ceph: section language fixup

[pve-docs.git] / pveceph.adoc

[[chapter_pveceph]]
ifdef::manvolnum[]
pveceph(1)
==========
:pve-toplevel:

NAME
----

pveceph - Manage Ceph Services on Proxmox VE Nodes

SYNOPSIS
--------

include::pveceph.1-synopsis.adoc[]

DESCRIPTION
-----------
endif::manvolnum[]
ifndef::manvolnum[]
Deploy Hyper-Converged Ceph Cluster
===================================
:pve-toplevel:
endif::manvolnum[]

[thumbnail="screenshot/gui-ceph-status.png"]

{pve} unifies your compute and storage systems, that is, you can use the same
physical nodes within a cluster for both computing (processing VMs and
containers) and replicated storage. The traditional silos of compute and
storage resources can be wrapped up into a single hyper-converged appliance.
Separate storage networks (SANs) and connections via network attached storage
(NAS) disappear. With the integration of Ceph, an open source software-defined
storage platform, {pve} has the ability to run and manage Ceph storage directly
on the hypervisor nodes.

Ceph is a distributed object store and file system designed to provide
excellent performance, reliability and scalability.

.Some advantages of Ceph on {pve} are:
- Easy setup and management via CLI and GUI
- Thin provisioning
- Snapshot support
- Self healing
- Scalable to the exabyte level
- Setup pools with different performance and redundancy characteristics
- Data is replicated, making it fault tolerant
- Runs on commodity hardware
- No need for hardware RAID controllers
- Open source

For small to medium-sized deployments, it is possible to install a Ceph server for
RADOS Block Devices (RBD) directly on your {pve} cluster nodes (see
xref:ceph_rados_block_devices[Ceph RADOS Block Devices (RBD)]). Recent
hardware has a lot of CPU power and RAM, so running storage services
and VMs on the same node is possible.

To simplify management, we provide 'pveceph' - a tool for installing and
managing {ceph} services on {pve} nodes.

.Ceph consists of multiple Daemons, for use as an RBD storage:
- Ceph Monitor (ceph-mon)
- Ceph Manager (ceph-mgr)
- Ceph OSD (ceph-osd; Object Storage Daemon)

TIP: We highly recommend to get familiar with Ceph
footnote:[Ceph intro {cephdocs-url}/start/intro/],
its architecture
footnote:[Ceph architecture {cephdocs-url}/architecture/]
and vocabulary
footnote:[Ceph glossary {cephdocs-url}/glossary].


Precondition
------------

To build a hyper-converged Proxmox + Ceph Cluster, you must use at least
three (preferably) identical servers for the setup.

Check also the recommendations from
{cephdocs-url}/start/hardware-recommendations/[Ceph's website].

.CPU
A high CPU core frequency reduces latency and should be preferred. As a simple
rule of thumb, you should assign a CPU core (or thread) to each Ceph service to
provide enough resources for stable and durable Ceph performance.

.Memory
Especially in a hyper-converged setup, the memory consumption needs to be
carefully monitored. In addition to the predicted memory usage of virtual
machines and containers, you must also account for having enough memory
available for Ceph to provide excellent and stable performance.

As a rule of thumb, for roughly **1 TiB of data, 1 GiB of memory** will be used
by an OSD. Especially during recovery, rebalancing or backfilling.

The daemon itself will use additional memory. The Bluestore backend of the
daemon requires by default **3-5 GiB of memory** (adjustable). In contrast, the
legacy Filestore backend uses the OS page cache and the memory consumption is
generally related to PGs of an OSD daemon.

.Network
We recommend a network bandwidth of at least 10 GbE or more, which is used
exclusively for Ceph. A meshed network setup
footnote:[Full Mesh Network for Ceph {webwiki-url}Full_Mesh_Network_for_Ceph_Server]
is also an option if there are no 10 GbE switches available.

The volume of traffic, especially during recovery, will interfere with other
services on the same network and may even break the {pve} cluster stack.

