This document is for a development version of Ceph.

mClock Config Reference

Mclock profiles mask the low level details from users, making it easier for them to configure mclock.

To use mclock, you must provide the following input parameters:

  • total capacity of each OSD

  • an mclock profile to enable

Using the settings in the specified profile, the OSD determines and applies the lower-level mclock and Ceph parameters. The parameters applied by the mclock profile make it possible to tune the QoS between client I/O, recovery/backfill operations, and other background operations (for example, scrub, snap trim, and PG deletion). These background activities are considered best-effort internal clients of Ceph.

mClock Profiles - Definition and Purpose

A mclock profile is “a configuration setting that when applied on a running Ceph cluster enables the throttling of the operations(IOPS) belonging to different client classes (background recovery, scrub, snaptrim, client op, osd subop)”.

The mclock profile uses the capacity limits and the mclock profile selected by the user to determine the low-level mclock resource control parameters.

Depending on the profile, lower-level mclock resource-control parameters and some Ceph-configuration parameters are transparently applied.

The low-level mclock resource control parameters are the reservation, limit, and weight that provide control of the resource shares, as described in the OSD Config Reference.

mClock Profile Types

mclock profiles can be broadly classified into two types,

  • Built-in: Users can choose between the following built-in profile types:

    • high_client_ops (default): This profile allocates more reservation and limit to external-client ops as compared to background recoveries and other internal clients within Ceph. This profile is enabled by default.

    • high_recovery_ops: This profile allocates more reservation to background recoveries as compared to external clients and other internal clients within Ceph. For example, an admin may enable this profile temporarily to speed-up background recoveries during non-peak hours.

    • balanced: This profile allocates equal reservation to client ops and background recovery ops.

  • Custom: This profile gives users complete control over all mclock and Ceph configuration parameters. Using this profile is not recommended without a deep understanding of mclock and related Ceph-configuration options.


Across the built-in profiles, internal clients of mclock (for example “scrub”, “snap trim”, and “pg deletion”) are given slightly lower reservations, but higher weight and no limit. This ensures that these operations are able to complete quickly if there are no other competing services.

mClock Built-in Profiles

When a built-in profile is enabled, the mClock scheduler calculates the low level mclock parameters [reservation, weight, limit] based on the profile enabled for each client type. The mclock parameters are calculated based on the max OSD capacity provided beforehand. As a result, the following mclock config parameters cannot be modified when using any of the built-in profiles:

  • osd_mclock_scheduler_client_res

  • osd_mclock_scheduler_client_wgt

  • osd_mclock_scheduler_client_lim

  • osd_mclock_scheduler_background_recovery_res

  • osd_mclock_scheduler_background_recovery_wgt

  • osd_mclock_scheduler_background_recovery_lim

  • osd_mclock_scheduler_background_best_effort_res

  • osd_mclock_scheduler_background_best_effort_wgt

  • osd_mclock_scheduler_background_best_effort_lim

The following Ceph options will not be modifiable by the user:

  • osd_max_backfills

  • osd_recovery_max_active

This is because the above options are internally modified by the mclock scheduler in order to maximize the impact of the set profile.

By default, the high_client_ops profile is enabled to ensure that a larger chunk of the bandwidth allocation goes to client ops. Background recovery ops are given lower allocation (and therefore take a longer time to complete). But there might be instances that necessitate giving higher allocations to either client ops or recovery ops. In order to deal with such a situation, you can enable one of the alternate built-in profiles mentioned above.

If a built-in profile is active, the following Ceph config sleep options will be disabled,

  • osd_recovery_sleep

  • osd_recovery_sleep_hdd

  • osd_recovery_sleep_ssd

  • osd_recovery_sleep_hybrid

  • osd_scrub_sleep

  • osd_delete_sleep

  • osd_delete_sleep_hdd

  • osd_delete_sleep_ssd

  • osd_delete_sleep_hybrid

  • osd_snap_trim_sleep

  • osd_snap_trim_sleep_hdd

  • osd_snap_trim_sleep_ssd

  • osd_snap_trim_sleep_hybrid

The above sleep options are disabled to ensure that mclock scheduler is able to determine when to pick the next op from its operation queue and transfer it to the operation sequencer. This results in the desired QoS being provided across all its clients.

Steps to Enable mClock Profile

The following sections outline the steps required to enable a mclock profile.

Determining OSD Capacity Using Benchmark Tests

To allow mclock to fulfill its QoS goals across its clients, it is most important to have a good understanding of each OSD’s capacity in terms of its baseline throughputs (IOPS) across the Ceph nodes. To determine this capacity, you must perform appropriate benchmarking tests. The steps for performing these benchmarking tests are broadly outlined below.

Any existing benchmarking tool can be used for this purpose. The following steps use the Ceph Benchmarking Tool (cbt). Regardless of the tool used, the steps described below remain the same.

As already described in the OSD Config Reference section, the number of shards and the bluestore’s throttle parameters have an impact on the mclock op queues. Therefore, it is critical to set these values carefully in order to maximize the impact of the mclock scheduler.

