mirror of
https://github.com/ceph/ceph
synced 2024-12-18 09:25:49 +00:00
147 lines
4.6 KiB
ReStructuredText
147 lines
4.6 KiB
ReStructuredText
Balancer plugin
|
|
===============
|
|
|
|
The *balancer* plugin can optimize the placement of PGs across OSDs in
|
|
order to achieve a balanced distribution, either automatically or in a
|
|
supervised fashion.
|
|
|
|
Enabling
|
|
--------
|
|
|
|
The *balancer* module is enabled with::
|
|
|
|
ceph mgr module enable balancer
|
|
|
|
(It is enabled by default.)
|
|
|
|
Status
|
|
------
|
|
|
|
The current status of the balancer can be check at any time with::
|
|
|
|
ceph balancer status
|
|
|
|
|
|
Automatic balancing
|
|
-------------------
|
|
|
|
The automatic balancing can be enabled, using the default settings, with::
|
|
|
|
ceph balancer on
|
|
|
|
The balancer can be turned back off again with::
|
|
|
|
ceph balancer off
|
|
|
|
This will use the ``crush-compat`` mode, which is backward compatible
|
|
with older clients, and will make small changes to the data
|
|
distribution over time to ensure that OSDs are equally utilized.
|
|
|
|
|
|
Throttling
|
|
----------
|
|
|
|
No adjustments will be made to the PG distribution if the cluster is
|
|
degraded (e.g., because an OSD has failed and the system has not yet
|
|
healed itself).
|
|
|
|
When the cluster is healthy, the balancer will throttle its changes
|
|
such that the percentage of PGs that are misplaced (i.e., that need to
|
|
be moved) is below a threshold of (by default) 5%. The
|
|
``max_misplaced`` threshold can be adjusted with::
|
|
|
|
ceph config-key set mgr/balancer/max_misplaced .07 # 7%
|
|
|
|
|
|
Modes
|
|
-----
|
|
|
|
There are currently two supported balancer modes:
|
|
|
|
#. **crush-compat**. The CRUSH compat mode uses the compat weight-set
|
|
feature (introduced in Luminous) to manage an alternative set of
|
|
weights for devices in the CRUSH hierarchy. The normal weights
|
|
should remain set to the size of the device to reflect the target
|
|
amount of data that we want to store on the device. The balancer
|
|
then optimizes the weight-set values, adjusting them up or down in
|
|
small increments, in order to achieve a distribution that matches
|
|
the target distribution as closely as possible. (Because PG
|
|
placement is a pseudorandom process, there is a natural amount of
|
|
variation in the placement; by optimizing the weights we
|
|
counter-act that natural variation.)
|
|
|
|
Notably, this mode is *fully backwards compatible* with older
|
|
clients: when an OSDMap and CRUSH map is shared with older clients,
|
|
we present the optimized weights as the "real" weights.
|
|
|
|
The primary restriction of this mode is that the balancer cannot
|
|
handle multiple CRUSH hierarchies with different placement rules if
|
|
the subtrees of the hierarchy share any OSDs. (This is normally
|
|
not the case, and is generally not a recommended configuration
|
|
because it is hard to manage the space utilization on the shared
|
|
OSDs.)
|
|
|
|
#. **upmap**. Starting with Luminous, the OSDMap can store explicit
|
|
mappings for individual OSDs as exceptions to the normal CRUSH
|
|
placement calculation. These `upmap` entries provide fine-grained
|
|
control over the PG mapping. This CRUSH mode will optimize the
|
|
placement of individual PGs in order to achieve a balanced
|
|
distribution. In most cases, this distribution is "perfect," which
|
|
an equal number of PGs on each OSD (+/-1 PG, since they might not
|
|
divide evenly).
|
|
|
|
Note that using upmap requires that all clients be Luminous or newer.
|
|
|
|
The default mode is ``crush-compat``. The mode can be adjusted with::
|
|
|
|
ceph balancer mode upmap
|
|
|
|
or::
|
|
|
|
ceph balancer mode crush-compat
|
|
|
|
Supervised optimization
|
|
-----------------------
|
|
|
|
The balancer operation is broken into a few distinct phases:
|
|
|
|
#. building a *plan*
|
|
#. evaluating the quality of the data distribution, either for the current PG distribution, or the PG distribution that would result after executing a *plan*
|
|
#. executing the *plan*
|
|
|
|
To evautate and score the current distribution,::
|
|
|
|
ceph balancer eval
|
|
|
|
You can also evaluate the distribution for a single pool with::
|
|
|
|
ceph balancer eval <pool-name>
|
|
|
|
Greater detail for the evaluation can be seen with::
|
|
|
|
ceph balancer eval-verbose ...
|
|
|
|
The balancer can generate a plan, using the currently configured mode, with::
|
|
|
|
ceph balancer optimize <plan-name>
|
|
|
|
The name is provided by the user and can be any useful identifying string. The contents of a plan can be seen with::
|
|
|
|
ceph balancer show <plan-name>
|
|
|
|
Old plans can be discarded with::
|
|
|
|
ceph balancer rm <plan-name>
|
|
|
|
Currently recorded plans are shown as part of the status command::
|
|
|
|
ceph balancer status
|
|
|
|
The quality of the distribution that would result after executing a plan can be calculated with::
|
|
|
|
ceph balancer eval <plan-name>
|
|
|
|
Assuming the plan is expected to improve the distribution (i.e., it has a lower score than the current cluster state), the user can execute that plan with::
|
|
|
|
ceph balancer execute <plan-name>
|