Re: Testing Tar (was Re: bad news for bsdtar..)

On 23 Apr, Tim Kientzle wrote:
> Don Lewis wrote:
>> On 23 Apr, Tim Kientzle wrote:
>>>Bruce Evans wrote:
>>>
>>>>At least the -current version of tar skips reading the
>>>>data when it is writing to /dev/null.
>>>
>>>I wonder why it does that?
>> 
>> This speeds up Amanda quite a bit.  Amanda will run tar with the
>> --totals option ...  to plan the best mixture of full and incremental
>> backups.
> 
> Hmmm... How accurate does such information need to be?

For use by Amanda, not terribly accurate.  Amanda archives are generally
done with compression, but the estimate is done without compression and
Amanda then calculates the compressed archive size based on the
historical compression ratio.  There will obviously be a lot of
uncertainty in this, but hopefully not so much as to cause Amanda to
underestimate the amount of tape needed, which might cause it to run out
of space before the backup is complete.

Any sort of systemic error in the estimate would probably get
compensated for when Amanda calculated the compression ratio.

On the other hand, it does look bad if you archive to a file and the
output of --totals doesn't match the archive file size.

> Getting accurate estimates out of bsdtar could be tricky.
> (Either bsdtar needs to know a lot more about archive formats
> than it does right now, or libarchive needs to have a way
> to export this kind of information.)
> 
> Tim
>