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Re: How does multisession work?
I'm going to answer your out-of-order.
On 12/19/06, Alejandro Weinstein <alejandro.weinstein@gmail.com> wrote:
> I don't quite understand how the multisession works. First of all, I
> believe that my cdwriter support multisession, since I see this in the
> log:
>
> 2006-12-19T13:37:41 CLST --> [INFO ] Device does support
> multisession discs.
Ok, this is good. Your hardware does support multiple sessions (most
modern hardware does, although when I started writing Cedar Backup in
1999 and 2000, it wasn't as common).
> "If the backup is running on the first day of the week, if the drive
> does not support multisession discs, or if the --full option is passed
> to the cback command, the disc will be rebuilt from scratch.
> Otherwise, a new ISO session will be added to the disc each day the
> backup runs."
>
> What "the disc will be rebuilt from scratch" mean?
> Does it mean that for a CD in particular, I can use the --full option
> only once?
> And, does it mean that I need to burn a CD once per week? (since once
> per week the CD "will be rebuilt from scratch") ?
The answer depends on whether you are using CD-R media or CD-RW media.
First, let's define "multisession". A simple CD has one session on
it. This session is created when the disc is burned. The disc is
"closed" and can never be modified.
A "multisession" disc is created a little differently. When the disc
is first created, it is not immediately "closed", making it possible
to add other sessions later. The disc remains fully usable after each
session is written.
If your drive supports it, Cedar Backup will write a multisession
disc. The disc is "created" on the first day of the week or when
--full is used. The first session on the disc will contain all of the
"weekly" backups, all of the "daily" backups and all of the
"incremental" backups.
Each day thereafter, Cedar Backup will write a new session to the
disc. This new session will contain just the "daily" backups and any
files that have changed among the "incremental" backups.
Every day, it will be possible to mount the CD media and see all of
the previously-written backups.
Now, if you are using CD-RW media, when the beginning of the week is
reached again (or when you use --full the next time), Cedar Backup
will wipe out (blank) the media and write a new first session to the
disc, starting the process over "from scratch".
If you are using just CD-R media, however, this does not work. CD-R
media cannot be erased once it has been written.
So, yes, you must use one CD per week, but if the CD is re-writable
(CD-RW), you can use the same disc multiple times.
> On the other hand, as my first try with Cedar Backup, I did a test
> with "cback --full collect subversion stage store purge", and
> everything go right, i.e., the backup was stored in the CD.
Ok, that's good.
> Later I decided to make some changes in /etc/cback.conf, and test the
> backup again with "cback --full collect subversion stage store purge",
> but this time I get the error (with the same CD in the cdwriter).
>
> "[ERROR ] Error executing backup: Directories contain a different set
> of files."
Well, this means that Cedar Backup was not able to successfully write
your backup to disc. However, for some reason, Cedar Backup did not
notice this until it did the consistency check.
Two questions:
1) What kind of media is in your backup device, CD-R or CD-RW?
2) What kind of media is Cedar Backup configured to use (i.e. cdr-74,
cdrw-74)?
If you can make this happen again, I would like to see the debug log.
> If i try with "cback collect subversion stage store purge", expecting
> an incremental backup), I get:
First, this probably won't do what you expect. You can certainly run
collect a second time to see that data is collected in incremental
mode. You would look in the log or look at the resulting files in the
collect directory to be sure.
However, you can't write this incremental backup to disc. Cedar
Backup is designed to write one directory to disc each day (one
directory per session). Since your full backup already includes a
backup directory for December 18, writing another one would cause
problems.
> "Error executing backup: Unable to find unused staging directory to
> store (tried today, yesterday, tomorrow)."
>
> I think that may be I screwed up by changing the name field of the
> stage section in /etc/cback.conf between the first time I executed the
> backup and the second.
I don't think that your configuration change caused this.
This behavior occurs because you ran store action twice on the same
day. If you look, you will see that the daily staging directory for
December 18 contains a file called 'cback.store'. This indicator file
tells Cedar Backup that the staging directory has already been written
to disc. Cedar Backup is refusing to write the same directory to disc
twice.
If you really want to see that the incremental backup contains what
you expect, execute just the collect and subversion actions and check
the contents of the collect directory afterwards. You can't write to
disc a second time, but you can see what would be in the incremental
backup.
Unfortunately, it's not easy to test the full backup cycle (including
the write to disc) on a single calendar day.
Hope that clears things up...
KEN
--
Kenneth J. Pronovici <pronovic@ieee.org>
http://www.cedar-solutions.com/
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