Furthermore, you should estimate your bandwidth needs. While one HDD might not
saturate a 1 Gb link, multiple HDD OSDs per node can, and modern NVMe SSDs will
even saturate 10 Gbps of bandwidth quickly. Deploying a network capable of even
more bandwidth will ensure that this isn't your bottleneck and won't be anytime
soon. 25, 40 or even 100 Gbps are possible.

.Disks
When planning the size of your Ceph cluster, it is important to take the
recovery time into consideration. Especially with small clusters, recovery
might take long. It is recommended that you use SSDs instead of HDDs in small
setups to reduce recovery time, minimizing the likelihood of a subsequent
failure event during recovery.

In general SSDs will provide more IOPs than spinning disks. With this in mind,
in addition to the higher cost, it may make sense to implement a
xref:pve_ceph_device_classes[class based] separation of pools. Another way to
speed up OSDs is to use a faster disk as a journal or
DB/**W**rite-**A**head-**L**og device, see xref:pve_ceph_osds[creating Ceph
OSDs]. If a faster disk is used for multiple OSDs, a proper balance between OSD
and WAL / DB (or journal) disk must be selected, otherwise the faster disk
becomes the bottleneck for all linked OSDs.

Aside from the disk type, Ceph performs best with an even sized and distributed
amount of disks per node. For example, 4 x 500 GB disks within each node is
better than a mixed setup with a single 1 TB and three 250 GB disk.

You also need to balance OSD count and single OSD capacity. More capacity
allows you to increase storage density, but it also means that a single OSD
failure forces Ceph to recover more data at once.

.Avoid RAID
As Ceph handles data object redundancy and multiple parallel writes to disks
(OSDs) on its own, using a RAID controller normally doesn’t improve
performance or availability. On the contrary, Ceph is designed to handle whole
disks on it's own, without any abstraction in between. RAID controllers are not
designed for the Ceph workload and may complicate things and sometimes even
reduce performance, as their write and caching algorithms may interfere with
the ones from Ceph.

WARNING: Avoid RAID controllers. Use host bus adapter (HBA) instead.

NOTE: The above recommendations should be seen as a rough guidance for choosing
hardware. Therefore, it is still essential to adapt it to your specific needs.
You should test your setup and monitor health and performance continuously.

[[pve_ceph_install_wizard]]
Initial Ceph Installation & Configuration
-----------------------------------------

[thumbnail="screenshot/gui-node-ceph-install.png"]

With {pve} you have the benefit of an easy to use installation wizard
for Ceph. Click on one of your cluster nodes and navigate to the Ceph
section in the menu tree. If Ceph is not already installed, you will see a
prompt offering to do so.

The wizard is divided into multiple sections, where each needs to
finish successfully, in order to use Ceph. After starting the installation,
the wizard will download and install all the required packages from {pve}'s Ceph
repository.

After finishing the first step, you will need to create a configuration.
This step is only needed once per cluster, as this configuration is distributed
automatically to all remaining cluster members through {pve}'s clustered
xref:chapter_pmxcfs[configuration file system (pmxcfs)].

The configuration step includes the following settings:

* *Public Network:* You can set up a dedicated network for Ceph. This
setting is required. Separating your Ceph traffic is highly recommended.
Otherwise, it could cause trouble with other latency dependent services,
for example, cluster communication may decrease Ceph's performance.

[thumbnail="screenshot/gui-node-ceph-install-wizard-step2.png"]

* *Cluster Network:* As an optional step, you can go even further and
separate the xref:pve_ceph_osds[OSD] replication & heartbeat traffic
as well. This will relieve the public network and could lead to
significant performance improvements, especially in large clusters.

You have two more options which are considered advanced and therefore
should only changed if you know what you are doing.

* *Number of replicas*: Defines how often an object is replicated
* *Minimum replicas*: Defines the minimum number of required replicas
for I/O to be marked as complete.

Additionally, you need to choose your first monitor node. This step is required.

