Re: [Bacula-users] Configuration reload for bacula-sd

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On 10/28/2014 07:39 PM, Dimitri Maziuk wrote:
> On 10/28/2014 01:04 PM, Alan Brown wrote:
>>
>> Block devices may be block devices, but a tape drive is a character device.
>
> Oops. Works either way, disks are char aka "raw" devices too.
>
>> People coming from a tape environment tend to see tapes (volumes) as
>> something you use for a pool of clients or filesets, because changing
>> volumes is NOT trivial, the tapes are only reated for a limited number
>> of load cycles and you can only access one volume at a time.
>>
>> If you have more tape drives you can access more volumes simultaneously,
>> but changing volumes is no less difficult/timeconsuming.
>
> As is changing disks. Skipping from one tarball to another on tape may
> take much longer, but that shouldn't make any difference to the
> higher-level code.
>
> There is no reason to see file volumes as dedicated to a client AFAICT;
> those who want it can do pool per client and volumes per pool -- works
> with tapes just as well.
>
> Yes, in general you can access multiple files on the same disk
> simultaneously. I don't expect that a backup application streaming data
> to or from that disk would -- or should -- actually do that, for any
> number of obvious reasons.

The above confuses me, because Bacula can stream multiple simultaneous job data to a disk -- they will of course be streamed into separate files, but nevertheless, you can have as many Bacula steams going simultaneously to a given disk drive. Bacula can also have multiple writes to the same disk file, but of course Bacula must sequence the writes to be able to capture correct disk addresses for the restore.

>
>
> Which is why I'll never understand the tape/disk dichotomy.
>
> (As an aside, bacula, specifically, seems to force you to use different
> Media Type for each physical device so I don't get how it would work
> with multiple drives in the same jukebox, either -- thankfully I don't
> need to.)

To be more precise, Bacula does not require you to use different Media Types for different devices. It only does so for different disk devices so that it can be sure where the Volume is actually stored. For a compatible tape drives in an autochanger, there is no need to have different Media Types. However, if you have multiple separate libraries the Media Type must be different so that Bacula is sure what library the Volume is in (it actually keeps a device index, but this is often insufficient).

Best regards,
Kern

>
>
>
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