The better
dd
for embedded projects, based on block maps.
bmaptool
is a generic tool for creating the block map (bmap) for a file and
copying files using the block map. The idea is that large files, like raw
system image files, can be copied or flashed a lot faster and more reliably
with bmaptool
than with traditional tools, like dd
or cp
.
bmaptool
was originally created for the "Tizen IVI" project and it was used for
flashing system images to USB sticks and other block devices. bmaptool
can also
be used for general image flashing purposes, for example, flashing Fedora Linux
OS distribution images to USB sticks.
Originally Tizen IVI images had been flashed using the dd
tool, but bmaptool
brought a number of advantages.
- Faster. Depending on various factors, like write speed, image size, how full
is the image, and so on,
bmaptool
was 5-7 times faster thandd
in the Tizen IVI project. - Integrity.
bmaptool
verifies data integrity while flashing, which means that possible data corruptions will be noticed immediately. - Usability.
bmaptool
can read images directly from the remote server, so users do not have to download images and save them locally. - Protects user's data. Unlike
dd
, if you make a mistake and specify a wrong block device name,bmaptool
will less likely destroy your data because it has protection mechanisms which, for example, preventbmaptool
from writing to a mounted block device.
bmaptool
supports 2 subcommands:
copy
- copy a file to another file using bmap or flash an image to a block devicecreate
- create a bmap for a file
You can get usage reference for bmaptool
and all the supported command using
the -h
or --help
options:
$ bmaptool -h # General bmaptool help
$ bmaptool <cmd> -h # Help on the <cmd> sub-command
You can also refer to the bmaptool
manual page:
$ man bmaptool
This section provides general information about the block map (bmap) necessary
for understanding how bmaptool
works. The structure of the section is:
- "Sparse files" - the bmap ideas are based on sparse files, so it is important to understand what sparse files are.
- "The block map" - explains what bmap is.
- "Raw images" - the main usage scenario for
bmaptool
is flashing raw images, which this section discusses. - "Usage scenarios" - describes various possible bmap and
bmaptool
usage scenarios.
One of the main roles of a filesystem, generally speaking, is to map blocks of file data to disk sectors. Different file-systems do this mapping differently, and filesystem performance largely depends on how well the filesystem can do the mapping. The filesystem block size is usually 4KiB, but may also be 8KiB or larger.
Obviously, to implement the mapping, the file-system has to maintain some kind of on-disk index. For any file on the file-system, and any offset within the file, the index allows you to find the corresponding disk sector, which stores the file's data. Whenever we write to a file, the filesystem looks up the index and writes to the corresponding disk sectors. Sometimes the filesystem has to allocate new disk sectors and update the index (such as when appending data to the file). The filesystem index is sometimes referred to as the "filesystem metadata".
What happens if a file area is not mapped to any disk sectors? Is this possible? The answer is yes. It is possible and these unmapped areas are often called "holes". And those files which have holes are often called "sparse files".
All reasonable file-systems like Linux ext[234], btrfs, XFS, or Solaris XFS, and even Windows' NTFS, support sparse files. Old and less reasonable filesystems, like FAT, do not support holes.
Reading holes returns zeroes. Writing to a hole causes the filesystem to allocate disk sectors for the corresponding blocks. Here is how you can create a 4GiB file with all blocks unmapped, which means that the file consists of a huge 4GiB hole:
$ truncate -s 4G image.raw
$ stat image.raw
File: image.raw
Size: 4294967296 Blocks: 0 IO Block: 4096 regular file
Notice that image.raw
is a 4GiB file, which occupies 0 blocks on the disk!
So, the entire file's contents are not mapped anywhere. Reading this file would
result in reading 4GiB of zeroes. If you write to the middle of the image.raw
file, you'll end up with 2 holes and a mapped area in the middle.
Therefore:
- Sparse files are files with holes.
- Sparse files help save disk space, because, roughly speaking, holes do not occupy disk space.
- A hole is an unmapped area of a file, meaning that it is not mapped anywhere on the disk.
- Reading data from a hole returns zeroes.
- Writing data to a hole destroys it by forcing the filesystem to map corresponding file areas to disk sectors.
- Filesystems usually operate with blocks, so sizes and offsets of holes are aligned to the block boundary.
