aic78
xx\Parameters\Device0\MaximumSGList
Use the following formula to calculate the value of the MaximumSGList:
Example The MaximumSGList value for a 256k block size is 65:
MaximumSGList = (265k/4k) + 1 = 64 + 1 = 65
If you have, for example, 3 aic78xx based SCSI channels on your system,
change the appropriate ...\Device0, ...\Device1 or ...\Device2 value. If
you want to set all adapters at the same time, specify MaximumSGList for
...\Device\.... Omitting the numerical reference sets the value for all
aic78xx adapters.
MaximumSGList
BlockSize
4096
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1+=