The Little Image Manager (LIM) is a package management system for OS images. It's intended for use with embedded Linux systems such as the Raspberry Pi, BeagleBoard, Pandaboard, etc. It allows you to easily save, archive, manage, and restore the disk images used on these devices. LIM has the advantage of a more targeted compression strategy than the typical dd/gzip combination used for storing such images. Due to this strategy LIM is able to produce images which can be significantly smaller than those produced by dd/gzip for many systems. (I've observed LIM images that are half the size of their dd/gzip equivalents, see 'How LIM Works' below for more detail.) Additionally, LIM can store meta-information about an image within its container format which can be useful for constructing image repositories and the like.

LIM's goal is to enable easier experimentation in building new OS images, porting OSes, writing custom kernels, and performing OS development, etc, without the requirement of managing dozens of SD cards.  LIM is intended to work with the microSD cards that often serve as the primary storage in embedded systems, but should work with anything that can present itself as a block device in a Linux system.

More info on LIM is at http://retrospectivelyobvious.org/lim
You can contact the author at bob.sherbert@gmail.com

To get started with LIM, first grab the source:

git clone git://github.com/retrospectivelyobvious/little-image-manager.git
cd little-image-manager

LIM makes use of a few common Linux utilities to get its job done, be sure you've satisfied the dependencies:

sudo apt-get install gzip coreutils binutils parted python-parted

Then install LIM by running setup.py:

sudo ./setup.py install

LIM's fundamental operation is to store and restore system images. To do this it requires you to communicate both source and destination for the operation. This is done through switches passed to the program. Generally speaking the letter used in the switch indicates what type of media is to be read from/written to, while the case of that switch indicates whether that option is communicating the source or destination. For example '-d /dev/sdX' indicates a device being used as the source for an operation while '-A somefile' indicates an archive file being used as a destination.

(NB: Unless you've been diligent about setting your device privileges, you'll probably need to be root to execute many of these commands. Treat LIM with the same respect you'd give dd/parted/fdisk/etc - it will ruin your day if you target the wrong device.)

Copy a disk image from an SD card to an archive:

lim -d /dev/sdX -A somefile.lim

Copy a disk image from an SD card to an archive (don't prompt for meta-info):

lim -M -d /dev/sdX -A somefile.lim

Copy a disk image from an SD card to an archive (use bzip2 compression):

lim -z bzip -M -d /dev/sdX -A somefile.lim

Restore a disk image from an archive to an SD card:

lim -a somefile.lim -D /dev/sdX

Restore a disk image from a url to an SD card:

lim -u someurl.lim -D /dev/sdX

When creating an archive, LIM will prompt you for some descriptive meta data about the disk image. Providing the information is optional, but it is useful for organizational purposes or if your image finds its way into a repository. To see a description of the types of meta data and suggested formats/conventions when providing them, see the section "The 'meta' file" at the end of this document.

When restoring an image, LIM can attempt to resize the image's partitions to fit the destination disk if the destination's size differs from that of the source image. It can both expand partitions and shrink them. Shrinking is obviously limited by the relative sizes of the data and the requested partition size (sorry, no magic here). You can also change the file system type when restoring the image.

Specifying a partition size/type is done with the following switch:

-pN:start:len:fstype

• N - the partition number to alter
• start - the location you want the partition to begin (in sectors, from start of disk)
• len - the size of the partition (in sectors)
• fstype - the file system type (ext2,ext3,ext4,fat16,fat32)

You can use the programs 'parted' or 'fdisk' to get information about the number of available sectors.

To get information about the geometry of the image's source disk, you can print out the meta file by using the command 'ar p somefile.lim meta' from the shell.

Example:
(a 1GB partition, starting at sector 2048, of type ext2)

lim -a somefile.lim -D /dev/sdX -p1:2048:2097152:ext2

CAUTION:
Be aware that if you change a partition's end boundary you will need to adjust the location of each subsequent partition's start so as not to create an overlap. Also be aware that changing partition types may interfere with the operation of certain boot loaders.

When creating archives from a physical disk, LIM's default behavior is to tarball and compress the contents of each partition while storing the metadata (partition size, filesystem type, etc) that will eventually be needed to reconstruct it. Generally this results in much better compression performance, but occasionally it doesn't work (e.g. if you're using a file system LIM doesn't understand, doing raw writes to the disk for whatever reason, etc). In these circumstances, you can tell LIM to store a particular partition as a data block (i.e. use 'dd' to store that partition's data).

lim -PN:block

where 'N' is the partition number

Make an archive named 'somefile.lim' out of sdd, where partition 1 is stored using 'dd' and other partitions are stored as tarballs:

lim -d /dev/sdd -P1:block -A somefile.lim

By default, LIM uses gzip to compress the tarballs or block files that contain the partition data. If desired, you can tell LIM to use a different compression program. (bzip, xz, gzip)

Specify alternation compression program:

lim -z bzip -d /dev/sdX -A somefile.lim

LIM takes its inspiration mostly from the Debian package management tools and their 'deb' format. LIM archives are simple 'ar' archives that contain a series of compressed partition images and a meta info file that describes each of those partitions (sizes of partitions, types of file systems, the MBR, etc). Each of the partitions can be stored either as a compressed tarball of the file system contents (default) or as a compressed block file read by 'dd'. The MBR and the interstitial space between the end of the MBR and the start of the first partition (which is sometimes used for bootloader code) are stored as compressed blocks.

Currently, LIM only supports devices with old-style DOS partition tables (pretty much all embedded systems use these), and only supports primary partitions (sorry, no logical or extended partition support yet - soon).

The reason that LIM can often outperform dd/gzip, and the reason that the gains are sometimes much better than others, is that LIM is storing only the files on the device as opposed to a full system image. When a full system image is stored (as with dd), all of the information between files as well as all of the 'dead-files' are picked up in the read. (Remember, most file systems don't zero out deleted files, they only delete the associated bookkeeping information.) This means that the more heavily used the file system was before the read, the more entropy exists in the dead space of the file system, and the more will be picked up by dd. This bloats the compressed image needlessly, as gzip will preserve the worthless information.

The following fields are user specified during image creation:

• name - A short name for the image. This should be one word (no spaces), be all lower case, and contain only alpha-numeric characters (no symbols)

• description - Up to a paragraph of descriptive text explaining what the image is

• url - A reference url where one can go for more information on the image, an author's homepage, etc. (This should not be the exact URL the image was retrieved from.)

• img_ver - A version number for the image itself. This is chosen at the author's discretion.

The remaining fields in the meta file are not user specified, they are based on the version of LIM used to create the image, and the settings given to it during creation:

• format_ver - The LIM archive format that the image matches. This is assigned by LIM when it creates the archive. It is used to ensure that different versions of LIM can deal appropriately with the archive's feature set.

• compression - The compression program used to store the partition tarball/blocks.

You can view the meta file for an archive using the 'ar' utility:

ar p archive.lim meta