pveceph: reword memory precondition

[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, i.e. 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 storages
(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 with CLI and GUI support
- Thin provisioning
- Snapshots 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 economical commodity hardware
- No need for hardware RAID controllers
- Open source

For small to mid 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 plenty 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 to install and
manage {ceph} services on {pve} nodes.

.Ceph consists of a couple of Daemons footnote:[Ceph intro https://docs.ceph.com/docs/{ceph_codename}/start/intro/], for use as a 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's architecture
footnote:[Ceph architecture https://docs.ceph.com/docs/{ceph_codename}/architecture/]
and vocabulary
footnote:[Ceph glossary https://docs.ceph.com/docs/{ceph_codename}/glossary].


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

To build a hyper-converged Proxmox + Ceph Cluster there should be at least
three (preferably) identical servers for the setup.

Check also the recommendations from
https://docs.ceph.com/docs/{ceph_codename}/start/hardware-recommendations/[Ceph's website].

.CPU
Higher CPU core frequency reduce 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 intended workload from virtual machines
and containers, Ceph needs enough memory available 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.

Further, 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 it 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, the 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. This fact and the
higher cost may make a xref:pve_ceph_device_classes[class based] separation of
pools appealing. Another possibility to speedup OSDs is to use a faster disk
as 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 best performs with an even sized and distributed
amount of disks per node. For example, 4 x 500 GB disks with in each node is
better than a mixed setup with a single 1 TB and three 250 GB disk.

One also need to balance OSD count and single OSD capacity. More capacity
allows 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 controller are not
designed for the Ceph use case 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 controller, use host bus adapter (HBA) instead.

NOTE: Above recommendations should be seen as a rough guidance for choosing
hardware. Therefore, it is still essential to adapt it to your specific needs,
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 be
offered to do so now.

The wizard is divided into different sections, where each needs to be
finished successfully in order to use Ceph. After starting the installation
the wizard will download and install all 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 should setup a dedicated network for Ceph, this
setting is required. Separating your Ceph traffic is highly recommended,
because it could lead to troubles with other latency dependent services,
e.g., cluster communication may decrease Ceph's performance, if not done.

[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 big clusters.

You have two more options which are considered advanced and therefore
should only changed if you are an expert.

* *Number of replicas*: Defines the how often a 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 is required.

That's it, you should see a success page as the last step with further
instructions on how to go on. You are now prepared to start using Ceph,
even though you will need to create additional xref:pve_ceph_monitors[monitors],
create some xref:pve_ceph_osds[OSDs] and at least one xref:pve_ceph_pools[pool].

The rest of this chapter will guide you on how to get the most out of
your {pve} based Ceph setup, this will include aforementioned and
more like xref:pveceph_fs[CephFS] which is a very handy addition to your
new Ceph cluster.

[[pve_ceph_install]]
Installation of Ceph Packages
-----------------------------
Use {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. That file is automatically distributed to
all {pve} nodes by using xref:chapter_pmxcfs[pmxcfs]. The command also
creates a symbolic link from `/etc/ceph/ceph.conf` pointing to that file.
So 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 https://docs.ceph.com/docs/{ceph_codename}/start/intro/]
maintains a master copy of the cluster map. For high availability you need to
have 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 midsize, only really large clusters will
need more than that.


[[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 it 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 Ceph luminous release at least one ceph-mgr
footnote:[Ceph Manager https://docs.ceph.com/docs/{ceph_codename}/mgr/] daemon is
required.

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

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

[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: A Ceph cluster can function without a Manager, but certain functions like
the cluster status or usage require a running Manager.


[[pve_ceph_osds]]
Ceph OSDs
---------
Ceph **O**bject **S**torage **D**aemons are storing 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"]

via GUI or via CLI as follows:

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

TIP: We recommend a Ceph cluster size, starting with 12 OSDs, distributed
evenly among your, at least three nodes (4 OSDs on each node).

If the disk was used before (eg. ZFS/RAID/OSD), to remove partition table, boot
sector and any OSD leftover the following command should be sufficient.

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

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

.Ceph Bluestore

Starting with the Ceph Kraken release, a new Ceph OSD storage type was
introduced, the so 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 for 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 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. Select the OSD to destroy. Next click the **OUT**
button. Once the OSD status changed from `in` to `out` click the **STOP**
button. As soon as the status 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 data on the disk!


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


Create 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** for serving objects in a degraded
state.

NOTE: The default number of PGs works for 2-5 disks. Ceph throws a
'HEALTH_WARNING' if you have too few or too many PGs in your cluster.

It is advised to calculate the PG number depending on your setup, you can find
the formula and the PG calculator footnote:[PG calculator
https://ceph.com/pgcalc/] online. While PGs can be increased later on, they can
never be decreased.


