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Re: [Bacula-devel] [Bacula-users] feature request


On Wed, Jul 30, 2008 at 11:45 AM, Kern Sibbald <kern@xxxxxxxxxxx> wrote:
> 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.
>
I totally agree that sending data blocks to more than 1 device at a
time for a single job at this time will be a very difficult task but
when I read this I see that there are two basic feature requests and
the second is not so bad. The second is to be able to continue a job
on a different tape than the job started on. I can see this is useful
if you have more than 1 drive of the same type.

John

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