[EMAIL PROTECTED] wrote:
> First, I have to add that the usage of the new compression mode of dump
> is essential when writing to a RAW cd device. The reason is that
> when writing in raw form (i.e. without an iso filesystem) the device
> cannot write variable blocks (only 2Kb blocks). Therefore restore
> cannot detect the correct end-of-volume.
> But when using the compressed mode, each block is prefixed by a
> 4 byte header containing a length byte which cannot be zero.
> Since cdrecord pads with zeroes this is a means to detect the
> last block.
I tried using dd with a block size of 2k, but obviously the dump method
is built-in and easier to use. Using dd does work, in a clumsy way.
> Second, by using different compression modes one can influence
> the load of the processor. And writing with speed 1 but compression
> ratio 3 is faster than writing at speed 2 with no compression.
Like all compression things this depends very much on the CPU speed and
the content of the data. Unfortunately no generalization will fit all
cases, as much as I wish I had a way to predict what to use.
Lastly, I wish I could find a compressor which uses all the CPUs in the
system. I hate to watch one CPU cook at 100 usage, while the other three
sit with their thumbs up their idle loop. I actually wrote a test
program to read 1MB of data and put it in a shared memory buffer. Four
processes compressed it using gzip with settings of 1, 3, 7, and 9. When
each finished it left the result in a different shared buffer and marked
its compression as done. When the next process downstream indicated that
it needed more data NOW, the best finished compression buffer was
selected and sent, and the rest of the processes were told to give it
up.
Actually, thinking back on when I did that, it wasn't gzip but old
compress running with 10, 12, 14 and 16 bit compression on a super
powerful SunOS server with four 68040's (or so). State of the art in
1990. :-(
None of this is practical in any case, you lose too much by having small
buffers at any compression. we need a new algorithm for compression to
benefit, and in the mean time I'll try dump+compress when I get a
chance.
--
-bill davidsen ([EMAIL PROTECTED])
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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