Re: [Dar-support] backing up directly to tape

[Denis C. <dar...@fr...>]

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On 18/02/2015 15:28, Gour wrote :
> Denis Corbin <dar...@fr...> writes:

[...]

>> Good question! Well this is something that is missing to dar, at
>> first I thought outputing dar to the split command would have
>> solve the issue but nop, split does not help sending again data
>> to a file/special device once end of file was met.
> 
> It means one is, in general, supposed to create all the slices with
> dar and then send them to 'remote' devices for storage?

dar is well adapted to disk by design (where from its name :-) ), thus
either you have local or network filesystem and dar creates files to
them or you can use the -E option to copy the slice to a remote
storage (scp, sftp, etc.) as soon as it is completed (the other way
for reading, as soon as it is required by dar, still using -E option).

In that case I recommend not choosing a tool large slicing for you can
avoid retransmitting too much data in case of network failure. Finding
the adequate size is defining how many time you are ready to loose
resending data thus knowing the expected network bandwidth with some
simple math you can find the expected slice size.

> 
[...]
> 
>> If that matches your need I will integrate that program (after
>> more intensive testing) into dar for next major release.
> 
> Thanks a lot...however, if next major dar relase won't be very
> soon, I believe we'll migrate to RDX in between. ,)

next major release (2.5.0) is expected for this year... but even if
you migrate to RDX before, or choose another backup solution, I would
appreciate if you could just tell me whether this "dar_split" is
suitable to work with tapes (I have no tape device here).

Last point about your feedback (first mail to this mailing-list),
while I can't help you about LTO and RDX media for backup with dar not
having any experience about this approach, you've been asking also
about par2:

Dar archive have redundancy checking (CRC) that will let it detect
corruptions in an archive and skip over such corruption (you may also
ask dar to restore the corrupted file anyway thanks to the 'lax
mode'). But dar cannot correct such corruption, par2 *can* most of the
time (if the amount of data lost is larger than the redundancy data
calculated by par2, obviously par2 cannot recreate the information it
hasn't).

Seen that point, as it does not hurt adding par2 data beside dar
slices, even 1% redundancy can help you preserve your data over time
or over bad media... however par2 works on files so you will not be
able to use tape except having first dar writing to disk, running par2
from -E option or using "par2" on command-line or even once all slices
have been created, then moving the slice(s) and its/their par2 file to
tape.


> 
> 
> Sincerely, Gour
> 
> 

Cheers,
Denis.
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