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Re: [Bacula-devel] feature request


Hello,

On Tuesday 29 July 2008 15:22:03 T. Horsnell wrote:
> What:
> That Bacula be modified to enable the simultaneous use of multiple tape
> drives, either as a bunch of freestanding units or as multiple drives in
> an autochanger.
>
> Why:
> For me personally, this would permit faster simpler backup of a large
> single filesystem. I have a two-drive LTO4 tapechanger and at present,
> in order to utilise both drives at once, I have to create two jobs each
> of which backs up part of the filesystem. This is not optimal as the two
> subparts can change size radically, meaning that one drive may spend
> much of its time idle. I also have to split my tapes into two pools, one
> for each job.
> I have seen one other similar request to the users list recently,
> whereby a user had a bynch of freestanding drives which he wanted to
> preload with a set of tapes once a week, and then have Bacula
> automatically organise his backup over the tapes as it saw fit.
> This may make a cheap alternative to a tapechanger.
>
> A device pool maybe?

Technically I don't know how to do this project.   The SD has blocks of data 
coming in from the FD, and sending those blocks to multiple drives would be 
extremely difficult to track -- I am not even sure how.  Currently, they are 
either sequential (if spooling is on) or at least written in order to a 
single drive, so it is very easy to track and find them for restores.

In addition, I don't think it is possible for any existing network connection 
to run fast enough to drive an LTO4 tape drive at full speed, so this project 
seems to add a lot of complexity to Bacula to give no improvement in 
performance.  If I am wrong about the network connection and an LTO4, please 
show me the math :-)

For the moment I cannot accept the Feature Request.  If you can give complete 
details of a design for tracking the blocks written to multiple tapes and how 
to send them back to the FD in the same order, and if you can show with a 
simulation that we would actually gain something, then you can resubmit it, 
and I will reconsider adding it to the projects.  After than, either someone 
must submit a patch or the Bacula users vote and it becomes a top rated 
project, in which case one of the developers would surely work on it.

Best regards,

Kern

PS: Normally, I don't veto a Feature Request even if I am not much in favor of 
it unless it involves license problems (using proprietary interfaces) or is 
technically not possible as is the current request as written.

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