That's it. You should now see a success page as the last step, with further
instructions on how to proceed. Your system is now ready to start using Ceph.
To get started, you will need to create some additional xref:pve_ceph_monitors[monitors],
xref:pve_ceph_osds[OSDs] and at least one xref:pve_ceph_pools[pool].

The rest of this chapter will guide you through getting the most out of
your {pve} based Ceph setup. This includes the aforementioned tips and
more, such as xref:pveceph_fs[CephFS], which is a helpful addition to your
new Ceph cluster.

[[pve_ceph_install]]
Installation of Ceph Packages
-----------------------------
Use the {pve} Ceph installation wizard (recommended) or run the following
command on each node:

[source,bash]
----
pveceph install
----

This sets up an `apt` package repository in
`/etc/apt/sources.list.d/ceph.list` and installs the required software.


Create initial Ceph configuration
---------------------------------

[thumbnail="screenshot/gui-ceph-config.png"]

Use the {pve} Ceph installation wizard (recommended) or run the
following command on one node:

[source,bash]
----
pveceph init --network 10.10.10.0/24
----

This creates an initial configuration at `/etc/pve/ceph.conf` with a
dedicated network for Ceph. This file is automatically distributed to
all {pve} nodes, using xref:chapter_pmxcfs[pmxcfs]. The command also
creates a symbolic link at `/etc/ceph/ceph.conf`, which points to that file.
Thus, you can simply run Ceph commands without the need to specify a
configuration file.


[[pve_ceph_monitors]]
Ceph Monitor
-----------
The Ceph Monitor (MON)
footnote:[Ceph Monitor {cephdocs-url}/start/intro/]
maintains a master copy of the cluster map. For high availability, you need at
least 3 monitors. One monitor will already be installed if you
used the installation wizard. You won't need more than 3 monitors, as long
as your cluster is small to medium-sized. Only really large clusters will
require more than this.


[[pveceph_create_mon]]
Create Monitors
~~~~~~~~~~~~~~~

[thumbnail="screenshot/gui-ceph-monitor.png"]

On each node where you want to place a monitor (three monitors are recommended),
create one by using the 'Ceph -> Monitor' tab in the GUI or run:


[source,bash]
----
pveceph mon create
----

[[pveceph_destroy_mon]]
Destroy Monitors
~~~~~~~~~~~~~~~~

To remove a Ceph Monitor via the GUI, first select a node in the tree view and
go to the **Ceph -> Monitor** panel. Select the MON and click the **Destroy**
button.

To remove a Ceph Monitor via the CLI, first connect to the node on which the MON
is running. Then execute the following command:
[source,bash]
----
pveceph mon destroy
----

NOTE: At least three Monitors are needed for quorum.


[[pve_ceph_manager]]
Ceph Manager
------------

The Manager daemon runs alongside the monitors. It provides an interface to
monitor the cluster. Since the release of Ceph luminous, at least one ceph-mgr
footnote:[Ceph Manager {cephdocs-url}/mgr/] daemon is
required.

[[pveceph_create_mgr]]
Create Manager
~~~~~~~~~~~~~~

Multiple Managers can be installed, but only one Manager is active at any given
time.

[source,bash]
----
pveceph mgr create
----

NOTE: It is recommended to install the Ceph Manager on the monitor nodes. For
high availability install more then one manager.


[[pveceph_destroy_mgr]]
Destroy Manager
~~~~~~~~~~~~~~~

To remove a Ceph Manager via the GUI, first select a node in the tree view and
go to the **Ceph -> Monitor** panel. Select the Manager and click the
**Destroy** button.

To remove a Ceph Monitor via the CLI, first connect to the node on which the
Manager is running. Then execute the following command:
[source,bash]
----
pveceph mgr destroy
----

NOTE: While a manager is not a hard-dependency, it is crucial for a Ceph cluster,
as it handles important features like PG-autoscaling, device health monitoring,
telemetry and more.

[[pve_ceph_osds]]
Ceph OSDs
---------
Ceph **O**bject **S**torage **D**aemons store objects for Ceph over the
network. It is recommended to use one OSD per physical disk.

