using tar with DAT drives

Simon Clayton Simon at
Fri Jun 1 08:25:14 BST 2001

OK, we did the backup yesterday with

	tar -cb 128 /etc /usr/home /usr/shared

When we did

	tar -t > filelist.txt

We got

  sa0:adw0:0:6:0) 65536 - byte tape record bigger than supplied buffer
  tar: read error  /dev/rsa0 : Input/output error

  May 31 16:56:35 bsd /kernel
  sa0:adw0:0:6:0) 65536 - byte tape recored bigger than supplied buffer

the last section starting with May 31 .... was repeated.

Any ideas what is happening as I don't understand what it is doing.


-----Original Message-----
From: Ian Pallfreeman [mailto:Ian.Pallfreeman at]
Sent: 31 May 2001 14:52
To: Simon Clayton
Cc: freebsd-users at
Subject: Re: using tar with DAT drives

> I have a machine running 4.0 to which I have fitted a 12/24 DAT drive.
> Using tar, the drive is recognized and seems to work perfectly but
> I have recently found that despite only having around 3Gb of data on
> the system and trying to backup to a 12Gb tape - only some of the
> data is actually being archived.

Meaning what, exactly? Are you getting "end of tape detected"? Some other
error? Forgetting to back up all your partitions?

> I have read the man page a number of times and been told that I can
> change block sizes or block counts but I don't actually know what
> the parameters should be.

It varies from drive to drive. Most DATs can handle 64K blocks, so you'll
want something like:

	tar cbf 128 /dev/nrsa0 /data/to/backup


	dump a0bf 128 /dev/nrsa0 /partition/to/dump

Bigger block sizes means more of the tape is written with real data, rather
than being used for inter-block gaps, and the drive will generally eat data
faster. The downside is that if you get an error on the tape, you'll lose
more data, but that usually isn't an issue. Also, I can't really see you
losing 9GB of tape even using small records, so it may be another problem.


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