[H-GEN] Linux Backup

Jason Parker-Burlingham jasonp at uq.net.au
Fri May 9 11:28:10 EDT 2003


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Robert Brockway <robert at timetraveller.org> writes:

> On Fri, 9 May 2003, Jason Parker-Burlingham wrote:
>> * Some people don't like to use even software compression because they
>>   worry that an error on the tape will invalidate all the data in
> Yes, this is the reason I don't like using it when backing up to tape.

Thought so.

> With an uncompressed backup you risk losing one file.

Well, hang on a minute.  Do you mean one "file" on the tape, or one
file as it would exist on the filesystem?  If we're talking about
dump, then surely a tape error would tend to invalidate the entire
dump output, making all the files contained it after the error
unretrievable?  So, then, how does one increase one's risk by
compressing dump output before sending it to tape?

> On the whole I've gone off tapes as a backup solution
> anyway.  Too many problems overall.

I think they're the only tenable solution for backups of large amounts
of data.

For home users---this being HUMBUG after all---you're probably right.
I am thinking of getting myself an IDE CD writer for making backups
(although I'll probably need a larger disk to hold the dumps).

> Getting back to compression, there is a middle path here - the zip utility
> compresses each file individually before archiving.

I see now.  You are talking about writing tar format backups to tape.
In that case your point above makes a lot more sense, but I was
assuming you were talking in your role as a dump-using person.

>> * Whatever you do, *never* turn on hardware and software compression
>>   at the same time:  instead of compressing the data even more, the
>>   hardware compression will only make the data slightly LARGER than
>>   before.
> Agreed - they often get in the way of each other.

Not just often.  It's practically guaranteed unless your software
compression is very bad.  What happens (I'm sure you know this, Rob)
is the software turns your data into basically random bits (this is
the definition of compression after all).  But these random bits by
definition can't be compressed more---if they could, your software
hasn't done a good job---and so whatever quoting system your hardware
compression uses will just get in the way and increase the size of the
file written to tape.

Anyone can see this for themself by trying to re-compress a file
compressed with gzip -9.

jason
-- 
``Oooh!  A gingerbread house!  Hansel and Gretel are set for life!''

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