NOTE: By default an object is 4 MiB in size.

[[pve_ceph_osd_create]]
Create OSDs
~~~~~~~~~~~

[thumbnail="screenshot/gui-ceph-osd-status.png"]

You can create an OSD either via the {pve} web-interface or via the CLI using
`pveceph`. For example:

[source,bash]
----
pveceph osd create /dev/sd[X]
----

TIP: We recommend a Ceph cluster with at least three nodes and at least 12
OSDs, evenly distributed among the nodes.

If the disk was in use before (for example, for ZFS or as an OSD) you first need
to zap all traces of that usage. To remove the partition table, boot sector and
any other OSD leftover, you can use the following command:

[source,bash]
----
ceph-volume lvm zap /dev/sd[X] --destroy
----

WARNING: The above command will destroy all data on the disk!

.Ceph Bluestore

Starting with the Ceph Kraken release, a new Ceph OSD storage type was
introduced called Bluestore
footnote:[Ceph Bluestore https://ceph.com/community/new-luminous-bluestore/].
This is the default when creating OSDs since Ceph Luminous.

[source,bash]
----
pveceph osd create /dev/sd[X]
----

.Block.db and block.wal

If you want to use a separate DB/WAL device for your OSDs, you can specify it
through the '-db_dev' and '-wal_dev' options. The WAL is placed with the DB, if
not specified separately.

[source,bash]
----
pveceph osd create /dev/sd[X] -db_dev /dev/sd[Y] -wal_dev /dev/sd[Z]
----

You can directly choose the size of those with the '-db_size' and '-wal_size'
parameters respectively. If they are not given, the following values (in order)
will be used:

* bluestore_block_{db,wal}_size from Ceph configuration...
** ... database, section 'osd'
** ... database, section 'global'
** ... file, section 'osd'
** ... file, section 'global'
* 10% (DB)/1% (WAL) of OSD size

NOTE: The DB stores BlueStore’s internal metadata, and the WAL is BlueStore’s
internal journal or write-ahead log. It is recommended to use a fast SSD or
NVRAM for better performance.


.Ceph Filestore

Before Ceph Luminous, Filestore was used as the default storage type for Ceph OSDs.
Starting with Ceph Nautilus, {pve} does not support creating such OSDs with
'pveceph' anymore. If you still want to create filestore OSDs, use
'ceph-volume' directly.

[source,bash]
----
ceph-volume lvm create --filestore --data /dev/sd[X] --journal /dev/sd[Y]
----

[[pve_ceph_osd_destroy]]
Destroy OSDs
~~~~~~~~~~~~

To remove an OSD via the GUI, first select a {PVE} node in the tree view and go
to the **Ceph -> OSD** panel. Then select the OSD to destroy and click the **OUT**
button. Once the OSD status has changed from `in` to `out`, click the **STOP**
button. Finally, after the status has changed from `up` to `down`, select
**Destroy** from the `More` drop-down menu.

To remove an OSD via the CLI run the following commands.

[source,bash]
----
ceph osd out <ID>
systemctl stop ceph-osd@<ID>.service
----

NOTE: The first command instructs Ceph not to include the OSD in the data
distribution. The second command stops the OSD service. Until this time, no
data is lost.

The following command destroys the OSD. Specify the '-cleanup' option to
additionally destroy the partition table.

[source,bash]
----
pveceph osd destroy <ID>
----

WARNING: The above command will destroy all data on the disk!


[[pve_ceph_pools]]
Ceph Pools
----------
A pool is a logical group for storing objects. It holds a collection of objects,
known as **P**lacement **G**roups (`PG`, `pg_num`).


