On Thu, Oct 6, 2011 at 5:42 PM, Holger Parplies <wbppc AT parplies DOT de> 
wrote:
>
>> [...]
>> With compressible data you increase both capacity and reliability by
>> compressing before storage.   There's no magical difference between the
>> reliability of 'cat' vs 'zcat'.  Either one could fail.
>
> the problem, I believe, is not 'cat' or 'zcat' failing, it's a *media* error,
> as you pointed out, rendering a complete compressed file unusable instead of
> only the erraneous bytes/sectors. Yes, there are compression algorithms that
> are able to recover after an error, but I don't think BackupPC uses any of
> these.
>
> Sure, the common case might be losing a complete disk rather than having a few
> bytes altered, but in that case, you can either recover from the remaining
> disks (presuming you have some form of redundancy), or you lose your complete
> pool, whether or not compressed.

I like RAID1 where you can recover from any singe surviving disk.

> While you might reduce the chances of failure with compression, you increase
> the impact of failure.

Maybe, maybe not.   You might find something usable if you scrape some
plain text or maybe even part of a tar file off a disk past a media
error which is pretty hard to do anyway, but most other file types
won't have much chance of working.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com

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