It is also useful to know that you should work with sparse files carefully. It
is easy to accidentally expand a sparse file, that is, to map all holes to
zero-filled disk areas. For example, scp
always expands sparse files, the
tar
and rsync
tools do the same, by default, unless you use the --sparse
option. Compressing and then decompressing a sparse file usually expands it.
There are 2 ioctl's in Linux which allow you to find mapped and unmapped areas:
FIBMAP
and FIEMAP
. The former is very old and is probably supported by all
Linux systems, but it is rather limited and requires root privileges. The
latter is a lot more advanced and does not require root privileges, but it is
relatively new (added in Linux kernel, version 2.6.28).
Recent versions of the Linux kernel (starting from 3.1) also support the
SEEK_HOLE
and SEEK_DATA
values for the whence
argument of the standard
lseek()
system call. They allow positioning to the next hole and the next
mapped area of the file.
Advanced Linux filesystems, in modern kernels, also allow "punching holes",
meaning that it is possible to unmap any aligned area and turn it into a hole.
This is implemented using the FALLOC_FL_PUNCH_HOLE
mode
of the
fallocate()
system call.
The bmap is an XML file, which contains a list of mapped areas, plus some additional information about the file it was created for, for example:
- SHA256 checksum of the bmap file itself
- SHA256 checksum of the mapped areas
- the original file size
- amount of mapped data
The bmap file is designed to be both easily machine-readable and human-readable. All the machine-readable information is provided by XML tags. The human-oriented information is in XML comments, which explain the meaning of XML tags and provide useful information like amount of mapped data in percent and in MiB or GiB.
So, the best way to understand bmap is to just to read it. Here is an example of a bmap file.
Raw images are the simplest type of system images which may be flashed to the
target block device, block-by-block, without any further processing. Raw images
just "mirror" the target block device: they usually start with the MBR sector.
There is a partition table at the beginning of the image and one or more
partitions containing filesystems, like ext4. Usually, no special tools are
required to flash a raw image to the target block device. The standard dd
command can do the job:
$ dd if=tizen-ivi-image.raw of=/dev/usb_stick
At first glance, raw images do not look very appealing because they are large and it takes a lot of time to flash them. However, with bmap, raw images become a much more attractive type of image. We will demonstrate this, using Tizen IVI as an example.
The Tizen IVI project uses raw images which take 3.7GiB in Tizen IVI 2.0 alpha. The images are created by the MIC tool. Here is a brief description of how MIC creates them:
- create a 3.7GiB sparse file, which will become the Tizen IVI image in the end
- partition the file using the
parted
tool - format the partitions using the
mkfs.ext4
tool - loop-back mount all the partitions
- install all the required packages to the partitions: copy all the needed files and do all the tweaks
- unmount all loop-back-mounted image partitions, the image is ready
- generate the block map file for the image
- compress the image using
bzip2
, turning them into a small file, around 300MiB
The Tizen IVI raw images are initially sparse files. All the mapped blocks represent useful data and all the holes represent unused regions, which "contain" zeroes and do not have to be copied when flashing the image. Although information about holes is lost once the image gets compressed, the bmap file still has it and it can be used to reconstruct the uncompressed image or to flash the image quickly, by copying only the mapped regions.
Raw images compress extremely well because the holes are essentially zeroes,
which compress perfectly. This is why 3.7GiB Tizen IVI raw images, which
contain about 1.1GiB of mapped blocks, take only 300MiB in a compressed form.
And the important point is that you need to decompress them only while
flashing. The bmaptool
does this "on-the-fly".
Therefore:
- raw images are distributed in a compressed form, and they are almost as small as a tarball (that includes all the data the image would take)
- the bmap file and the
bmaptool
make it possible to quickly flash the compressed raw image to the target block device - optionally, the
bmaptool
can reconstruct the original uncompressed sparse raw image file
And, what is even more important, is that flashing raw images is extremely fast because you write directly to the block device, and write sequentially.
Another great thing about raw images is that they may be 100% ready-to-go and all you need to do is to put the image on your device "as-is". You do not have to know the image format, which partitions and filesystems it contains, etc. This is simple and robust.
Flashing or copying large images is the main bmaptool
use case. The idea is
that if you have a raw image file and its bmap, you can flash it to a device by
writing only the mapped blocks and skipping the unmapped blocks.