Create and Edit Pools
~~~~~~~~~~~~~~~~~~~~~

You can create pools from the command line or the web-interface of any {pve}
host under **Ceph -> Pools**.

[thumbnail="screenshot/gui-ceph-pools.png"]

When no options are given, we set a default of **128 PGs**, a **size of 3
replicas** and a **min_size of 2 replicas**, to ensure no data loss occurs if
any OSD fails.

WARNING: **Do not set a min_size of 1**. A replicated pool with min_size of 1
allows I/O on an object when it has only 1 replica, which could lead to data
loss, incomplete PGs or unfound objects.

It is advised that you calculate the PG number based on your setup. You can
find the formula and the PG calculator footnote:[PG calculator
https://ceph.com/pgcalc/] online. From Ceph Nautilus onward, you can change the
number of PGs footnoteref:[placement_groups,Placement Groups
{cephdocs-url}/rados/operations/placement-groups/] after the setup.

In addition to manual adjustment, the PG autoscaler
footnoteref:[autoscaler,Automated Scaling
{cephdocs-url}/rados/operations/placement-groups/#automated-scaling] can
automatically scale the PG count for a pool in the background.

.Example for creating a pool over the CLI
[source,bash]
----
pveceph pool create <name> --add_storages
----

TIP: If you would also like to automatically define a storage for your
pool, keep the `Add as Storage' checkbox checked in the web-interface, or use the
command line option '--add_storages' at pool creation.

.Base Options
Name:: The name of the pool. This must be unique and can't be changed afterwards.
Size:: The number of replicas per object. Ceph always tries to have this many
copies of an object. Default: `3`.
PG Autoscale Mode:: The automatic PG scaling mode footnoteref:[autoscaler] of
the pool. If set to `warn`, it produces a warning message when a pool
has a non-optimal PG count. Default: `warn`.
Add as Storage:: Configure a VM or container storage using the new pool.
Default: `true` (only visible on creation).

.Advanced Options
Min. Size:: The minimum number of replicas per object. Ceph will reject I/O on
the pool if a PG has less than this many replicas. Default: `2`.
Crush Rule:: The rule to use for mapping object placement in the cluster. These
rules define how data is placed within the cluster. See
xref:pve_ceph_device_classes[Ceph CRUSH & device classes] for information on
device-based rules.
# of PGs:: The number of placement groups footnoteref:[placement_groups] that
the pool should have at the beginning. Default: `128`.
Target Size Ratio:: The ratio of data that is expected in the pool. The PG
autoscaler uses the ratio relative to other ratio sets. It takes precedence
over the `target size` if both are set.
Target Size:: The estimated amount of data expected in the pool. The PG
autoscaler uses this size to estimate the optimal PG count.
Min. # of PGs:: The minimum number of placement groups. This setting is used to
fine-tune the lower bound of the PG count for that pool. The PG autoscaler
will not merge PGs below this threshold.

Further information on Ceph pool handling can be found in the Ceph pool
operation footnote:[Ceph pool operation
{cephdocs-url}/rados/operations/pools/]
manual.


Destroy Pools
~~~~~~~~~~~~~

To destroy a pool via the GUI, select a node in the tree view and go to the
**Ceph -> Pools** panel. Select the pool to destroy and click the **Destroy**
button. To confirm the destruction of the pool, you need to enter the pool name.

Run the following command to destroy a pool. Specify the '-remove_storages' to
also remove the associated storage.

[source,bash]
----
pveceph pool destroy <name>
----

NOTE: Pool deletion runs in the background and can take some time.
You will notice the data usage in the cluster decreasing throughout this
process.


