The Windows SCSI drivers that use the miniport driver often are limited to 
64K.   The limit is in the SCSI driver, and can be raised by adjusting the 
'maxiumum scatter-gather list' size.  This is in the registry under the 
parameters for the specific SCSI driver, so step 1 is to work out which 
driver entry it is.  maximumsglist is I think the parameter - google can 
confirm this.  You can up it to about 1MB...but of course not above the 
maximum for the drive.

As mentioned, the DLTs used to be limited to 64K.  May no longer apply to 
newer SDLT, best thing is to try.

The only real gotcha is that while you can write a tape at whatever 
blocksize suits, and on restore the block size is read off the tape, that 
size on the tape must not exceed the maximum the restore server is capable 
of.  So it is just possible to write a tape at 256K blocks, and fail to 
restore it on a Windows box with a 64k limit set in the SCSI driver.

Fibre cards mostly don't have this limit, but some do still use the 
miniport driver - HP seem to even on Emulex.   They do in their install 
increase the maximumsglist from the default.

William D L Brown
S



"Bob Stump" <stumpb AT michigan DOT gov> 
Sent by: veritas-bu-admin AT mailman.eng.auburn DOT edu
16-Nov-2005 15:55
 
To
veritas-bu AT mailman.eng.auburn DOT edu
cc

Subject
[Veritas-bu] limitations on SIZE_DATA_BUFFERS






Thank ypu to all who answered my earlier post.
The correct answer given to me was  256K (262144)

Here is a followup question.
Can this value also be used in W2K and W2K3 servers?
I seem to remember limitations in earlier versions of Windows/NetBackup.


>>> Bob Stump 11/15/2005 4:47 PM >>>
Because the data buffer size equals the tape I/O size, the value specified 
in SIZE_DATA_BUFFERS must not exceed the maximum tape I/O size supported 
by the tape drive or operating system. This is usually 256 KB or 128 KB.

For Digital Linear Tape (DLT) drives, the number that seems to give the 
best performance for SIZE_DATA_BUFFERS is 65536. Further performance 
improvements can be achieved by increasing the value in the 
NUMBER_DATA_BUFFERS file.

What is the optimum for STK 9940B?


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