AMTAPETYPE(8)AMTAPETYPE(8)NAMEamtapetype - generate a tapetype definition.
SYNOPSISamtapetype [-h] [-c] [-o] [-b blocksize] -e estsize [-f tapedev]
[-t typename]
DESCRIPTIONamtapetype 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 (No default!)
-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 -e 150G
NOTES
Hardware compression is detected by measuring the writing speed
difference 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,
quoting 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
happens 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
temperature/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
happens now we just report it as zero.
SEE ALSOamanda(8)
06/06/2007 AMTAPETYPE(8)