PG Autoscaler
~~~~~~~~~~~~~

The PG autoscaler allows the cluster to consider the amount of (expected) data
stored in each pool and to choose the appropriate pg_num values automatically.

You may need to activate the PG autoscaler module before adjustments can take
effect.

[source,bash]
----
ceph mgr module enable pg_autoscaler
----

The autoscaler is configured on a per pool basis and has the following modes:

[horizontal]
warn:: A health warning is issued if the suggested `pg_num` value differs too
much from the current value.
on:: The `pg_num` is adjusted automatically with no need for any manual
interaction.
off:: No automatic `pg_num` adjustments are made, and no warning will be issued
if the PG count is not optimal.

The scaling factor can be adjusted to facilitate future data storage with the
`target_size`, `target_size_ratio` and the `pg_num_min` options.

WARNING: By default, the autoscaler considers tuning the PG count of a pool if
it is off by a factor of 3. This will lead to a considerable shift in data
placement and might introduce a high load on the cluster.

You can find a more in-depth introduction to the PG autoscaler on Ceph's Blog -
https://ceph.io/rados/new-in-nautilus-pg-merging-and-autotuning/[New in
Nautilus: PG merging and autotuning].


[[pve_ceph_device_classes]]
Ceph CRUSH & device classes
---------------------------
The footnote:[CRUSH
https://ceph.com/wp-content/uploads/2016/08/weil-crush-sc06.pdf] (**C**ontrolled
**R**eplication **U**nder **S**calable **H**ashing) algorithm is at the
foundation of Ceph.

CRUSH calculates where to store and retrieve data from. This has the
advantage that no central indexing service is needed. CRUSH works using a map of
OSDs, buckets (device locations) and rulesets (data replication) for pools.

NOTE: Further information can be found in the Ceph documentation, under the
section CRUSH map footnote:[CRUSH map {cephdocs-url}/rados/operations/crush-map/].

This map can be altered to reflect different replication hierarchies. The object
replicas can be separated (eg. failure domains), while maintaining the desired
distribution.

A common configuration is to use different classes of disks for different Ceph
pools.  For this reason, Ceph introduced device classes with luminous, to
accommodate the need for easy ruleset generation.

The device classes can be seen in the 'ceph osd tree' output. These classes
represent their own root bucket, which can be seen with the below command.

[source, bash]
----
ceph osd crush tree --show-shadow
----

Example output form the above command:

[source, bash]
----
ID  CLASS WEIGHT  TYPE NAME
-16  nvme 2.18307 root default~nvme
-13  nvme 0.72769     host sumi1~nvme
 12  nvme 0.72769         osd.12
-14  nvme 0.72769     host sumi2~nvme
 13  nvme 0.72769         osd.13
-15  nvme 0.72769     host sumi3~nvme
 14  nvme 0.72769         osd.14
 -1       7.70544 root default
 -3       2.56848     host sumi1
 12  nvme 0.72769         osd.12
 -5       2.56848     host sumi2
 13  nvme 0.72769         osd.13
 -7       2.56848     host sumi3
 14  nvme 0.72769         osd.14
----

To instruct a pool to only distribute objects on a specific device class, you
first need to create a ruleset for the device class:

[source, bash]
----
ceph osd crush rule create-replicated <rule-name> <root> <failure-domain> <class>
----

[frame="none",grid="none", align="left", cols="30%,70%"]
|===
|<rule-name>|name of the rule, to connect with a pool (seen in GUI & CLI)
|<root>|which crush root it should belong to (default ceph root "default")
|<failure-domain>|at which failure-domain the objects should be distributed (usually host)
|<class>|what type of OSD backing store to use (eg. nvme, ssd, hdd)
|===

Once the rule is in the CRUSH map, you can tell a pool to use the ruleset.

[source, bash]
----
ceph osd pool set <pool-name> crush_rule <rule-name>
----

TIP: If the pool already contains objects, these must be moved accordingly.
Depending on your setup, this may introduce a big performance impact on your
cluster. As an alternative, you can create a new pool and move disks separately.


Ceph Client
-----------

[thumbnail="screenshot/gui-ceph-log.png"]

Following the setup from the previous sections, you can configure {pve} to use
such pools to store VM and Container images. Simply use the GUI to add a new
`RBD` storage (see section xref:ceph_rados_block_devices[Ceph RADOS Block
Devices (RBD)]).

You also need to copy the keyring to a predefined location for an external Ceph
cluster. If Ceph is installed on the Proxmox nodes itself, then this will be
done automatically.

NOTE: The filename needs to be `<storage_id> + `.keyring`, where `<storage_id>` is
the expression after 'rbd:' in `/etc/pve/storage.cfg`. In the following example,
`my-ceph-storage` is the `<storage_id>`:

[source,bash]
----
mkdir /etc/pve/priv/ceph
cp /etc/ceph/ceph.client.admin.keyring /etc/pve/priv/ceph/my-ceph-storage.keyring
----

[[pveceph_fs]]
CephFS
------

Ceph also provides a filesystem, which runs on top of the same object storage as
RADOS block devices do. A **M**eta**d**ata **S**erver (`MDS`) is used to map the
RADOS backed objects to files and directories, allowing Ceph to provide a
POSIX-compliant, replicated filesystem. This allows you to easily configure a
clustered, highly available, shared filesystem. Ceph's Metadata Servers
guarantee that files are evenly distributed over the entire Ceph cluster. As a
result, even cases of high load will not overwhelm a single host, which can be
an issue with traditional shared filesystem approaches, for example `NFS`.

[thumbnail="screenshot/gui-node-ceph-cephfs-panel.png"]

{pve} supports both creating a hyper-converged CephFS and using an existing
xref:storage_cephfs[CephFS as storage] to save backups, ISO files, and container
templates.


[[pveceph_fs_mds]]
Metadata Server (MDS)
~~~~~~~~~~~~~~~~~~~~~

