Disk Image Tools

An assortment of tools for producing disk images.

These tools are designed to work on both real disk device nodes and on sparse files.

gptimage

The gptimage tool produces GPT- or DOS-formatted disk images. It is designed to be used with the execline scripting language.

Usage:

gptimage [-d] [-a align] [-s size] [-u label] [-b base] { partitions ... } prog ...

Command line arguments:

• -d: use DOS instead of the default partitioning scheme (GPT)
• -a bits: use bits as the alignment for partitions (default 20, or 1M)
• -s size: use size as the size of the disk; the default is to make the image as small as possible while still preserving alignment
• -u label: use label as the disk lable; either a 4-byte hex number for DOS or a GPT UUID for GPT
• -b base: use base as the lowest available offset for partitions

The { partitions ... } spec are pairs of content-plus-type indicators. The "content" can be specified by a file name, or it can be the character '+' plus a size to indicate an empty partition of a particular size. As a special case, "content" can be * when specifying the last partition, which indicates that the partition should consume the rest of the image. The "type" can be the literal characters L or U, which mean Linux filesystem data and EFI System Partition, respectively, or it can be a literal GPT partition type UUID.

For example:

#!/bin/execlineb -P

# create a DOS (-d) disk0.img of size 16G (-s 16G)
# with a disk label of 0xdeadbeef
gptimage -d -s 16G -u 0xdeadbeef disk0.img {
	 +32M       L # p1: empty, 32M in size, Linux filesystem partition type
	 rootfs.img L # p2: contents should come from rootfs.img
	 *          L # p3: rest of the disk, no contents
}

# create disk1.img, sized automatically, using GPT, with
# a UEFI ESP and an ordinary Linux filesystem partition
gptimage disk1.img {
	 efi.img  U # p1: UEFI EFI system partition, using efi.img
	 root.img L # p2: Linux filesystem data, from root.img
}

Note that GPT disks will be formatted with a protective MBR, so tools that only recognize DOS partitions will see the disk as containing a single partition of type 0xee.

dosextend and gptextend

The dosextend and gptextend tools append partitions to their respective partitioning schemes.

These help simplify deployment of variable-sized disk images (often in virtualized environments) where the size of the target disk isn't known in advance.

For example, if you use gptimage to produce a 128M image with two partitions, and then run truncate -s 1G my.img && ./gptextend my.img, the disk will now have three partitions with the third partition consuming the remaining disk space. (The distill project uses this trick for creating the /var partition in virtualized environments.)

Usage:

gptextend [-n num] [-k] <file-or-disk>
dosextend [-n num] [-k] <file-or-disk>

Command line options (for both tools):

• -n num: Expect to create partition num, and fail otherwise.
• -k: Inform the kernel of the new partition. This is only useful if the target is actually a disk device node. You likely don't need this option unless one of the paritions on this disk is already mounted.

alignsize

The alignsize tool prints file sizes in various units and alignments.

Command line options:

• -a align: align the result to 2^align bits
• -s sector: right-shift the result by sector
• -e addend: add addend to the result before alignment

Usage:

alignsize [-a align] [-s sector] [-e addend] files...

For example:

# print the size of 'foo.img' rounded up to mebibytes in 512-byte sectors:
$ alignsize -a20 -s9 foo.img
# print the size of 'foo.img' in mebibytes, rounded up to the nearest mebibyte:
$ alignsize -a20 -s20 foo.img
# round up the sum of the sizes of a bunch of files to mebibytes:
$ find . | xargs alignsize -a20