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