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Re: [Bacula-devel] Virtual backup


In find_next_volume_for_append() in next_vol.c, there is a check to make 
sure the volume selected has not expired. Would it be possible to also 
add a check to make sure the volume is not in jcr->VolList? Assuming the 
list of input volumes for a virtual full job is in jcr->VolList and has 
been built by the time the next appendable volume is searched for, of 
course. The idea being to simply exclude any input volumes from being 
selected as an output volume.

Kern Sibbald wrote:
> Hello,
>
> As many of you know Virtual Backup (consolidation, synthetic full, ...) is a 
> new feature that is implemented in the development trunk and scheduled to be 
> released later this year.  It essentially copies what would be a "full 
> current" restore to a new Volume thus creating an virtual backup that can 
> serve as a Full backup.  This has a lot of advantages, particularly for sites 
> with full backups that run long times or for remote sites where the time to 
> transmit a full backup is excessive.
>
> The Virtual Backup feature works much like Migration and Copy.  It reads from 
> the required Volumes and writes to a Volume specified in the pool as "Next 
> Pool".  This ensures that the read and write Volumes are different.
>
> Everything seems to work fine with the Virtual Backup.  However, thinking 
> about longer term operations, it has occurred to me that when you want to 
> make a second Virtual Backup things will become very complicated.  First, the 
> Virtual backup will want to read the previous Virtual backup volume, and then 
> if that Volume is not full, it will want to write to the same Volume.  Even 
> if the volume is full, you will be in a situation where the Job will want to 
> read and write to volumes in the same pool.  In all those cases, there is no 
> guarantee that there will not be a deadlock situation (actually Bacula 
> currently cancels any job attempting to read and write from the same Storage 
> device).
>
> I am not 100% sure what to do to resolve this issue.  I suppose one could run 
> a Migration job to "move" the Virtual Backup back to the Pool from which it 
> originally came, then the next Virtual Backup would work fine (the same as 
> the first one), but that sounds a bit kludgie.  
>
> If anyone has any suggestions, I would appreciate to hear them.  However, 
> suggestions that require implementing significant amounts of code or complex 
> new algorithms such as deadlock detection won't be very helpful since there 
> is no time left to do such implementations between now and release time.  In 
> addition, deadlock detection won't help, what we really need is deadlock 
> resolution, and that is an even more difficult subject.
>
> Best regards,
>
> Kern
>
>
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