1. This work is licensed under a Creative Commons Attribution 3.0 Unported License.


Block Device Setup Level 1: Partitioning

During the creation of a disk image (e.g. for a VM), there is the need to create, setup, configure and afterwards detach some kind of storage where the newly installed OS can be copied to or directly installed in.


The implementation for this proposed changed already exists, was discussed and is currently waiting for reviews [1]. To have a complete overview over the block device setup, this document is provided.

The dependencies are not implemented as they should be, because

  • the spec process is currently in the phase of discussion and not finalized [2],

  • the implementation was finished and reviewed before the spec process was described. [1]

Problem description

When setting up a block device there is the need to partitioning the block device.

Use Cases

User (Actor: End User) wants to create multiple partitions in multiple block devices where the new system is installed in.

The user wants to specify if the image should be optimized for speed or for size.

The user wants the same behavior independently of the current host or target OS.

Proposed change

Move the partitioning functionality from elements/vm/block-device.d/10-partition to a new block_device python module: level1/partitioning.py.

Instead of using a program or a library, the data is written directly with the help of python file.write() into the disk image.


The existing implementation uses the parted program (old versions of DIB were using sfdisk). The first implementations of this change used the python-parted library.

All these approaches have a major drawback: they automatically optimize based on information collected on the host system - and not of the target system. Therefore the resulting partitioning layout may lead to a degradation of performance on the target system. A change in these external programs and libraries also lead to errors during a DIB run [4] or there are general issues [7].

Also everything build around GNU parted falls under the GPL2 (not LGPL2) license - which is incompatible with the currently used Apache license in diskimage-builder.

API impact

Extends the (optional) environment variable DIB_BLOCK_DEVICE_CONFIG: a JSON structure to configure the (complete) block device setup. For this proposal the second entry in the original list will be used (the first part (as described in [5]) is used by the level 0 modules).

The name of this module is partitioning (element[0]). The value (element[1]) is a dictionary.

For each disk that should be partitioned there exists one entry in the dictionary. The key is the name of the disk (see [5] how to specify names for block device level 0). The value is a dictionary that defines the partitioning of each disk.

There are the following key / value pairs to define one disk:


(mandatory) Possible values: ‘mbr’ This uses the Master Boot Record (MBR) layout for the disk. (Later on this can be extended, e.g. using GPT).


(optional - default value ‘1MiB’) Set the alignment of the partition. This must be a multiple of the block size (i.e. 512 bytes). The default of 1MiB (~ 2048 * 512 bytes blocks) is the default for modern systems and known to perform well on a wide range of targets [6]. For each partition there might be some space that is not used - which is align - 512 bytes. For the default of 1MiB exactly 1048064 bytes (= 1 MiB - 512 byte) are not used in the partition itself. Please note that if a boot loader should be written to the disk or partition, there is a need for some space. E.g. grub needs 63 * 512 byte blocks between the MBR and the start of the partition data; this means when grub will be installed, the align must be set at least to 64 * 512 byte = 32 KiB.


(mandatory) A list of dictionaries. Each dictionary describes one partition.

The following key / value pairs can be given for each partition:


(mandatory) The name of the partition. With the help of this name, the partition can later be referenced, e.g. while creating a file system.


(optional) List of flags for the partition. Default: empty. Possible values:


Sets the boot flag for the partition


(mandatory) The size of the partition. The size can either be an absolute number using units like 10GiB or 1.75TB or relative (percentage) numbers: in the later case the size is calculated based on the remaining free space.


   {"rootdisk": {
     "label": "mbr",
        [{"name": "part-01",
          "flags": ["boot"],
          "size": "100%"}]}}]

Security impact

None - functionality stays the same.

Other end user impact


Performance Impact

Measurements showed there is a performance degradation for the target system of the partition table is not correctly aligned: writing takes about three times longer on an incorrect aligned system vs. one that is correctly aligned.



Primary assignee:

ansreas (andreas@florath.net)

Work Items

None - this is already a small part of a bigger change [1].




The refactoring introduces no new test cases: the functionality is tested during each existing test building VM images.

Documentation Impact

End user: the additional environment variable is described.


[1] Refactor: block-device handling (partitioning)


[2] Add specs dir


[3] Old implementation using parted-lib


[4] ERROR: embedding is not possible, but this is required

for cross-disk install http://lists.openstack.org/pipermail/openstack-dev/2016-June/097789.html

[5] Refactor: block-device handling (local loop)


[6] Proper alignment of partitions on an Advanced Format HDD using Parted


[7] Red Hat Enterprise Linux 6 - Creating a 7TB Partition Using

parted Always Shows “The resulting partition is not properly aligned for best performance” http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=emr_na-c03479326&DocLang=en&docLocale=en_US&jumpid=reg_r11944_uken_c-001_title_r0001

[8] Spec for changing the block device handling