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CRUSH Maps¶

The CRUSH algorithm determines how to store and retrieve data by computing storage locations. CRUSH empowers Ceph clients to communicate with OSDs directly rather than through a centralized server or broker. With an algorithmically determined method of storing and retrieving data, Ceph avoids a single point of failure, a performance bottleneck, and a physical limit to its scalability.

CRUSH uses a map of your cluster (the CRUSH map) to pseudo-randomly map data to OSDs, distributing it across the cluster according to configured replication policy and failure domain. For a detailed discussion of CRUSH, see CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data

CRUSH maps contain a list of OSDs, a hierarchy of ‘buckets’ for aggregating devices and buckets, and rules that govern how CRUSH replicates data within the cluster’s pools. By reflecting the underlying physical organization of the installation, CRUSH can model (and thereby address) the potential for correlated device failures. Typical factors include chassis, racks, physical proximity, a shared power source, and shared networking. By encoding this information into the cluster map, CRUSH placement policies distribute object replicas across failure domains while maintaining the desired distribution. For example, to address the possibility of concurrent failures, it may be desirable to ensure that data replicas are on devices using different shelves, racks, power supplies, controllers, and/or physical locations.

When you deploy OSDs they are automatically added to the CRUSH map under a host bucket named for the node on which they run. This, combined with the configured CRUSH failure domain, ensures that replicas or erasure code shards are distributed across hosts and that a single host or other failure will not affect availability. For larger clusters, administrators must carefully consider their choice of failure domain. Separating replicas across racks, for example, is typical for mid- to large-sized clusters.

CRUSH Location¶

The location of an OSD within the CRUSH map’s hierarchy is referred to as a CRUSH location. This location specifier takes the form of a list of key and value pairs. For example, if an OSD is in a particular row, rack, chassis and host, and is part of the ‘default’ CRUSH root (which is the case for most clusters), its CRUSH location could be described as:

root=default row=a rack=a2 chassis=a2a host=a2a1

Note:

Note that the order of the keys does not matter.
The key name (left of =) must be a valid CRUSH type. By default these include root, datacenter, room, row, pod, pdu, rack, chassis and host. These defined types suffice for almost all clusters, but can be customized by modifying the CRUSH map.
Not all keys need to be specified. For example, by default, Ceph automatically sets an OSD’s location to be root=default host=HOSTNAME (based on the output from hostname -s).

The CRUSH location for an OSD can be defined by adding the crush location option in ceph.conf. Each time the OSD starts, it verifies it is in the correct location in the CRUSH map and, if it is not, it moves itself. To disable this automatic CRUSH map management, add the following to your configuration file in the [osd] section:

osd crush update on start = false

Note that in most cases you will not need to manually configure this.

Custom location hooks¶

A customized location hook can be used to generate a more complete CRUSH location on startup. The CRUSH location is based on, in order of preference:

A crush location option in ceph.conf
A default of root=default host=HOSTNAME where the hostname is derived from the hostname -s command

A script can be written to provide additional location fields (for example, rack or datacenter) and the hook enabled via the config option:

crush location hook = /path/to/customized-ceph-crush-location

This hook is passed several arguments (below) and should output a single line to stdout with the CRUSH location description.:

--cluster CLUSTER --id ID --type TYPE

where the cluster name is typically ceph, the id is the daemon identifier (e.g., the OSD number or daemon identifier), and the daemon type is osd, mds, etc.

For example, a simple hook that additionally specifies a rack location based on a value in the file /etc/rack might be:

#!/bin/sh
echo "host=$(hostname -s) rack=$(cat /etc/rack) root=default"

CRUSH structure¶

The CRUSH map consists of a hierarchy that describes the physical topology of the cluster and a set of rules defining data placement policy. The hierarchy has devices (OSDs) at the leaves, and internal nodes corresponding to other physical features or groupings: hosts, racks, rows, datacenters, and so on. The rules describe how replicas are placed in terms of that hierarchy (e.g., ‘three replicas in different racks’).

Devices¶

Devices are individual OSDs that store data, usually one for each storage drive. Devices are identified by an id (a non-negative integer) and a name, normally osd.N where N is the device id.

