On Monday 29 August 2005 13:39, Arno Lehmann wrote:
> Hi,
>
> Florian Schnabel wrote:
> > i'm curious ..
> > what mechanism(s) uses bacula to verify that the tape (or any other
> > storage medium) has written the data correctly ?
> > e.g. would it notice if my tape is damaged ?
>
> That depends.
>
> Basically, Bacula relies on the drives abilities to detect write errors
> (Read after Write, current drives should do this automatically and
> report an error to controller, which goes through the driver to the
> application).
>
> If you don't want to rely on this, you can only re-read and compare the
> complete drive's contents. This is not something bacula supports,
> (although you can compare the metadata - see Verify Jobs in the manual)
> and I think this is ok:
As you point out, Bacula does not compare the complete contents of the drive
(i.e. the file data), but in order to get at the "metadata" or what I call
the attributes, which Bacula does compare, it must read all the blocks
containing the data. In doing so, the drive must first read the data and
return a good status, and secondly, Bacula will verify it's own checksum on
each block. So end the end, you may not be sure that the data is correct,
but you can be sure that it can be re-read.
> First, modern tape drives are quite reliable.
> Second, todays tape capacity makes re-reading a whole backup rather
> useless, because they can conatin an amount of data that makes it quite
> probable that some of the original data is modified between passes, and
> the whole operation would take just too long.
>
> In short, you have to rely on the tape drive itself.
>
> Arno
--
Best regards,
Kern
(">
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