Re: blu-ray recommendations?

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

deloptes wrote:
> the nature of the CD/DVD implies degradation of material.

This is where our experience differs.


If i had such media in hand i would run

  img_buffer=...path.for.emerging.media.image.file...
  sector_map=...path.for.emerging.map.file...

  xorriso -outdev /dev/sr0 \
          -check_media use=outdev \
                       sector_map="$sector_map" \
                       data_to="$img_buffer" \
                       --

This run will try to create a pair of files with retrieved image data
and info about successfully read sectors and yet unreadable sectors.
Repeated runs will only try to retrieve the previously unreadable sectors.

The file pair may be moved to other computers (or accessed via network)
in order to make retrieval attempts with other reader drives.
But it is also worth to make more tries with the same drive, because a
degrading drive tends to retrieve more sectors if it tries reading with
different start sectors.

Depending on the drive's effort, unreadable sectors can slow down the
retrieval substantially. By default -check_media will try for 8 hours.
(See -check_media option "time_limit=seconds" in man xorriso.)

If the attempt shall be aborted, then this should be done by touching
file
  /var/opt/xorriso/do_abort_check_media
in order to let the retrieval run store its sector map before ending.
(The file path settable by -check_media option "abort_file=disk_path".)

In case of success, the report text will contain only quality marks "+"
and "0, not "-"'. Subsequent attempts with the same sector_map file will
finish quickly without reading attempts.

  Media checks :        lba ,       size , quality
  Media region :          0 ,    1209009 , + valid
  Media region :    1209009 ,         15 , 0 off_track
  Media region :    1209024 ,      20491 , + valid
  Media region :    1229515 ,         21 , 0 off_track
  Media region :    1229536 ,      21186 , + valid
  ...
  Media region :    1800992 ,      21006 , + valid
  Media region :    1821998 ,         18 , 0 off_track
  Media region :    1822016 ,      21838 , + valid

("off_track" are sectors in the gaps between tracks or sessions.
 With CD-R and DVD-R those gaps can have several MB of size.)

The resulting image file can be read or mounted like the medium.
If the storage format contains own checksums, then this is the state to
verify them.


Curt wrote:
> Here's a decade-old (and a little more) opinion from UNESCO (?)
> [...] data tape and hard disc systems are made reliable because
> technological testing [...]
> No viable automatic testing and management system exists to make
> optical disc reliable [...]

It depends on whom you ask. Of course, owners of LTO robots or giant disk
arrays are not impressed by optical media.

Testing is much a matter of popularity. Insofar tapes are substandard.
(The reason why i began to make my backups on CD was the miserable
 quality delivered by QIC and DAT tapes and drives in conjunction with
 normal users.)

Whether testing needs to be automatical depends on the ratio of people
and data. I am one people and have data for one people.
If you have the money for tape robot or drive array, then you also can
afford a big jukebox for opticals.

Maybe the quoted text also mentions the relevance of ubiquity of reader
drives for old media. If so, then it should also note that optical drives
are still manufactured for all old media types.

I have seen as backup media: removable hard disks, MOs, and various tapes.
They all were unreliable in the hands of $WORK users. Optical media
brought relief twofold: They don't fail as often and the user can be held
responsible for verifying that important data are indeed in the backup.

Optical media cannot really compete with spinning hard disks when it comes
to price and reliability. But the properties of both are so different
that hard disk cannot easily replace all typical backup use cases.


Have a nice day :)

Thomas