On Jul 10, 2008, at 7:45 AM, Kern Sibbald wrote:
> Hello,
>
> I'm a bit burned out from intensive bug fixing over the last couple
> of months,
> so decided to do something totally new yesterday. I started
> implementing
> what I call Virtual Backup or Vbackup, which is essentially project
> #3 "Merge
> multiple backups (Synthetic Backup or Consolidation)".
>
> In attempting to implement it, I've realized a few things:
>
> 1. It is probably better to implement it as a new "level" under the
> normal
> Backup code, for example "level=vbackup". The resulting output
> will be
> recorded in the catalog as a "Full".
>
> 2. In most all respects it must behave much like a Migration job in
> that it
> does not use a FD, it reads an existing set of backups, and writes
> them to a
> new Volume.
>
> 3. One difference from a Migration job is that all the old jobs remain
> unchanged (i.e. like a Copy).
>
> 4. Another difference is that it has many fewer features in that it
> simply
> finds all the current backup records and copies them. There are no
> complicated selection criteria.
>
> 5. Like the Migration and Copy jobs, the input Pool (from where it
> reads the
> currently backed up data) and the output Pool (where it writes the
> merged
> data) must be different. This ensures that the job does not
> attempt to read
> and write to the same device, which just will not work.
>
> Well the problem with the above -- principally item #5 is consider the
> following:
>
> You have a job J1, which does a Full, one or more Diff backups,
> then any
> number of Inc backups all going to Pool P1. At some point in time
> (possibly
> via the Schedule), you run a vbackup level, so it finds all the
> current
> backup files (Full, last Diff, and all later Inc) and copies the
> data from
> the input Pool (P1) to the output Pool (P2).
>
> Now, if you then redo a normal Full backup and restart with Diff
> and Inc jobs
> again, all will work.
>
> However, it is much more likely that you will then continue doing
> incremental
> backups (no more Full or Diffs). At some point later, you want to
> do another
> vbackup to "consolidate" all the Inc backups, and now the process
> fails,
> because you are going to need to read from Pools P2 (Full produced
> by the
> vbackup) and P1 (new Incs), and you will attempt to write to P2,
> which will
> not work.
>
> Thus without some other mechanism to move Volumes from Pool to
> Pool, a setup
> like described above won't work, and I suspect this is what will be
> done the
> most frequently (i.e. do only one Full and there after vbackups
> when there
> are enough Incs to warrant a consolidation).
How does the above strategy cope with deleted files? For example,
foo.bar is
included in the Full backup, but removed from disk before the next
backup job.
How does this strategy deal with the above is:
- foo.bar is not explicitly listed in the FileSet
- foo.bar is explicity listed
e.g. File = /usr
versus File = /usr/foo.bar
My thoughts: In the First case, the Vbackup job would not include
foo.bar. In the
second case, it should. In both cases, the file could be retrieved
from the original
Full backup.
Critics who think it should always be included should not be using
Vbackup.
--
Dan Langille -- http://www.langille.org/
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