Writing zeros when the restore target is a block device might be a better
approach, rather than doing it based on the type of the backup.
__Martin
>>>>> On Sat, 28 Jul 2012 08:50:53 -0700, Kern Sibbald said:
>
> Hello Bastian,
>
> On the documentation issue, the best would be for you to submit
> a bug report marked [DOC] in the subject line, with perhaps some
> sample text that you would propose, otherwise your request is
> likely to be lost.
>
> Also, for this patch, you should do the same: submit it as a
> bug report with [PATCH] in the subject. I am not really sure we
> want to do this, at least under the "sparse" key word as it would
> mean that certain devices that may have holes in them will
> actually consume much more space -- this would apply particularly
> to VMs where they tend to use sparse allocation techniques.
>
> The sparse option is intended to work only for *real* sparse
> files, and you are trying to apply it to reduce or compress zeros
> in certain images. This could probably be better done through a
> plugin where one could tune it to do exactly what you want.
>
> If you do want to submit the patch, we will consider it, but please
> if you have not already done so, fill out an FLA and send it in. You
> can find it at www.bacula.org -> FLA License.
>
> Thanks for your interest in this feature ...
>
> Best regards,
> Kern
>
> On 07/27/2012 02:33 AM, Bastian Friedrich wrote:
> > Hi,
> >
> > Am Dienstag, 24. Juli 2012, 15:28:54 schrieb Bastian Friedrich:
> >> we are using Bacula's raw block device backup features for backing up
> >> virtual machines that use raw devices (logical volumes, or DRBD) as their
> >> storage. As advised by the bacula documentation (obviously, for the purpose
> >> of sparing storage space), we set "sparse = yes" for the respective file
> >> sets.
> >>
> >> Recently, we discovered a problem with systems backed up in this way.
> >>
> >> Bacula does not only back up sparse data; it also restores the data in such
> >> a way, which can result in problems. Imagine a virtual machine that has a
> >> (non- sparse!) file with large chunks of zero data in its file system. When
> >> these data are backed up, these null data are stored as a sparse stream.
> >>
> >> During restore, however, this can result in a problem: The sparse data (all
> >> zeros) are not written to the disk; instead, the respective chunk is just
> >> skipped (via seek). As a result, the virtual machine now has random data in
> >> its (formerly all-zero) file.
> > the attached patch modifies Bacula's behavior during restore of data that
> > originates from a block device with the "sparse" option set. Instead of
> > simply
> > seeking to the requested file position, zero bytes are written to the
> > file/device in chunks of 64k.
> >
> > This results in sparse blocks (i.e., blocks that only contain zeros) on the
> > original disk being reproduced identically to the original data during
> > restore
> > to a new block device (of identical size). Restoring to a file ("image")
> > results in it having the correct size, and of course identical data (e.g.,
> > identical md5sums).
> >
> > On the downside, restored data of block devices always require the full
> > amount
> > of space on disk, even if they contain sparse blocks.
> >
> > Comments welcome. The patch has NOT yet been tested to full extend; I'd
> > welcome any feedback.
> >
> > Thx
> > Bastian
> >
> >
> >
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> >
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>
>
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