The hidden email

[Lennart Sorensen]

On Wed, Oct 19, 2005 at 02:59:33PM -0400, Robert Brockway wrote:
> On Thu, 13 Oct 2005, Lennart Sorensen wrote:
> 
> > I use afio rather than tar/gzip since afio compresses each file in the
> > archive with gzip rather than creating the archive and then compressing
> > it.  You loose some compression as a result but not much, but you gain
> > the ability to quickly seek through the file to the head for the file
> > you want and then start decompressing the file (and only the file)
> > rather than having to decompress the whole thing to find the header and
> 
> This approach (also taken by the zip algorithm) provides for improved data 
> reliability too.

zip unfortunately seems to store the archive directory at the end of the
file, so if you are missing the end of the file, it can be very hard to
get much useful out of zip.  Corrupt content in the middle is ok though.
zip is far from ideal as an archiver.

> If you use tar.gz ("archive then compress") to dump data to a tape and the 
> archive suffers from an error on restore your archive is basically toast.  
> You may have archive restore utilities available if you are lucky.

Well at least unlike zip, everything up to the error will be fine
without extra work.

> If you use a "compress then archive" approach (used by afio and zip) you 
> will only lose a minority of files if you suffer an error when restoring.

Well if you can seek to the next header, then yes it will recover the
rest.

> When it comes to tape technologies that are not as reliable as many people 
> think this is a big deal.
> 
> Like many sysadmins I never use "archive then compress" to tape but I use 
> it on harddrives all the time as the level of reliability is much greater.

Lennart Sorensen
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