Suggestion: bmap files and bmaptool

[Till Maas]

On Wed, Aug 14, 2013 at 09:31:22AM +0300, Artem Bityutskiy wrote:

> Other things like reading from remote sites, progress indicator,
> protecting your mounted disks, uncompressing on-the-fly, checking sha1
> of the data ond of the bmap file itself - are goodies, although
> important ones.

Why sha1? If the check is there for security reasons, please use at
least sha256. You can also encode the checksum as base64, base85 or
base91 to reduce the size of the bmap file.

> But the base principle is to utilize the inherent sparseness most raw
> images have a lot of, record this in the bmap file before it is lost,
> then publish the image in any form (compressed or not), and use the bmap
> file for fetching the sparseness information and writing/copying only
> the real data, and leaving out the zeroes.

This does not sound safe, because it does not ensure that all data that
should be zero actually is a zero. It works well for unassigned file
systems blocks, but if there is a file containing zeroes in the file
system (that is not a sparse file) it might not contains zeroes
afterwards as far as I understand bmap. This does not sound like
something that is safe to do.

Regards
Till