You can create pools through command line or on the GUI on each PVE host under
**Ceph -> Pools**.

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

If you would like to automatically also get a storage definition for your pool,
mark the checkbox "Add storages" in the GUI or use the command line option
'--add_storages' at pool creation.

Further information on Ceph pool handling can be found in the Ceph pool
operation footnote:[Ceph pool operation
https://docs.ceph.com/docs/{ceph_codename}/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: Deleting the data of a pool is a background task and can take some time.
You will notice that the data usage in the cluster is decreasing.

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

CRUSH calculates where to store to and retrieve data from, this has the
advantage that no central index service is needed. CRUSH works with 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 https://docs.ceph.com/docs/{ceph_codename}/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 use case is to use different classes of disks for different Ceph pools.
For this reason, Ceph introduced the 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 let a pool distribute its objects only on a specific device class, you need
to create a ruleset with the specific class first.

[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, all of these have to be moved
accordingly. Depending on your setup this may introduce a big performance hit
on your cluster. As an alternative, you can create a new pool and move disks
separately.


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

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

You can then configure {pve} to use such pools to store VM or
Container images. Simply use the GUI too 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 file name needs to be `<storage_id> + `.keyring` - `<storage_id>` is
the expression after 'rbd:' in `/etc/pve/storage.cfg` which is
`my-ceph-storage` in the following example:

[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 provides also a filesystem running 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 to provide a
POSIX-compliant replicated filesystem. This allows one to have a clustered
highly available shared filesystem in an easy way if ceph is already used.  Its
Metadata Servers guarantee that files get balanced out over the whole Ceph
cluster, this way even high load will not overload a single host, which can be
an issue with traditional shared filesystem approaches, like `NFS`, for
example.

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

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


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

CephFS needs at least one Metadata Server to be configured and running to be
able to work. One can simply create one through the {pve} web GUI's `Node ->
CephFS` panel or on 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 any time. If an MDS, or its node, becomes
unresponsive (or crashes), another `standby` MDS will get promoted to `active`.
One can speed up the hand-over between the active and a standby MDS up by using
the 'hotstandby' parameter option on create, or if you have already created it
you may set/add:

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

in the ceph.conf respective MDS section. With this enabled, this specific MDS
will always poll the active one, so that it can take over faster as it is in a
`warm` state. But naturally, the active polling will cause some additional
performance impact on your system and active `MDS`.

.Multiple Active MDS

Since Luminous (12.2.x) you can also have multiple active metadata servers
running, but this is normally only useful for a high count on parallel clients,
as else the `MDS` seldom is the bottleneck. If you want to set this up please
refer to the ceph documentation. footnote:[Configuring multiple active MDS
daemons https://docs.ceph.com/docs/{ceph_codename}/cephfs/multimds/]

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

With {pve}'s CephFS integration into you can create a CephFS easily over the
Web GUI, the 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 might want to rerun it on an up to date system to ensure that
  also 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 got all checked and done you can simply create a CephFS through
either the Web GUI's `Node -> CephFS` panel or the command line tool `pveceph`,
for example with:

----
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 pools placement groups (`32`).
Check the xref:pve_ceph_pools[{pve} managed Ceph pool chapter] or visit the
Ceph documentation for more information regarding a fitting placement group
number (`pg_num`) for your setup footnote:[Ceph Placement Groups
https://docs.ceph.com/docs/{ceph_codename}/rados/operations/placement-groups/].
Additionally, the `'--add-storage'' parameter will add the CephFS to the {pve}
storage configuration after it was created successfully.

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

WARNING: Destroying a CephFS will render all 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 over the Web
GUI or the command line interface, with:

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

Then, you can remove (destroy) CephFS by issuing a:

----
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 common maintenance tasks in Ceph is to replace a 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 still functioning disk, on 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 save to remove the OSD, you can
continue with 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 a good measure to run 'fstrim' (discard) regularly on VMs or 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 https://docs.ceph.com/docs/{ceph_codename}/rados/configuration/osd-config-ref/#scrubbing]
are executed.


Ceph monitoring and troubleshooting
-----------------------------------
A good start is to continuosly monitor the ceph health from the start of
initial deployment. Either through the ceph tools itself, but also by accessing
the status through the {pve} link:api-viewer/index.html[API].

The following ceph commands below 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/` and if there is not enough detail, the log level can be
adjusted footnote:[Ceph log and debugging https://docs.ceph.com/docs/{ceph_codename}/rados/troubleshooting/log-and-debug/].

You can find more information about troubleshooting
footnote:[Ceph troubleshooting https://docs.ceph.com/docs/{ceph_codename}/rados/troubleshooting/]
a Ceph cluster on the official website.


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