FW: Question concerning PartitionMagic 3.0

[mikebw at bilow.bilow.uu.ids.net (Mike Bilow)]

Bosco So wrote in a message to Mike Bilow:

 BS> PS:  I seem to recall reading somewhere that tar files do
 BS> not contain error correction/detection information.  So if
 BS> the huge tar file that I made in step #1 is corrupt, and I
 BS> whack the original  partition in step #2, then I'm in deep
 BS> doo-doo, right?  Will gzip compression add verification
 BS> info?  If so, can I tune the compression ratio way low so I
 BS> don't take any performance hits?

No!  Using gzip will *reduce* resistance to errors.  By default, tar stores
data in fixed blocks, so a single fixed block being damaged will only cause the
loss of the file or files contained in that block.  The usual block size is
about 10 KB, although it can be made larger.  All new tar implementations have
error detection, but not error correction.

However, if you pass tar blocks through a gzip filter, than all of the data
past the point of corruption will be lost.  Because of this, gzip filtering
should only be used with tar where the media provides its own error control,
and even then it is risky.

Dealing with a bad tar block can be a mildly involved manual procedure, but tar
was originally designed for magnetic tape where this sort of thing is not
unusual and provisions were therefore put into tar to handle it.  In fact, tar
is more robust than QIC-117 or similar formats because tar stores directory
information physically adjacent to the files, while QIC-117 tacks on a global
directory at the end.
 
-- Mike