sendmail -bv davidsen@tmr.comThomas
Schmitt wrote:
Hi,
Question: is this currently capable of updating a CD-R or DVD-R using
multi-session as the basis for updating selected files?
Yes. xorriso does multi-session on nearly any media
(except the infamous fast-blanked DVD-RW).
You may overwrite existing files in the ISO image,
or delete them, or move them to another address path.
If you add directories then they get merged with
existing ones. So it might be desireable to delete
the old one first.
The according example from the man page:
Manipulating an existing ISO image on the same media
Load image from drive. Remove (i.e. hide) directory /sounds and its
subordinates. Rename directory /pictures/confidential to
/pictures/restricted. Change access permissions of directory
/pictures/restricted. Add new directory trees /sounds and /movies.
Burn to the same media and eject.
$ xorriso -dev /dev/sr2 \
-rm_r /sounds -- \
-mv \
/pictures/confidential \
/pictures/restricted \
-- \
-chmod go-rwx /pictures/restricted -- \
-map /home/me/prepared_for_dvd/sounds_dummy /sounds \
-map /home/me/prepared_for_dvd/movies /movies \
-commit -eject all
I will have to think on this. Currently I backup a source directory,
then detect the files changed since the last update, use a small script
to generate a file suitable for mkisofs with -graft-points, and update
only those files. The source is ~3GB, and on a good day we generate
100MB of changed files, so fitting on a DVD isn't an issue.
But if you want to mirror some hard disk subtrees
by some trees on optical media, try this example:
Incremental backup of a few directory trees
This does the following to directories /open_source_project and
/personal_mail in the ISO image: create them if not existing yet,
compare them with their disk counterparts, add disk file objects which
are missing yet, overwrite those which are different on disk, and
delete those which have vanished on disk. But do not add or overwrite
files matching *.o, *.swp.
$ xorriso -dev /dev/sr0 \
-volid PROJECTS_MAIL_"$(date '+%Y_%m_%d_%H%M%S')" \
-not_leaf '*.o' -not_leaf '*.swp' \
-update_r /home/thomas/open_source_projects /open_source_projects \
-update_r /home/thomas/personal_mail /personal_mail \
-commit -toc -eject all
To be used several times on the same media, whenever an update of the
two disk trees to the media is desired. Begin with blank media and
start a new blank media when the run fails due to lack of remaining
space on the old one.
The above example is more what I have in mind, this particular backup
needs no fancy hiding or such, I just want to (a) not lose data and (b)
be able to roll back to any point in time by mounting a session. Since
the builds are usually done by shadow tree I don't even have to exclude
anything in the real source.
This makes most sense with backups on non-erasable media like CD-R,
DVD-R, DVD+R if the full backup leaves substantial remaining capacity
on media and if the expected changes are much smaller than the full
backup. An update run will probably save no time but last longer than
a full backup. Other good reasons may be given if read speed is much
higher than write speed or if file size changes happen too often within
the write time of a full backup.
With mount option "sbsector=" it is possible to access the session
trees which represent the older backup versions. With CD media, Linux
mount accepts session numbers directly by its option "session=".
Multi-session media and most overwriteable media written by xorriso can
tell the sbsector of a session by xorriso option -toc.
Sessions on multi-session media are separated by several MB of unused
blocks. So with small sessions the payload capacity can become
substantially lower than the overall media capacity. If the remaining
space on media does not suffice for the next gap, the drive is supposed
to close the media automatically.
Better do not use your youngest backup for -update_r. Have at least
two media which you use alternatingly. So only older backups get
endangered by the new write operation, while the newest backup is
stored safely on a different media. Always have a blank media ready to
perform a full backup in case the update attempt fails due to
insufficient remaining capacity.
Clearly I could write whole new media, that's not optimal from a time or
storage space perspective, writing a new DVD for a 3-4kB file change is
undesirable.
The waste of space on multi-session media is worst on CD-R[W]
and DVD-R[W] where session gaps can be up to 20 MB. On DVD+R
it is only 4 MB. Best it is with overwriteables: DVD-RAM, DVD+RW,
formatted DVD-RW. There the waste is between 0 and 62 KiB.
Another overhead is caused by the fact that with each session
a complete new directory structure is written.
The size depends on the number of file objects in the image
and usually is in the range of a single percent of the
combined image size.
So the wish to write in granularity of few KiB will hardly
be fulfillable by xorriso.
My backups produce one base session of 200 MB and 20 small
update sessions on CD media, one base session of 800 MB and
40 update sessions on DVD-R, 50 on DVD+R and 60 on DVD+RW.
About 4500 blocks of each session hold the directory tree.
(One can measure by making two runs with only a small data
file changed inbetween.)
Exanmple of a DVD-R:
TOC layout : Idx , sbsector , Size , Volume Id
ISO session : 1 , 0 , 870113s , UPDATE_HOME_2008_04_18_220030
ISO session : 2 , 908176 , 22632s , UPDATE_HOME_2008_04_20_133303
ISO session : 3 , 938656 , 19169s , UPDATE_HOME_2008_04_21_153459
...
ISO session : 41 , 2233968 , 60302s , UPDATE_HOME_2008_06_06_114440
DVD+-R multi-session info might be ignored or misunderstood
by DVD-ROM drives. In that case one has to help mount
to find the youngest session.
This problem does not apply to overwriteable media.
Have a nice day :)
Thomas
--
Bill Davidsen <davidsen@tmr.com>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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