[Simh] Reading VMS Backup tapes on Linux

[Larry Baker]

Jon,

It is not always the case that the record length for a save set will  
be 8192.  That is the default value for save sets written directly to  
tape.  Save sets written directly to disk use 32760 (I think).  The  
value can be specified in the backup command.

I found a script a while ago that sets the proper record length  
attributes using an undocumented filed in the backup save set header  
that contains the record length.  It uses F$CVUI to convert the  
unsigned 32 bit field at byte offset 0x28 to the integer record  
length.  You could do something similar to read the actual record  
length on Linux using od or hexdump.  Here's that DCL command procedure.

Larry Baker
US Geological Survey
650-329-5608
baker at usgs.gov

$! RESET_BACKUP_SAVESET_ATTRIBUTES.COM
$!
$! P1  is the specification of the BACKUP saveset
$!
$! This procedure resets the record format and record
$! length attributes of a BACKUP saveset -- savesets
$! can get "broken" during certain sorts of file
$! transfers -- such as FTP.  This procedure reads the
$! (undocumented) saveset record attributes directly
$! out of the target file.
$!
$! First render the saveset readable, and implicitly
$! check that the file exists.
$!
$ Set File -
     /Attributes=(RFM:FIX,MRS:512,LRL=512,ORG=SEQ,RAT=NONE) -
     'p1'
$
$ Open/Error=whoops/Read BckSaveset 'p1'
$ Read/Error=whoops/End=whoops BckSaveset Record
$ Close/Nolog BckSaveset
$
$! Find the blocksize from within the record...
$
$ BlockSize = 0
$ BBH_L_BLOCKSIZE = %x28*8
$ BlockSize = F$CVUI(BBH_L_BLOCKSIZE, 32, Record)
$ If BlockSize .lt. 2048 .or. BlockSize .gt. 65535
$ Then
$   Write sys$output "Unexpected block size"
$   Goto whoops
$ Else
$   Set File /Attributes=(RFM:FIX,LRL='BlockSize', -
        MRS='BlockSize',RAT=none) -
        'p1'
$ endif
$ exit
$WHOOPS:
$ Write sys$output "Error"
$ exit

On 6 Apr 2012, at 9:00 AM, simh-request at trailing-edge.com wrote:

> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 05 Apr 2012 20:03:08 -0500
> From: Jon Elson <elson at pico-systems.com>
> To: simh at trailing-edge.com
> Subject: [Simh] Reading VMS Backup tapes on Linux
> Message-ID: <4F7E40CC.6070509 at pico-systems.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Well, for those NOT running VMS in simulation, but wanting
> to get at some archival programs, I finally figured out how to
> make vmsbackup work.  First, you need to extract the backup
> save set from the tape container file, so it is just a stream of
> bytes, without the block size and file mark information.
> In other words, just the bytes from the data blocks, all strung
> together.  I already had a program that did this for cr/lf
> ASCII files.
>
> Then, the command is :
> vmsbackup -xe -b 8192 -f <filename>
> where the 8192 is the blocksize of the saveset file on the tape.
>
> Jon
>
>
>
>
> ------------------------------