mirror of
https://github.com/ceph/ceph
synced 2024-12-19 01:46:00 +00:00
90 lines
4.2 KiB
ReStructuredText
90 lines
4.2 KiB
ReStructuredText
.. _ceph-volume-overview:
|
|
|
|
Overview
|
|
--------
|
|
The ``ceph-volume`` tool aims to be a single purpose command line tool to deploy
|
|
logical volumes as OSDs, trying to maintain a similar API to ``ceph-disk`` when
|
|
preparing, activating, and creating OSDs.
|
|
|
|
It deviates from ``ceph-disk`` by not interacting or relying on the udev rules
|
|
that come installed for Ceph. These rules allow automatic detection of
|
|
previously setup devices that are in turn fed into ``ceph-disk`` to activate
|
|
them.
|
|
|
|
.. _ceph-disk-replaced:
|
|
|
|
Replacing ``ceph-disk``
|
|
-----------------------
|
|
The ``ceph-disk`` tool was created at a time were the project was required to
|
|
support many different types of init systems (upstart, sysvinit, etc...) while
|
|
being able to discover devices. This caused the tool to concentrate initially
|
|
(and exclusively afterwards) on GPT partitions. Specifically on GPT GUIDs,
|
|
which were used to label devices in a unique way to answer questions like:
|
|
|
|
* is this device a Journal?
|
|
* an encrypted data partition?
|
|
* was the device left partially prepared?
|
|
|
|
To solve these, it used ``UDEV`` rules to match the GUIDs, that would call
|
|
``ceph-disk``, and end up in a back and forth between the ``ceph-disk`` systemd
|
|
unit and the ``ceph-disk`` executable. The process was very unreliable and time
|
|
consuming (a timeout of close to three hours **per OSD** had to be put in
|
|
place), and would cause OSDs to not come up at all during the boot process of
|
|
a node.
|
|
|
|
It was hard to debug, or even replicate these problems given the asynchronous
|
|
behavior of ``UDEV``.
|
|
|
|
Since the world-view of ``ceph-disk`` had to be GPT partitions exclusively, it meant
|
|
that it couldn't work with other technologies like LVM, or similar device
|
|
mapper devices. It was ultimately decided to create something modular, starting
|
|
with LVM support, and the ability to expand on other technologies as needed.
|
|
|
|
|
|
GPT partitions are simple?
|
|
--------------------------
|
|
Although partitions in general are simple to reason about, ``ceph-disk``
|
|
partitions were not simple by any means. It required a tremendous amount of
|
|
special flags in order to get them to work correctly with the device discovery
|
|
workflow. Here is an example call to create a data partition::
|
|
|
|
/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
|
|
|
|
Not only creating these was hard, but these partitions required devices to be
|
|
exclusively owned by Ceph. For example, in some cases a special partition would
|
|
be created when devices were encrypted, which would contain unencrypted keys.
|
|
This was ``ceph-disk`` domain knowledge, which would not translate to a "GPT
|
|
partitions are simple" understanding. Here is an example of that special
|
|
partition being created::
|
|
|
|
/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
|
|
|
|
|
|
Modularity
|
|
----------
|
|
``ceph-volume`` was designed to be a modular tool because we anticipate that
|
|
there are going to be lots of ways that people provision the hardware devices
|
|
that we need to consider. There are already two: legacy ceph-disk devices that
|
|
are still in use and have GPT partitions (handled by :ref:`ceph-volume-simple`),
|
|
and lvm. SPDK devices where we manage NVMe devices directly from userspace are
|
|
on the immediate horizon, where LVM won't work there since the kernel isn't
|
|
involved at all.
|
|
|
|
``ceph-volume lvm``
|
|
-------------------
|
|
By making use of :term:`LVM tags`, the :ref:`ceph-volume-lvm` sub-command is
|
|
able to store and later re-discover and query devices associated with OSDs so
|
|
that they can later activated. This includes support for lvm-based technologies
|
|
like dm-cache as well.
|
|
|
|
For ``ceph-volume``, the use of dm-cache is transparent, there is no difference
|
|
for the tool, and it treats dm-cache like a plain logical volume.
|
|
|
|
LVM performance penalty
|
|
-----------------------
|
|
In short: we haven't been able to notice any significant performance penalties
|
|
associated with the change to LVM. By being able to work closely with LVM, the
|
|
ability to work with other device mapper technologies (for example ``dmcache``)
|
|
was a given: there is no technical difficulty in working with anything that can
|
|
sit below a Logical Volume.
|