ABSTRACT: Determining <Tape> <Block> <Size> and Format

 ABOUT THIS DOCUMENT

       The AIX Operating System supports <tape> devices with various
       <block> <sizes>.  <Tapes> written at one particular  physical
       <block> <size> may not be readable unless the <tape> device is
       reconfigured for that same <block> <size>.  This document
       describes how to determine the correct physical <block> <size>
       and how to determine the <tape> format (such as tar or
       backup/restore) in all versions of AIX.

  <BLOCK> <SIZE> DETERMINATION

       You can find the <tape> <block> <size> by configuring the device
       to accept a very large <block>, performing a single read, and
       determining the <size> of the read <block>, as in the following
       steps:

       1.  Begin by configuring the device for variable <block> <size>
           data:

              chdev -a block_size=0 -l <<tape>>

           where <<tape>> is the device, such as rmt0 or rmt1.

       2.  Next, read a single <block> from the device and write the
           <block> to disk.  This will allow you to see exactly how
           large the <block> is and what data is contained on the
           <tape>.

              dd if=/dev/<<tape>> of=/tmp/block bs=128k count=1

       3.  The data from the <tape> <block> will be in the file
           /tmp/block.  You can determine the <block> <size> by looking
           at the byte count with

              wc -c /tmp/block

           The <size> in bytes is the <size> of the physical <blocks> on
           the <tape>, assuming the <tape> was written with a single
           physical <block> <size>.

       4.  You can then use this <size> in the chdev command given
           above to configure the <tape> drive for the loaded <tape>.

      Please Note:  If you only need to determine the blocksize of
      the archive and want to avoid creating the temporary file,
      you can issue the following command:

      dd if=/dv/<<tape>> bs=128k count=1 | wc -c

  FORMAT DETERMINATION

       The "file" command can be used on the <block> of data that was
       read from the <tape> in the previous section (the /tmp/block
       file) to determine the format type of data on the <tape>.

          file /tmp/block

       The file command attempts to match header information in the
       file against known header types described in the /etc/magic
       file.  After a change to the /etc/magic file (described
       below), the file command will recognize tar, cpio, and
       backup/restore formats.

       However, on AIX versions previous to 4.1 and later, the file
       command will not recognize format types that do not have
       header information, such as dd and back-by-inode.  For
       example, if you used dd to put a compressed file on a <tape>
       and then followed the procedures in this document, the
       format would be recognized as compressed.

  Updating the /etc/magic file (not necessary for AIX 4.1 and later)

       To update the /etc/magic file so that tar and backup/restore
       formats can be recognized by the file command, do the fol-
       lowing:

       1.  Become the root user.

       2.  Make a backup copy of the /etc/magic file by running

              cp /etc/magic /etc/magic.backup

       3.  Add the following two lines to the bottom of the
           /etc/magic file.  Note that each line must start in the
           first column.

              257   string    0x7573746172   tar format file
              0     long      0x09006bea     backup/restore by name format file

  SHELL SCRIPT TO AUTOMATE <BLOCK> <SIZE> DETERMINATION

       This shell script provides a means of performing the <block>-
       <size> determination steps automatically.

       #!/bin/ksh
       #
       #        lstape -- List a <tape>'s <block> <size>
       #
       if [ $# -ne 1 ] ; then
               /usr/bin/echo 'usage: lstape device'
               exit 1
       fi
       #
       #        Check the name and put it in a standard format
       #
       if [[ "$1" = /dev/* ]]; then
               <TAPE>=${1##/dev/}
       else
               <TAPE>=$1
       fi
       #
       #        Set the <block> <size> to variable
       #
       /usr/sbin/chdev -l ${<TAPE>} -a block_size=0 >/dev/null
       if [ $? -ne 0 ]; then
               exit 1
       fi
       #
       #        Read a single <block> and write to /tmp
       #
       /usr/bin/dd if=/dev/${<TAPE>} bs=128k of=/tmp/lst$$ count=1
       if [ $? -ne 0 ]; then
               /usr/bin/rm -f /tmp/lst$$
               exit 1
       fi
       #
       #        Get the <block> <size> of the resultant file
       #
       /usr/bin/wc -c /tmp/lst$$ | read <SIZE> FILE
       /usr/bin/echo "$<TAPE>:        ${<SIZE>} bytes per <block>."
       if [ -x /usr/bin/file ] ; then
               /usr/bin/file /tmp/lst$$ | sed -e "s!/tmp/lst$$!/dev/${<TAPE>}!"
       fi
       #
       #        Set <tape> drive to correct physical <block> <size>
       #
       /usr/sbin/chdev -l ${<TAPE>} -a block_size=${<SIZE>} >/dev/null
       /usr/bin/rm -f /tmp/lst$$
       exit 0



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