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90 lines
4.2 KiB
ReStructuredText
.. _ceph-volume-overview:
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Overview
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--------
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The ``ceph-volume`` tool aims to be a single purpose command line tool to deploy
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logical volumes as OSDs, trying to maintain a similar API to ``ceph-disk`` when
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preparing, activating, and creating OSDs.
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It deviates from ``ceph-disk`` by not interacting or relying on the udev rules
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that come installed for Ceph. These rules allow automatic detection of
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previously setup devices that are in turn fed into ``ceph-disk`` to activate
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them.
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.. _ceph-disk-replaced:
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Replacing ``ceph-disk``
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-----------------------
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The ``ceph-disk`` tool was created at a time were the project was required to
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support many different types of init systems (upstart, sysvinit, etc...) while
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being able to discover devices. This caused the tool to concentrate initially
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(and exclusively afterwards) on GPT partitions. Specifically on GPT GUIDs,
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which were used to label devices in a unique way to answer questions like:
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* is this device a Journal?
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* an encrypted data partition?
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* was the device left partially prepared?
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To solve these, it used ``UDEV`` rules to match the GUIDs, that would call
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``ceph-disk``, and end up in a back and forth between the ``ceph-disk`` systemd
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unit and the ``ceph-disk`` executable. The process was very unreliable and time
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consuming (a timeout of close to three hours **per OSD** had to be put in
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place), and would cause OSDs to not come up at all during the boot process of
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a node.
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It was hard to debug, or even replicate these problems given the asynchronous
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behavior of ``UDEV``.
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Since the world-view of ``ceph-disk`` had to be GPT partitions exclusively, it meant
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that it couldn't work with other technologies like LVM, or similar device
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mapper devices. It was ultimately decided to create something modular, starting
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with LVM support, and the ability to expand on other technologies as needed.
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GPT partitions are simple?
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--------------------------
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Although partitions in general are simple to reason about, ``ceph-disk``
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partitions were not simple by any means. It required a tremendous amount of
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special flags in order to get them to work correctly with the device discovery
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workflow. Here is an example call to create a data partition::
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/sbin/sgdisk --largest-new=1 --change-name=1:ceph data --partition-guid=1:f0fc39fd-eeb2-49f1-b922-a11939cf8a0f --typecode=1:89c57f98-2fe5-4dc0-89c1-f3ad0ceff2be --mbrtogpt -- /dev/sdb
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Not only creating these was hard, but these partitions required devices to be
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exclusively owned by Ceph. For example, in some cases a special partition would
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be created when devices were encrypted, which would contain unencrypted keys.
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This was ``ceph-disk`` domain knowledge, which would not translate to a "GPT
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partitions are simple" understanding. Here is an example of that special
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partition being created::
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/sbin/sgdisk --new=5:0:+10M --change-name=5:ceph lockbox --partition-guid=5:None --typecode=5:fb3aabf9-d25f-47cc-bf5e-721d181642be --mbrtogpt -- /dev/sdad
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Modularity
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----------
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``ceph-volume`` was designed to be a modular tool because we anticipate that
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there are going to be lots of ways that people provision the hardware devices
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that we need to consider. There are already two: legacy ceph-disk devices that
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are still in use and have GPT partitions (handled by :ref:`ceph-volume-simple`),
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and lvm. SPDK devices where we manage NVMe devices directly from userspace are
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on the immediate horizon, where LVM won't work there since the kernel isn't
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involved at all.
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``ceph-volume lvm``
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-------------------
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By making use of :term:`LVM tags`, the :ref:`ceph-volume-lvm` sub-command is
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able to store and later re-discover and query devices associated with OSDs so
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that they can later activated. This includes support for lvm-based technologies
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like dm-cache as well.
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For ``ceph-volume``, the use of dm-cache is transparent, there is no difference
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for the tool, and it treats dm-cache like a plain logical volume.
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LVM performance penalty
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-----------------------
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In short: we haven't been able to notice any significant performance penalties
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associated with the change to LVM. By being able to work closely with LVM, the
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ability to work with other device mapper technologies (for example ``dmcache``)
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was a given: there is no technical difficulty in working with anything that can
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sit below a Logical Volume.
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