From: Kern S. <ke...@si...> - 2008-07-31 19:29:32
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On Thursday 31 July 2008 16:32:49 Josh Fisher wrote: > 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. Whatever way such a project is done, as a separate driver or as part of Bacula, I agree with the above sentence, and it will require additional programmers to implement it. Kern |