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The Wednesday 2007-10-17 at 04:15 +0100, David Bolt wrote:
...
see if I could find anything about it, just observed it. Here's a quick
example[0]:
[EMAIL PROTECTED]:/usr/src/packages/SOURCES> ls -l
total 173652
-rw------- 1 davjam users 67072000 2007-10-17 02:22 home.tar
....
67076096 bytes (67 MB) copied, 33.386 s, 2.0 MB/s
Ah! I understand now, it's padded up to complete an exact number of
sectors. That's clear.
...
disc equivalent of dvd.home.tar
Insert disc equivalent of dvd.home.tar.bz2
disc equivalent of dvd.home.tar.bz2
bzip2: (stdin): trailing garbage after EOF ignored
of course...
It would mean that md5sum wouldn't read back the same bits that were
supposedly written, there is some thing else... padding?
That's what it seems to be. A quick check shows that you can get around
it by using either of these tar options:
--blocking-factor=64
--record-size=32768
A quick test shows they both produce tar archives where the length is
exactly divisible by the growisofs block length:
[EMAIL PROTECTED]:/usr/src/packages/SOURCES> tar cf home.2.tar
--record-size=32768 home
[EMAIL PROTECTED]:/usr/src/packages/SOURCES> tar cf home.3.tar
--blocking-factor=64 home
-rw-r--r-- 1 davjam users 67076096 2007-10-17 04:01 home.2.tar
-rw-r--r-- 1 davjam users 67076096 2007-10-17 04:02 home.3.tar
...
131008+0 records in
131008+0 records out
67076096 bytes (67 MB) copied, 50.1566 s, 1.3 MB/s
Right.
[EMAIL PROTECTED]:/usr/src/packages/SOURCES> md5sum home.2.tar dvd.home.2.tar
/dev/hdc
377f9a42287c631d7367f7ee53e681df home.2.tar
377f9a42287c631d7367f7ee53e681df dvd.home.2.tar
377f9a42287c631d7367f7ee53e681df /dev/hdc
And, as you can see above, using either of those two options results in
all three test subjects being binary identical.
Yep. I must remember this, not only with tars, but when creating images
files.
Forgot to mention, the "tar xjf /dev/dvd" pops up a warning about
ignoring extra data after the EOF marker whereas the other two don't.
Curious.
I'll save this for future reference. When I wanted to know the exact
size of a dvd I found the 4.7 GB figure.
I always use a figure of 4.2GiB. Gives me a little leeway, just in case.
[0] Had to use DVD+R for this. Trying it with a +RW resulted in the
entire contents of the disc being read, all 4.3GiB of it :|
Maybe because it overwrites the whole disk to erase the old contents.
- --
Cheers,
Carlos E. R.
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