Number of Operational Shards

We recommend using the default number of shards as defined by the configuration options osd_op_num_shards, osd_op_num_shards_hdd, and osd_op_num_shards_ssd. In general, a lower number of shards will increase the impact of the mclock queues.

Bluestore Throttle Parameters

We recommend using the default values as defined by bluestore_throttle_bytes and bluestore_throttle_deferred_bytes. But these parameters may also be determined during the benchmarking phase as described below.

Benchmarking Test Steps Using CBT

The steps below use the default shards and detail the steps used to determine the correct bluestore throttle values.


These steps, although manual in April 2021, will be automated in the future.

  1. On the Ceph node hosting the OSDs, download cbt from git.

  2. Install cbt and all the dependencies mentioned on the cbt github page.

  3. Construct the Ceph configuration file and the cbt yaml file.

  4. Ensure that the bluestore throttle options ( i.e. bluestore_throttle_bytes and bluestore_throttle_deferred_bytes) are set to the default values.

  5. Ensure that the test is performed on similar device types to get reliable OSD capacity data.

  6. The OSDs can be grouped together with the desired replication factor for the test to ensure reliability of OSD capacity data.

  7. After ensuring that the OSDs nodes are in the desired configuration, run a simple 4KiB random write workload on the OSD(s) for 300 secs.

  8. Note the overall throughput(IOPS) obtained from the cbt output file. This value is the baseline throughput(IOPS) when the default bluestore throttle options are in effect.

  9. If the intent is to determine the bluestore throttle values for your environment, then set the two options, bluestore_throttle_bytes and bluestore_throttle_deferred_bytes to 32 KiB(32768 Bytes) each to begin with. Otherwise, you may skip to the next section.

  10. Run the 4KiB random write workload as before on the OSD(s) for 300 secs.

  11. Note the overall throughput from the cbt log files and compare the value against the baseline throughput in step 8.

  12. If the throughput doesn’t match with the baseline, increment the bluestore throttle options by 2x and repeat steps 9 through 11 until the obtained throughput is very close to the baseline value.

For example, during benchmarking on a machine with NVMe SSDs, a value of 256 KiB for both bluestore throttle and deferred bytes was determined to maximize the impact of mclock. For HDDs, the corresponding value was 40 MiB, where the overall throughput was roughly equal to the baseline throughput. Note that in general for HDDs, the bluestore throttle values are expected to be higher when compared to SSDs.

Specifying Max OSD Capacity

The steps in this section may be performed only if the max osd capacity is different from the default values (SSDs: 21500 IOPS and HDDs: 315 IOPS). The option osd_mclock_max_capacity_iops_[hdd, ssd] can be set by specifying it in either the [global] section or in a specific OSD section ([osd.x] of your Ceph configuration file).

Alternatively, commands of the following form may be used:

ceph config set [global, osd] osd_mclock_max_capacity_iops_[hdd,ssd] <value>

For example, the following command sets the max capacity for all the OSDs in a Ceph node whose underlying device type is SSDs:

ceph config set osd osd_mclock_max_capacity_iops_ssd 25000

To set the capacity for a specific OSD (for example “osd.0”) whose underlying device type is HDD, use a command like this:

ceph config set osd.0 osd_mclock_max_capacity_iops_hdd 350

Specifying Which mClock Profile to Enable

As already mentioned, the default mclock profile is set to high_client_ops. The other values for the built-in profiles include balanced and high_recovery_ops.

If there is a requirement to change the default profile, then the option osd_mclock_profile may be set in the [global] or [osd] section of your Ceph configuration file before bringing up your cluster.

Alternatively, to change the profile during runtime, use the following command:

ceph config set [global,osd] osd_mclock_profile <value>

For example, to change the profile to allow faster recoveries, the following command can be used to switch to the high_recovery_ops profile:

ceph config set osd osd_mclock_profile high_recovery_ops


The custom profile is not recommended unless you are an advanced user.

And that’s it! You are ready to run workloads on the cluster and check if the QoS requirements are being met.

mClock Config Options



This sets the type of mclock profile to use for providing QoS based on operations belonging to different classes (background recovery, scrub, snaptrim, client op, osd subop). Once a built-in profile is enabled, the lower level mclock resource control parameters [reservation, weight, limit] and some Ceph configuration parameters are set transparently. Note that the above does not apply for the custom profile.



Valid Choices

high_client_ops, high_recovery_ops, balanced, custom





Max IOPS capacity (at 4KiB block size) to consider per OSD (overrides _ssd and _hdd if non-zero)







Max IOPS capacity (at 4KiB block size) to consider per OSD (for rotational media)







Max IOPS capacity (at 4KiB block size) to consider per OSD (for solid state media)







Cost per IO in microseconds to consider per OSD (overrides _ssd and _hdd if non-zero)







Cost per IO in microseconds to consider per OSD (for rotational media)







Cost per IO in microseconds to consider per OSD (for solid state media)







Cost per byte in microseconds to consider per OSD (overrides _ssd and _hdd if non-zero)







Cost per byte in microseconds to consider per OSD (for rotational media)







Cost per byte in microseconds to consider per OSD (for solid state media)