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Re: [Bacula-devel] feature request


Kern Sibbald wrote:
> On Wednesday 30 July 2008 18:27:42 David Boyes wrote:
>   
>>> I initially suggested RAIT to the submitter, but he said he was not
>>> considering that -- if I remember right RAIT actually requires
>>>       
>> something
>>
>>     
>>> like
>>> 5 tape drives. Maybe I am wrong on that though, because I looked at
>>>       
>> the
>>
>>     
>>> code
>>> about 5 years ago.
>>>       
>> It's like RAID -- you need one more drive than you want to use to store
>> data. The current LLNL RAIT library code supports 2, 3, and 5 drive
>> stripes (1+1, 2+1, and 4+1 data+parity). It wouldn't be hard to expand
>> that to larger stripe sizes if need be, but probably not all that
>> interesting unless done as described below.
>>
>>     
>>>  In any case, once you are talking about an LTO4 drive, it is
>>>       
>> currently
>>
>>     
>>> impossible or nearly so to drive it full speed (except possibly by
>>>       
>> some
>>
>>     
>>> high
>>> end IBM mainframes), so why complicate things when Bacula still lacks
>>>       
>> a
>>
>>     
>>> lot
>>> of nice features ... :-)
>>>       
>> True, although with multiple 10G Ethernets and RTSP you can get pretty
>> close to making a LTO4 sweat a little....8-). FICON Express
>> multi-pathing is a lovely thing.
>>
>> Something like this would be more profitably be implemented as a virtual
>> volume storage manager independent of a tool like Bacula. Hmm...
>>     
>
> Well, we are almost there (to your Virtual Storage Manager) ...   :-)
>   

Nothing against RAIT, but I get the impression that rather than a 
RAID-like tape subsystem, what is needed is a LVM-like subsystem for 
tape. The key word is 'subsystem', meaning that maybe it is not 
something that should be built into Bacula, and indeed would be best 
implemented as a device driver, just as lvm and md are. For example, one 
can easily create a LVM volume with striping across two or more RAID1 md 
devices and use that LVM volume with Bacula exactly the same as an 
ordinary disk drive partition. With RAIT and a LVM_for_tape driver, the 
resulting logical tape device could be used with Bacula exactly the same 
as an ordinary SCSI tape device.

Just my opinion, I realize, but my opinion is that this low level device 
handling should not be built into Bacula, allowing Bacula to focus on 
what it does so well, the actual backup of networked machines. Besides, 
something like this is a huge undertaking, deserving of its own project.

>
>
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