[Gllug] Tape block size confusion

[Tom Schutzer-Weissmann]

Something odd when trying to read a tape, I wondered if anyone knew what
was going on.

- The tape has a block size of 64k. 

- dd without a block size (bs or ibs) specified, or with one lower than
64k, fails with "failed to allocate memory". 

- dd with a block size of 128k or more works: that is it reads each file
on the tape up to its file marker and then stops, reading the next file
when re-invoked. 
So if the file is only 64k, dd if=$TAPE of=$DUMPFILE bs=1M still only
reads 64k into $DUMPFILE.

So far so normal. But since 64k was the lowest block size at which dd
would work, and since I knew the tape itself was in blocks of 64k, I
used bs=64k to read the entire tape into a set of files. All these files
contained nothing but 0s! 30-odd gigabytes of zeroes split into files of
the same size as the files on the tape.

I have re-run the process using a block size of 128k and get the same
set of files, only they actually contain data.

TomSW



	
	
		
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