CephFS needs at least one Metadata Server to be configured and running, in order
to function. You can create an MDS through the {pve} web GUI's `Node
-> CephFS` panel or from the command line with:

----
pveceph mds create
----

Multiple metadata servers can be created in a cluster, but with the default
settings, only one can be active at a time. If an MDS or its node becomes
unresponsive (or crashes), another `standby` MDS will get promoted to `active`.
You can speed up the handover between the active and standby MDS by using
the 'hotstandby' parameter option on creation, or if you have already created it
you may set/add:

----
mds standby replay = true
----

in the respective MDS section of `/etc/pve/ceph.conf`. With this enabled, the
specified MDS will remain in a `warm` state, polling the active one, so that it
can take over faster in case of any issues.

NOTE: This active polling will have an additional performance impact on your
system and the active `MDS`.

.Multiple Active MDS

Since Luminous (12.2.x) you can have multiple active metadata servers
running at once, but this is normally only useful if you have a high amount of
clients running in parallel. Otherwise the `MDS` is rarely the bottleneck in a
system. If you want to set this up, please refer to the Ceph documentation.
footnote:[Configuring multiple active MDS daemons
{cephdocs-url}/cephfs/multimds/]

[[pveceph_fs_create]]
Create CephFS
~~~~~~~~~~~~~

With {pve}'s integration of CephFS, you can easily create a CephFS using the
web interface, CLI or an external API interface. Some prerequisites are required
for this to work:

.Prerequisites for a successful CephFS setup:
- xref:pve_ceph_install[Install Ceph packages] - if this was already done some
time ago, you may want to rerun it on an up-to-date system to
ensure that all CephFS related packages get installed.
- xref:pve_ceph_monitors[Setup Monitors]
- xref:pve_ceph_monitors[Setup your OSDs]
- xref:pveceph_fs_mds[Setup at least one MDS]

After this is complete, you can simply create a CephFS through
either the Web GUI's `Node -> CephFS` panel or the command line tool `pveceph`,
for example:

----
pveceph fs create --pg_num 128 --add-storage
----

This creates a CephFS named 'cephfs', using a pool for its data named
'cephfs_data' with '128' placement groups and a pool for its metadata named
'cephfs_metadata' with one quarter of the data pool's placement groups (`32`).
Check the xref:pve_ceph_pools[{pve} managed Ceph pool chapter] or visit the
Ceph documentation for more information regarding an appropriate placement group
number (`pg_num`) for your setup footnoteref:[placement_groups].
Additionally, the '--add-storage' parameter will add the CephFS to the {pve}
storage configuration after it has been created successfully.

Destroy CephFS
~~~~~~~~~~~~~~

WARNING: Destroying a CephFS will render all of its data unusable. This cannot be
undone!

