README.md

bmaptool-scan

Scan for unallocated blocks in a disk image, and export this information for further use by bmaptool, either by punching holes in the image or by generating a bmap file.

Usage

The script needs quite some Perl packages, which you can easily install using cpanm:

$ cpanm XML::Writer IPC::System::Simple Number::Bytes::Human Switch Clone

Afterwards, run the script on an image, selecting either --bmap or --sparse mode of operation:

# modprobe loop
# bmaptool-scan --bmap $FILENAME.img > $FILENAME.bmap

Running the script as root is necessary to losetup the image, which also needs to have the loop kernel module loaded.

If you're running the script through sudo while using local::lib, it is possible that the PERL5LIB variable is scrubbed and packages are not found. A workaround:

$ sudo PERL5LIB=$PERL5LIB bmaptool-scan ...

Motivation

See this blog post for an explanation. The short version: bmaptool create relies on file sparseness to generate a bmap file, but these semantics do not map what bmaptool uses them for (return 0 upon read vs. skip writing and return garbage upon read). This script parses the underlying file systems in order to discover the actually unused blocks, which can be safely skipped when writing an image.

TODO

• Add support for partition images (currently only full-disk images containing partitions are supported).
• Add support for more file systems.
• Detect funky partition tables (hybrid images, overlapping partitions), and abort.
• More robust way of parsing partition table (i.e. don't rely on fdisk output).