Documentation/admin-guide/device-mapper/vdo.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/device-mapper/vdo.rst

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Documentation/admin-guide/device-mapper/vdo.rst
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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.. SPDX-License-Identifier: GPL-2.0-only

======
dm-vdo
======

The dm-vdo (virtual data optimizer) device mapper target provides
block-level deduplication, compression, and thin provisioning. As a device
mapper target, it can add these features to the storage stack, compatible
with any file system. The vdo target does not protect against data
corruption, relying instead on integrity protection of the storage below
it. It is strongly recommended that lvm be used to manage vdo volumes. See
lvmvdo(7).

Userspace component
===================

Formatting a vdo volume requires the use of the 'vdoformat' tool, available
at:

https://github.com/dm-vdo/vdo/

In most cases, a vdo target will recover from a crash automatically the
next time it is started. In cases where it encountered an unrecoverable
error (either during normal operation or crash recovery) the target will
enter or come up in read-only mode. Because read-only mode is indicative of
data-loss, a positive action must be taken to bring vdo out of read-only
mode. The 'vdoforcerebuild' tool, available from the same repo, is used to
prepare a read-only vdo to exit read-only mode. After running this tool,
the vdo target will rebuild its metadata the next time it is
started. Although some data may be lost, the rebuilt vdo's metadata will be
internally consistent and the target will be writable again.

The repo also contains additional userspace tools which can be used to
inspect a vdo target's on-disk metadata. Fortunately, these tools are
rarely needed except by dm-vdo developers.

Metadata requirements
=====================

Each vdo volume reserves 3GB of space for metadata, or more depending on
its configuration. It is helpful to check that the space saved by
deduplication and compression is not cancelled out by the metadata
requirements. An estimation of the space saved for a specific dataset can
be computed with the vdo estimator tool, which is available at:

https://github.com/dm-vdo/vdoestimator/

Target interface
================

Table line
----------

::

	<offset> <logical device size> vdo V4 <storage device>
	<storage device size> <minimum I/O size> <block map cache size>
	<block map era length> [optional arguments]


Required parameters:

	offset:
		The offset, in sectors, at which the vdo volume's logical
		space begins.

	logical device size:
		The size of the device which the vdo volume will service,
		in sectors. Must match the current logical size of the vdo

Annotation

Implementation Notes