tar tape blocking factor

tar tape blocking factor

Post by Tom Metzge » Sat, 27 Feb 1999 04:00:00



I have an 8mm tape written on a Sun system using tar. My problem is reading
the tapes. I can read the tape on a Linux(intel) computer using GNU tar with
my 8mm Exabyte Eliant 820 tape drive connected. But only if I set  the
blocking mode to zero( mt -f /dev/rmt0 setblk 0 ). How can I tell what the
blocking factor the tape was written in on the Sun system? The person I am
getting the tapes from doesn't know how to tell.  The command used to create
the tape was "tar cvf /dev/rmt/0c".

I called Exabyte tech support, but they said there tape drives can write
using any blocking factor and that I needed to check with my backup software
vendor.

Any help would be greatly appreciated.

 
 
 

tar tape blocking factor

Post by Derek Smi » Sun, 28 Feb 1999 04:00:00



>I have an 8mm tape written on a Sun system using tar. My problem is reading
>the tapes. I can read the tape on a Linux(intel) computer using GNU tar with
>my 8mm Exabyte Eliant 820 tape drive connected. But only if I set  the
>blocking mode to zero( mt -f /dev/rmt0 setblk 0 ). How can I tell what the
>blocking factor the tape was written in on the Sun system? The person I am
>getting the tapes from doesn't know how to tell.  The command used to create
>the tape was "tar cvf /dev/rmt/0c".

>I called Exabyte tech support, but they said there tape drives can write
>using any blocking factor and that I needed to check with my backup software
>vendor.

>Any help would be greatly appreciated.

I've found that, on some of the Sun OS versions, if you quote a large enough
blocking factor in your tar command and use 'tar -tvf ...' then you'll get
feedback on thefiles and the blocking factor which was used.

HTH

Derek

 
 
 

tar tape blocking factor

Post by Wolfgang Den » Wed, 03 Mar 1999 04:00:00



>I have an 8mm tape written on a Sun system using tar. My problem is reading
>the tapes. I can read the tape on a Linux(intel) computer using GNU tar with
>my 8mm Exabyte Eliant 820 tape drive connected. But only if I set  the
>blocking mode to zero( mt -f /dev/rmt0 setblk 0 ). How can I tell what the

...which means setting the drive to use variable record size.

Quote:>blocking factor the tape was written in on the Sun system? The person I am
>getting the tapes from doesn't know how to tell.  The command used to create
>the tape was "tar cvf /dev/rmt/0c".

`tar' by convention defaults to a blocking factor  of  20,  with  512
bytes per block. So the default record size on `tar' tapes is usually
10 kB.

If you have an unknown tape format, you can try to find out this way:

        1. set tape drive to variable record size (see above)
        2. think about the biggest possible (supported on your system)
           record size MAX_SIZE
        3. Try:
                dd if=/dev/rmt/0 bs=MAX_SIZE count=1 2>/dev/null | wc -c

This gives the number of bytes in the first record. Assuming that all
records have the same size, you can use this size. Be careful if  the
size  you  get  is  identical to the value of MAX_SIZE - in this case
MAX_SIZE was probably not big enough.

Wolfgang Denk

--


Do not simplify the design of a program if a way can be found to make
it complex and wonderful.

 
 
 

1. Best tar blocking factor for HP DDS tape drive ?

There is blocking-factor option for the tar cmd.  It seems the tar write
to the tape drive for every blocking-factor (default is 20) X 512 bytes.
And the tape drive write and skip to next block even if the data size
is less than tape drive's block size.

Since DDS is using rotary head, I guess the block size of such tape
is the data stored during half round of the head turning.  Which may
be 10K bytes to 200K bytes.  Anyone knows the block size of DDS-1, 2, 3
such that we can tune to best blocking-factor when using tar ?

Or the efficiency was optimized internally by the drive (since it also
perform compression) and the user doesn't need to care about it ?
Then, why you have to match the blocking-factor during create and extract ?

Thanks in advance.

SK

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