What this basically means is that with bmap it is not necessary to try to
minimize the raw image size by making the partitions small, which would require
resizing them. The image can contain huge multi-gigabyte partitions, just like
the target device requires. The image will then be a huge sparse file, with
little mapped data. And because unmapped areas "contain" zeroes, the huge image
will compress extremely well, so the huge image will be very small in
compressed form. It can then be distributed in compressed form, and flashed
very quickly with bmaptool
and the bmap file, because bmaptool
will decompress
the image on-the-fly and write only mapped areas.
The additional benefit of using bmap for flashing is the checksum verification.
Indeed, the bmaptool create
command generates SHA256 checksums for all mapped
block ranges, and the bmaptool copy
command verifies the checksums while
writing. Integrity of the bmap file itself is also protected by a SHA256
checksum and bmaptool
verifies it before starting flashing.
On top of this, the bmap file can be signed using OpenPGP (gpg) and bmaptool automatically verifies the signature if it is present. This allows for verifying the bmap file integrity and authoring. And since the bmap file contains SHA256 checksums for all the mapped image data, the bmap file signature verification should be enough to guarantee integrity and authoring of the image file.
The second usage scenario is reconstructing sparse files Generally speaking, if you had a sparse file but then expanded it, there is no way to reconstruct it. In some cases, something like:
$ cp --sparse=always expanded.file reconstructed.file
would be enough. However, a file reconstructed this way will not necessarily be the same as the original sparse file. The original sparse file could have contained mapped blocks filled with all zeroes (not holes), and, in the reconstructed file, these blocks will become holes. In some cases, this does not matter. For example, if you just want to save disk space. However, for raw images, flashing it does matter, because it is essential to write zero-filled blocks and not skip them. Indeed, if you do not write the zero-filled block to corresponding disk sectors which, presumably, contain garbage, you end up with garbage in those blocks. In other words, when we are talking about flashing raw images, the difference between zero-filled blocks and holes in the original image is essential because zero-filled blocks are the required blocks which are expected to contain zeroes, while holes are just unneeded blocks with no expectations regarding the contents.
bmaptool
may be helpful for reconstructing sparse files properly. Before the
sparse file is expanded, you should generate its bmap (for example, by using
the bmaptool create
command). Then you may compress your file or, otherwise,
expand it. Later on, you may reconstruct it using the bmaptool copy
command.
If running on the ZFS file system, the Linux ZFS kernel driver parameters configuration can cause the finding of mapped and unmapped areas to fail. This can be fixed temporarily by doing the following:
$ echo 1 | sudo tee -a /sys/module/zfs/parameters/zfs_dmu_offset_next_sync
However, if a permanent solution is required then perform the following:
$ echo "options zfs zfs_dmu_offset_next_sync=1" | sudo tee -a /etc/modprobe.d/zfs.conf
Depending upon your Linux distro, you may also need to do the following to ensure that the permanent change is updated in all your initramfs images:
$ sudo update-initramfs -u -k all
To verify the temporary or permanent change has worked you can use the following
which should return 1
:
$ cat /sys/module/zfs/parameters/zfs_dmu_offset_next_sync
More details can be found in the OpenZFS documentation.
bmaptool
uses hatch
to build python packages. If you would like to make
changes to the project, first create a new virtual environment and activate it:
python3 -m venv .venv
. .venv/bin/activate
Next install the project in editable mode with development dependencies:
pip install -m .[dev]
Finally, to run tests use unittest
:
python3 -m unittest -bv
The bmaptool project implements bmap-related tools and API modules. The entire project is written in python.
The project author is Artem Bityutskiy ([email protected]). The project is currently maintained by:
- Trevor Woerner ([email protected])
- Joshua Watt ([email protected])
- Tim Orling ([email protected])
Project git repository is here: https://github.com/yoctoproject/bmaptool
- Ed Bartosh ([email protected]) for helping me with learning python (this is my first python project) and working with the Tizen IVI infrastructure. Ed also implemented the packaging.
- Alexander Kanevskiy ([email protected]) and Kevin Wang ([email protected]) for helping with integrating this stuff to the Tizen IVI infrastructure.
- Simon McVittie ([email protected]) for improving Debian packaging and fixing bmaptool.