If you really want to destroy an existing CephFS, you first need to stop or
destroy all metadata servers (`M̀DS`). You can destroy them either via the web
interface or via the command line interface, by issuing

----
pveceph mds destroy NAME
----
on each {pve} node hosting an MDS daemon.

Then, you can remove (destroy) the CephFS by issuing

----
ceph fs rm NAME --yes-i-really-mean-it
----
on a single node hosting Ceph. After this, you may want to remove the created
data and metadata pools, this can be done either over the Web GUI or the CLI
with:

----
pveceph pool destroy NAME
----


Ceph maintenance
----------------

Replace OSDs
~~~~~~~~~~~~

One of the most common maintenance tasks in Ceph is to replace the disk of an
OSD. If a disk is already in a failed state, then you can go ahead and run
through the steps in xref:pve_ceph_osd_destroy[Destroy OSDs]. Ceph will recreate
those copies on the remaining OSDs if possible. This rebalancing will start as
soon as an OSD failure is detected or an OSD was actively stopped.

NOTE: With the default size/min_size (3/2) of a pool, recovery only starts when
`size + 1` nodes are available. The reason for this is that the Ceph object
balancer xref:pve_ceph_device_classes[CRUSH] defaults to a full node as
`failure domain'.

To replace a functioning disk from the GUI, go through the steps in
xref:pve_ceph_osd_destroy[Destroy OSDs]. The only addition is to wait until
the cluster shows 'HEALTH_OK' before stopping the OSD to destroy it.

On the command line, use the following commands:

----
ceph osd out osd.<id>
----

You can check with the command below if the OSD can be safely removed.

----
ceph osd safe-to-destroy osd.<id>
----

Once the above check tells you that it is safe to remove the OSD, you can
continue with the following commands:

----
systemctl stop ceph-osd@<id>.service
pveceph osd destroy <id>
----

Replace the old disk with the new one and use the same procedure as described
in xref:pve_ceph_osd_create[Create OSDs].

Trim/Discard
~~~~~~~~~~~~

It is good practice to run 'fstrim' (discard) regularly on VMs and containers.
This releases data blocks that the filesystem isn’t using anymore. It reduces
data usage and resource load. Most modern operating systems issue such discard
commands to their disks regularly. You only need to ensure that the Virtual
Machines enable the xref:qm_hard_disk_discard[disk discard option].

[[pveceph_scrub]]
Scrub & Deep Scrub
~~~~~~~~~~~~~~~~~~

Ceph ensures data integrity by 'scrubbing' placement groups. Ceph checks every
object in a PG for its health. There are two forms of Scrubbing, daily
cheap metadata checks and weekly deep data checks. The weekly deep scrub reads
the objects and uses checksums to ensure data integrity. If a running scrub
interferes with business (performance) needs, you can adjust the time when
scrubs footnote:[Ceph scrubbing {cephdocs-url}/rados/configuration/osd-config-ref/#scrubbing]
are executed.


Ceph Monitoring and Troubleshooting
-----------------------------------

It is important to continuously monitor the health of a Ceph deployment from the
beginning, either by using the Ceph tools or by accessing
the status through the {pve} link:api-viewer/index.html[API].

The following Ceph commands can be used to see if the cluster is healthy
('HEALTH_OK'), if there are warnings ('HEALTH_WARN'), or even errors
('HEALTH_ERR'). If the cluster is in an unhealthy state, the status commands
below will also give you an overview of the current events and actions to take.

----
# single time output
pve# ceph -s
# continuously output status changes (press CTRL+C to stop)
pve# ceph -w
----

To get a more detailed view, every Ceph service has a log file under
`/var/log/ceph/`. If more detail is required, the log level can be
adjusted footnote:[Ceph log and debugging {cephdocs-url}/rados/troubleshooting/log-and-debug/].

You can find more information about troubleshooting
footnote:[Ceph troubleshooting {cephdocs-url}/rados/troubleshooting/]
a Ceph cluster on the official website.


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