If you look back in the list archives you will see this explained.

HBAs and SCSI cards that use the windows miniport driver have a 64k 
limitation set by the maximum_sg_list parameter - or rather its default 
value as it is usually not defined.  You can raise this in the registry 
settings for the particular driver, up to a maximum of 1MB.

Search the internet for the sg_list or 'scatter-gather list'.

256k is consistently reported as the best size for LTO by all LTO vendors.

HBAs that don't use the miniport driver usually don't have this 
restriction.  Note that HP use the miniport driver with Emulex OEM'd HBAs, 
where the native Emulex driver is a port driver.   So you can have the 
identical hardware behave differently.

Always check you can restore data where you need to.  If your DR site is 
limited to 64k by the drivers there, backing up at 256k will hurt.

William D L Brown




"Daryl Katauskas" <DKatauskas AT nst-inc DOT com> 
Sent by: veritas-bu-admin AT mailman.eng.auburn DOT edu
31-Jan-2006 20:28
 
To
veritas-bu AT mailman.eng.auburn DOT edu
cc

Subject
[Veritas-bu] HP LTO 3 Ultrium & WIN2K03 NBU Optimization?






Hello all:
 
Am in the midst of deploying new HP LTO3 (Fibre) drives in our NBU 
environment and am finding some conflicting information regarding NBU 
buffer configuration.
 
(Using Qlogic & Emulex HBAâ??s).
 
HP recommends the following settings: 
 
<install path>\ NetBackup\db\config\SIZE_DATA_BUFFERS = 256KB (262144)
 
I have read (MS Technet Article Q152518) which states that the Windows O/S 
has a limitation of 64KB tape variable blocksize.
 
If this is the case, how does one configure NBU 5.x/6.x to optimally 
perform under WIN2K03 or WIN2K with Fibre LTO3 drives?
 
Any insight/info would be most appreciated.
 
 
Daryl Katauskas
Consultant, Storage Solutions
2050 Finley Road, Suite 80
Lombard, IL  60148
vox 630.281.7041
fax  630.904.9601