My experience is Win2k networker uses the registry variable to set the MAXIMUM 
block size allowed to read/write the tapes. Any block size lower than the 
maximum value set in the registry is fully supported (read/write).
Furthermore, if you have tapes previously written with the default block size 
higher than the one you get setting the NSR_DEV_BLOCK... variable, with scanner 
you are able to detect the block size fo the tape, but you cannot read data!

Hope it helps
Claudio Ruzza

P.S. following there is a part of a message that i found in listmail archives 
describing how to set up in windows the registry key (sorry for non mentioning 
the author)

Hi, 
To change the maximum block size in NT/Win2K environments from the default 64k 
follow the following procedure. 
1. Find out your adapter card type (from Control Panel/SCSI) the tapes are 
connected to. 
2. Backup your registry. 
3. Open the registry for editing. 
4. Goto \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\AdapterType 
5. Create a key named Parameters by selecting Add Key from the Edit menu and 
entering Parameters for Key Name. 
   Leave Class blank. 
6. Open the Parameters key and create a key named Device by selecting Add Key 
from the Edit menu. Enter Device for Key Name. 
   Leave Class blank. 
7. Open the Device key and select Add Value from the Edit menu. In the Value 
Name edit-box, enter MaximumSGList. 
   Select REG_DWORD as the type. 
   For Value, enter a value of 0x41(hexadecimal). This will allow for block 
sizes up to 256k. 
8. Reboot your system for the new values to take place. 
Any tape that has been labeled with a 32/64KB blocksize should be relabeled or 
at least erased. 

> -----Original Message-----
> From: Kit Cunningham [mailto:kit AT 4F.CO DOT NZ]
> Sent: lunedì 11 agosto 2003 08:17
> To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
> Subject: Re: setting block size
> 
> 
> My experince is windows2k networker uses the variable setting 
> for writing only and the registry
> change for writing/reading. I do not know why.
> 
> Thus if you only set the variable you cant read the tapes. We 
> set the registry entry only to get
> both settings.
> 
> I suggest ( if not already suggested by someone) you use 
> st.conf entries to set your blocksize.
> 
> Kit
> 
> On 8 Aug 2003 at 10:57, Teresa Biehler wrote:
> 
> > So, I set the environment variable on the Solaris server to 
> lower the =
> > block size to 128KB so that Windows and UNIX agree.  I added the =
> > following to the top of the networker start up script:
> >
> > NSR_DEV_BLOCK_SIZE_8MM_AIT_3=3D128
> > export NSR_DEV_BLOCK_SIZE_8MM_AIT_3
> >
> > After making this change, I restarted the server.  I states 
> that the =
> > block size for all the AIT-3 devices is 128KB as expected.  Then I =
> > labeled a new tape, ran a full back up to it - so far so 
> good.  When I =
> > ran scanner on the tape, however, I get this:
> >
> > scanner -v /dev/rmt/5cbn
> > scanner: Opened /dev/rmt/5cbn for read
> > scanner: Rewinding...
> > scanner: Rewinding done
> > scanner: Reading the label...
> > scanner: Reading the label done
> > scanner: SYSTEM error: Tape label read: Bad file number
> > scanner: SYSTEM error: Tape label read: Bad file number
> > scanner: scanning for valid records...
> > scanner: read: -1 bytes : Not enough space
> > scanner: Opened /dev/rmt/5cbn for read
> > scanner: read: -1 bytes : Not enough space
> > .
> > .
> > .
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > scanner: read: 0 bytes
> > .
> > .
> > .
> >
> > Does anyone have any ideas what is going on?
> >
> > Thanks.
> > Teresa
> >
> >
> >
> > -----Original Message-----
> > From: Ruzza, Claudio [mailto:claudio.ruzza AT HP DOT COM]=20
> > Sent: Friday, August 08, 2003 8:54 AM
> > To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
> > Subject: Re: [Networker] setting block size
> >
> > Teresa,=20
> > Needless to say, the variable name should be modified 
> according to your =
> > drives technology.
> > Example: assuming you have SDLT drives, the variable name 
> will become
> > NSR_DEV_BLOCK_SIZE_SDLT
> >
> > In windows 2000 you can set this variable doing some easy steps:
> > Click right on "My computer" and choose Properties
> > Choose "Advanced"
> > Click the "Environment variables" button
> > Click the button "new" in the lower part of "system variables" pane
> > In "variable name" enter the name you have "build up" once 
> you have =
> > identified your drive's technology
> > In "variable value" just enter 128
> > Click OK and that's all.
> >
> > Please check that after having set the variable at least one media =
> > initialized by those Server/SN has reaaly labeled at 128K. 
> In case it =
> > doesn't, there are some additional steps to do, involving 
> registry edit. =
> > Check listmail archives, there's a wonderful page about that.
> >
> > In Solaris and any other UNIX flavour the variable must be 
> set as any =
> > other environment variable. I'm not a UNIX expert, so I 
> can't be more =
> > precise.
> >
> > Hope it helps
> >
> > Claudio Ruzza
> >
> > > -----Original Message-----
> > > From: Teresa Biehler [mailto:tpbsys AT RIT DOT EDU]
> > > Sent: venerd=EC 8 agosto 2003 14:05
> > > To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
> > > Subject: setting block size
> > >=20
> > >=20
> > > I've got a Solaris Networker server and I'm trying to set 
> up a Windows
> > > storage node that will share the same tape library.  So, I=20
> > > want the each
> > > system to be able to read/write tapes written by the 
> other.  Based on
> > > running scanner -v on tapes written by both hosts, I've=20
> > > determined that
> > > I need to set the block size to 128KB.  So, how do I do 
> this?  In the
> > > admin guide is says to set the environment variable
> > > NSR_DEV_BLOCK_SIZE_DEVICE_NAME.  Where/how do I do this (for both
> > > Solaris and Windows)?
> > >=20
> > >=20
> > >=20
> > > Thanks!
> > >=20
> > > Teresa
> > >=20
> > >=20
> > > --
> > > Note: To sign off this list, send a "signoff networker"=20
> > > command via email
> > > to listserv AT listmail.temple DOT edu or visit the list's Web site at
> > > http://listmail.temple.edu/archives/networker.html where you can
> > > also view and post messages to the list.
> > > =
> > 
> =3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3D*=3
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> 
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