Re: Using dar for archives to optical discs

Thank you, Denis.  This is helpful.  Some more comments/questions
inline:

On Wed, May 31 2023, Denis Corbin wrote:

> On 31/05/2023 17:54, John Goerzen wrote:
>> Now, the question of restoration arises.  dar_manager can ingest these
>> archives (or isolated catalogs).  Is dar_manager slice-aware?
>
> No, dar_manager does not need to be slice-aware. Furthermore,
> dar/dar_manager do support re-slicing a backup/archive (using dar_xform)
> and the re-sliced backup/archive can be used in place of the original in
> total transparency with the same isolated catalogue.
>
>> In other
>> words, can I get it to tell me, for a given restoration, "I will be
>> needing slices 5, 3, and 10 of backup A and slice 2 of backup B"?
>
> This is dar's task (not dar_manager's) to do that, see -Tslice option:
>
>   dar -l <backup/archive> -Tslice
>   dar -l <backup/archive> -Tslice -g some/file

Very nice!  I did some experiments with dar_manager and all this, and it
is looking promising.

The one thing I'd like to be able to do is easily find what slice is
needed for a given restoration.  I can do that somewhat manually; with:

dar_manager -B foo -f bar

that will give me archive numbers, which dar_manager ... -l will
translate into filename for me.  From that, I can use dar -l -Tslice -g
bar to get the slices for each required backup file.

I did note that dar_manager -r is very helpful for actually doing the
restore!  Nicely prompts for the last slice, then the slices it needs.
One foible: it complains of removing a non-existent file (presumably it
wouldn't if I used -k) but it did work correctly.

> Note that location of data and metadata stored in a catalogue correspond
> to the amount of byte from the beginning of the backup as if there was
> only a single slice. If the backup is sliced this is the duty of the
> underlying "sar" object to abstract this slicing and convert these
> absolutes offsets to slice number + relative offset, knowing the slicing
> information of the backup (= initial slice size and other slices size).

Ahh; I gather that this is why dar_split can't be used with random
access, because there are only two sizes stored (the size of slice 1,
and the size of all slices >1)?

- John