Balancer

The balancer can optimize the allocation of placement groups (PGs) across OSDs in order to achieve a balanced distribution. The balancer can operate either automatically or in a supervised fashion.

Status

To check the current status of the balancer, run the following command:

ceph balancer status

Automatic balancing

When the balancer is in upmap mode, the automatic balancing feature is enabled by default. For more details, see Using pg-upmap. To disable the balancer, run the following command:

ceph balancer off

The balancer mode can be changed from upmap mode to crush-compat mode. crush-compat mode is backward compatible with older clients. In crush-compat mode, the balancer automatically makes small changes to the data distribution in order to ensure that OSDs are utilized equally.

Throttling

If the cluster is degraded (that is, if an OSD has failed and the system hasn’t healed itself yet), then the balancer will not make any adjustments to the PG distribution.

When the cluster is healthy, the balancer will incrementally move a small fraction of unbalanced PGs in order to improve distribution. This fraction will not exceed a certain threshold that defaults to 5%. To adjust this target_max_misplaced_ratio threshold setting, run the following command:

ceph config set mgr target_max_misplaced_ratio .07   # 7%

The balancer sleeps between runs. To set the number of seconds for this interval of sleep, run the following command:

ceph config set mgr mgr/balancer/sleep_interval 60

To set the time of day (in HHMM format) at which automatic balancing begins, run the following command:

ceph config set mgr mgr/balancer/begin_time 0000

To set the time of day (in HHMM format) at which automatic balancing ends, run the following command:

ceph config set mgr mgr/balancer/end_time 2359

Automatic balancing can be restricted to certain days of the week. To restrict it to a specific day of the week or later (as with crontab, 0 is Sunday, 1 is Monday, and so on), run the following command:

ceph config set mgr mgr/balancer/begin_weekday 0

To restrict automatic balancing to a specific day of the week or earlier (again, 0 is Sunday, 1 is Monday, and so on), run the following command:

ceph config set mgr mgr/balancer/end_weekday 6

Automatic balancing can be restricted to certain pools. By default, the value of this setting is an empty string, so that all pools are automatically balanced. To restrict automatic balancing to specific pools, retrieve their numeric pool IDs (by running the ceph osd pool ls detail command), and then run the following command:

ceph config set mgr mgr/balancer/pool_ids 1,2,3

Modes

There are two supported balancer modes:

  1. crush-compat. This mode uses the compat weight-set feature (introduced in Luminous) to manage an alternative set of weights for devices in the CRUSH hierarchy. When the balancer is operating in this mode, the normal weights should remain set to the size of the device in order to reflect the target amount of data intended to be stored on the device. The balancer will then optimize 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, it is subject to a natural amount of variation; optimizing the weights serves to counteract that natural variation.)

    Note that this mode is fully backward compatible with older clients: when an OSD Map and CRUSH map are shared with older clients, Ceph presents the optimized weights as the “real” weights.

    The primary limitation 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. (Such sharing of OSDs is not typical and, because of the difficulty of managing the space utilization on the shared OSDs, is generally not recommended.)

  2. upmap. In Luminous and later releases, 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 balancer mode optimizes the placement of individual PGs in order to achieve a balanced distribution. In most cases, the resulting distribution is nearly perfect: that is, there is an equal number of PGs on each OSD (±1 PG, since the total number might not divide evenly).

    To use``upmap``, all clients must be Luminous or newer.

The default mode is upmap. The mode can be changed to crush-compat by running the following command:

ceph balancer mode crush-compat

Supervised optimization

Supervised use of the balancer can be understood in terms of three distinct phases:

  1. building a plan

  2. evaluating the quality of the data distribution, either for the current PG distribution or for the PG distribution that would result after executing a plan

  3. executing the plan

To evaluate the current distribution, run the following command:

ceph balancer eval

To evaluate the distribution for a single pool, run the following command:

ceph balancer eval <pool-name>

To see the evaluation in greater detail, run the following command:

ceph balancer eval-verbose ...

To instruct the balancer to generate a plan (using the currently configured mode), make up a name (any useful identifying string) for the plan, and run the following command:

ceph balancer optimize <plan-name>

To see the contents of a plan, run the following command:

ceph balancer show <plan-name>

To display all plans, run the following command:

ceph balancer ls

To discard an old plan, run the following command:

ceph balancer rm <plan-name>

To see currently recorded plans, examine the output of the following status command:

ceph balancer status

To evaluate the distribution that would result from executing a specific plan, run the following command:

ceph balancer eval <plan-name>

If a plan is expected to improve the distribution (that is, the plan’s score is lower than the current cluster state’s score), you can execute that plan by running the following command:

ceph balancer execute <plan-name>