Re: Archiving content of a directory on a DVD-R.

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

Hi,

peter@easthope.ca wrote:
> Media current: DVD-R sequential recording
> [...]
> xorriso : UPDATE :  742.1m content bytes read in 112 seconds = 5.0xD

It seems that next the joy of incremental backups could be explored.


> http://easthope.ca/DebianPage.html

First precondition would be to give up xorriso command
  -close on
to keep the DVD-R writable on its yet unused area.

Next replace -outdev by -dev and -map by -update_r.
I.e.:

  xorriso -for_backup -dev /dev/sr0 \
          -update_r . / \ 
          -commit \
          -toc -check_md5 failure -- \ 
          -eject all 

Now you can write to the DVD-R until it is full. Each run will produce a
complete directory tree of the current state. Data file content will
only be written if the file is new or its content changed since the
previous backup session (if there is such a session on DVD already).
If not many files changed, the add-on session 

When you mount the DVD on GNU/Linux, the youngest session will be used by
default. Older sessions can be mounted by mount(8) option "-o sbsector="
using the numbers told by column "sbsector" of xorriso command -toc:

  $ sudo mount -o sbsector=567890 /dev/sr0 /mnt/iso

Each session shows the complete directory state as it was at the time
when it was written to DVD.

When the DVD-R has not enough room for the upcomming session, xorriso will
fail without altering the DVD-R content. In this case, just insert a new
blank DVD-R to write a new first session.

------------------------------------------------------------------------

The update decision is based on the timestamps and recorded MD5s on DVD.
For the comparison, the MD5s of the files in /home/peter/MY0.Bak have
to be computed by reading the full file content. If the storage device
is slow or your CPU runs hot, then consider to bet on device numbers and
inode numbers:

If the device number of the storage device is persistent over unmounting
or reboot, then insert
  -disk_dev_ino on
between -for_backup and -outdev.

If the device number changes, but the inode numbers are persistent, then
use
  -disk_dev_ino ino_only

Whether those numbers are persistent can be inquired by printing them
with shell command "stat" before and after reboot or re-mounting:

  $ stat --format="device=%d  inode=%i" /.../some_file
  device=2051  inode=3540166

(Device number changes are a matter of the operating system's bus
 management, inode number changes are mostly a matter of the filesystem's
 type. extN should be safe at least with inode numbers.)


Have a nice day :)

Thomas