Merge "Spec for changing the block device handling: partitioning"
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doc/source/specs/v1/approved/block-device-lvl1-partitioning.rst
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doc/source/specs/v1/approved/block-device-lvl1-partitioning.rst
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..
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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========================================
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Block Device Setup Level 1: Partitioning
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========================================
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During the creation of a disk image (e.g. for a VM), there is the need
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to create, setup, configure and afterwards detach some kind of storage
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where the newly installed OS can be copied to or directly installed
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in.
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Remark
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------
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The implementation for this proposed changed already exists, was
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discussed and is currently waiting for reviews [1]. To have a
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complete overview over the block device setup, this document is
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provided.
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The dependencies are not implemented as they should be, because
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* the spec process is currently in the phase of discussion and not
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finalized [2],
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* the implementation was finished and reviewed before the spec process
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was described. [1]
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Problem description
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===================
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When setting up a block device there is the need to partitioning the
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block device.
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Use Cases
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---------
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User (Actor: End User) wants to create multiple partitions in multiple
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block devices where the new system is installed in.
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The user wants to specify if the image should be optimized for speed
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or for size.
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The user wants the same behavior independently of the current host or
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target OS.
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Proposed change
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===============
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Move the partitioning functionality from
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`elements/vm/block-device.d/10-partition` to a new block_device
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python module: `level1/partitioning.py`.
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Instead of using a program or a library, the data is written directly
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with the help of python `file.write()` into the disk image.
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Alternatives
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------------
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The existing implementation uses the `parted` program (old versions of
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DIB were using `sfdisk`). The first implementations of this change
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used the python-parted library.
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All these approaches have a major drawback: they automatically
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*optimize* based on information collected on the host system - and not
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of the target system. Therefore the resulting partitioning layout may
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lead to a degradation of performance on the target system. A change
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in these external programs and libraries also lead to errors during a
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DIB run [4] or there are general issues [7].
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Also everything build around GNU parted falls under the GPL2 (not
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LGPL2) license - which is incompatible with the currently used Apache
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license in diskimage-builder.
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API impact
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----------
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Extends the (optional) environment variable
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``DIB_BLOCK_DEVICE_CONFIG``: a JSON structure to configure the
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(complete) block device setup. For this proposal the second entry in
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the original list will be used (the first part (as described in [5])
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is used by the level 0 modules).
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The name of this module is `partitioning` (element[0]). The value
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(element[1]) is a dictionary.
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For each disk that should be partitioned there exists one entry in the
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dictionary. The key is the name of the disk (see [5] how to specify
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names for block device level 0). The value is a dictionary that
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defines the partitioning of each disk.
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There are the following key / value pairs to define one disk:
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label
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(mandatory) Possible values: 'mbr'
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This uses the Master Boot Record (MBR) layout for the disk.
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(Later on this can be extended, e.g. using GPT).
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align
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(optional - default value '1MiB')
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Set the alignment of the partition. This must be a multiple of the
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block size (i.e. 512 bytes). The default of 1MiB (~ 2048 * 512
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bytes blocks) is the default for modern systems and known to
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perform well on a wide range of targets [6]. For each partition
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there might be some space that is not used - which is `align` - 512
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bytes. For the default of 1MiB exactly 1048064 bytes (= 1 MiB -
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512 byte) are not used in the partition itself. Please note that
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if a boot loader should be written to the disk or partition,
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there is a need for some space. E.g. grub needs 63 * 512 byte
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blocks between the MBR and the start of the partition data; this
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means when grub will be installed, the `align` must be set at least
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to 64 * 512 byte = 32 KiB.
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partitions
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(mandatory) A list of dictionaries. Each dictionary describes one
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partition.
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The following key / value pairs can be given for each partition:
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name
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(mandatory) The name of the partition. With the help of this name,
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the partition can later be referenced, e.g. while creating a
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file system.
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flags
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(optional) List of flags for the partition. Default: empty.
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Possible values:
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boot
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Sets the boot flag for the partition
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size
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(mandatory) The size of the partition. The size can either be an
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absolute number using units like `10GiB` or `1.75TB` or relative
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(percentage) numbers: in the later case the size is calculated
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based on the remaining free space.
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Example:
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::
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["partitioning",
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{"rootdisk": {
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"label": "mbr",
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"partitions":
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[{"name": "part-01",
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"flags": ["boot"],
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"size": "100%"}]}}]
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Security impact
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---------------
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None - functionality stays the same.
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Other end user impact
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---------------------
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None.
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Performance Impact
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------------------
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Measurements showed there is a performance degradation for the target
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system of the partition table is not correctly aligned: writing takes
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about three times longer on an incorrect aligned system vs. one that
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is correctly aligned.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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ansreas (andreas@florath.net)
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Work Items
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----------
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None - this is already a small part of a bigger change [1].
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Dependencies
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============
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None.
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Testing
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=======
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The refactoring introduces no new test cases: the functionality is
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tested during each existing test building VM images.
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Documentation Impact
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====================
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End user: the additional environment variable is described.
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References
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==========
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[1] Refactor: block-device handling (partitioning)
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https://review.openstack.org/322671
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[2] Add specs dir
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https://review.openstack.org/336109
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[3] Old implementation using parted-lib
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https://review.openstack.org/#/c/322671/1..7/elements/block-device/pylib/block-device/level1/Partitioning.py
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[4] ERROR: embedding is not possible, but this is required
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for cross-disk install
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http://lists.openstack.org/pipermail/openstack-dev/2016-June/097789.html
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[5] Refactor: block-device handling (local loop)
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https://review.openstack.org/319591
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[6] Proper alignment of partitions on an Advanced Format HDD using Parted
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http://askubuntu.com/questions/201164/proper-alignment-of-partitions-on-an-advanced-format-hdd-using-parted
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[7] Red Hat Enterprise Linux 6 - Creating a 7TB Partition Using
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parted Always Shows "The resulting partition is not properly
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aligned for best performance"
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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
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[8] Spec for changing the block device handling
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https://review.openstack.org/336946
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