amtapetype man page on YellowDog

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AMTAPETYPE(8)							 AMTAPETYPE(8)

NAME
       amtapetype - generate a tapetype definition.

SYNOPSIS
       amtapetype  [-h]	 [-c]  [-o]  [-b�blocksize]  [-e�estsize] [-f�tapedev]
		  [-t�typename]

DESCRIPTION
       amtapetype generates a tapetype entry for Amanda.

OPTIONS
       -h     Display an help message.

       -c     Run only the hardware compression detection heuristic  test  and
	      stop. This takes a few minutes only.

       -o     Overwrite the tape, even if it's an Amanda tape.

       -b blocksize
	      record block size (default: 32k)

       -e estsize
	      estimated tape size (default: 1g == 1024m)

       -f tapedev
	      tape  device  name  (default:  $TAPE)  The device to perform the
	      test.

       -t typename
	      tapetype name (default: unknown-tapetype)

EXAMPLE
       Generate a tapetype definition for your tape device:

       % amtapetype -f /dev/nst0

NOTES
       Hardware compression is detected by measuring the writing speed differ‐
       ence  of	 the  tape  drive  when	 writing an amount of compressable and
       uncompresseable data. It does not rely on the status bits of  the  tape
       drive  or  the OS parameters. If your tape drive has very large buffers
       or is very fast, the program could fail to detect hardware  compression
       status reliably.

       During  the  first pass, it writes files that are estimated to be 1% of
       the expected tape capacity. It gets the expected capacity from  the  -e
       command	line  flag,  or defaults to 1 GByte. In a perfect world (which
       means there is zero chance of this  happening  with  tapes  :-),	 there
       would be 100 files and 100 file marks.

       During  the  second  pass,  the	file size is cut in half. In that same
       fairyland world, this means 200 files and 200 file marks.

       In both passes the total amount of data written is summed  as  well  as
       the  number of file marks written. At the end of the second pass, quot‐
       ing from the code:

       * Compute the size of a filemark as  the	 difference  in	 data  written
       between	pass  1 and pass 2 divided by the difference in number of file
       marks written between pass 1 and pass 2. ... *

       So if we wrote 1.0 GBytes on the first pass and 100 file marks, and 0.9
       GBytes  on  the	second	pass with 200 file marks, those additional 100
       file marks in the second pass took 0.1 GBytes and therefor a file  mark
       is 0.001 GBytes (1 MByte).

       Note  that if the estimated capacity is wrong, the only thing that hap‐
       pens is a lot more (or less, but unlikely) files, and thus, file marks,
       get  written.  But  the	math  still works out the same. The -e flag is
       there to keep the number of file marks down because they	 can  be  slow
       (since  they  force  the	 drive	to  flush  all its buffers to physical
       media).

       All sorts of things might happen to cause the amount of data written to
       vary enough to generate a big file mark size guess. A little more "shoe
       shining" because of the additional file marks (and flushes), dirt  left
       on  the	heads  from  the  first pass of a brand new tape, the tempera‐
       ture/humidity changed during the multi-hour run, a different amount  of
       data was written after the last file mark before EOT was reported, etc.

       Note  that  the file mark size might really be zero for whatever device
       this is, and it was just the measured capacity  variation  that	caused
       amtapetype  to  think those extra file marks in pass 2 actually took up
       space.

       It also explains why amtapetype used to	sometimes  report  a  negative
       file  mark size if the math happened to end up that way. When that hap‐
       pens now we just report it as zero.

SEE ALSO
       amanda(8)

								 AMTAPETYPE(8)
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