Am Montag, 30. Juli 2012, 11:31:32 schrieb Martin Simmons:
> 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.
That would still result in broken file images when restoring a raw device
backup to an image.
Restoring a "hole" at the end of such image files would solve the problem as
well; I don't know how to do that, however. Additionally, knowing the restore
target type is not as simple as requesting the stream type (and, thus, the
backup type).
> >>>>> 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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