Since the Luminous release, devices may also have a device class assigned (e.g., hdd or ssd or nvme), allowing them to be conveniently targeted by CRUSH rules. This is especially useful when mixing device types within hosts.

Types and Buckets¶

A bucket is the CRUSH term for internal nodes in the hierarchy: hosts, racks, rows, etc. The CRUSH map defines a series of types that are used to describe these nodes. Default types include:

osd (or device)
host
chassis
rack
row
pdu
pod
room
datacenter
zone
region
root

Most clusters use only a handful of these types, and others can be defined as needed.

The hierarchy is built with devices (normally type osd) at the leaves, interior nodes with non-device types, and a root node of type root. For example,

Each node (device or bucket) in the hierarchy has a weight that indicates the relative proportion of the total data that device or hierarchy subtree should store. Weights are set at the leaves, indicating the size of the device, and automatically sum up the tree, such that the weight of the root node will be the total of all devices contained beneath it. Normally weights are in units of terabytes (TB).

You can get a simple view the of CRUSH hierarchy for your cluster, including weights, with:

ceph osd tree

Rules¶

CRUSH Rules define policy about how data is distributed across the devices in the hierarchy. They define placement and replication strategies or distribution policies that allow you to specify exactly how CRUSH places data replicas. For example, you might create a rule selecting a pair of targets for two-way mirroring, another rule for selecting three targets in two different data centers for three-way mirroring, and yet another rule for erasure coding (EC) across six storage devices. For a detailed discussion of CRUSH rules, refer to CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data, and more specifically to Section 3.2.

CRUSH rules can be created via the CLI by specifying the pool type they will be used for (replicated or erasure coded), the failure domain, and optionally a device class. In rare cases rules must be written by hand by manually editing the CRUSH map.

You can see what rules are defined for your cluster with:

ceph osd crush rule ls

You can view the contents of the rules with:

ceph osd crush rule dump

Device classes¶

Each device can optionally have a class assigned. By default, OSDs automatically set their class at startup to hdd, ssd, or nvme based on the type of device they are backed by.

The device class for one or more OSDs can be explicitly set with:

ceph osd crush set-device-class <class> <osd-name> [...]

Once a device class is set, it cannot be changed to another class until the old class is unset with:

ceph osd crush rm-device-class <osd-name> [...]

This allows administrators to set device classes without the class being changed on OSD restart or by some other script.

A placement rule that targets a specific device class can be created with:

ceph osd crush rule create-replicated <rule-name> <root> <failure-domain> <class>

A pool can then be changed to use the new rule with:

ceph osd pool set <pool-name> crush_rule <rule-name>

Device classes are implemented by creating a “shadow” CRUSH hierarchy for each device class in use that contains only devices of that class. CRUSH rules can then distribute data over the shadow hierarchy. This approach is fully backward compatible with old Ceph clients. You can view the CRUSH hierarchy with shadow items with:

ceph osd crush tree --show-shadow

For older clusters created before Luminous that relied on manually crafted CRUSH maps to maintain per-device-type hierarchies, there is a reclassify tool available to help transition to device classes without triggering data movement (see Migrating from a legacy SSD rule to device classes).

Weights sets¶

A weight set is an alternative set of weights to use when calculating data placement. The normal weights associated with each device in the CRUSH map are set based on the device size and indicate how much data we should be storing where. However, because CRUSH is a “probabilistic” pseudorandom placement process, there is always some variation from this ideal distribution, in the same way that rolling a die sixty times will not result in rolling exactly 10 ones and 10 sixes. Weight sets allow the cluster to perform numerical optimization based on the specifics of your cluster (hierarchy, pools, etc.) to achieve a balanced distribution.

There are two types of weight sets supported:

A compat weight set is a single alternative set of weights for each device and node in the cluster. This is not well-suited for correcting for all anomalies (for example, placement groups for different pools may be different sizes and have different load levels, but will be mostly treated the same by the balancer). However, compat weight sets have the huge advantage that they are backward compatible with previous versions of Ceph, which means that even though weight sets were first introduced in Luminous v12.2.z, older clients (e.g., firefly) can still connect to the cluster when a compat weight set is being used to balance data.

A per-pool weight set is more flexible in that it allows placement to be optimized for each data pool. Additionally, weights can be adjusted for each position of placement, allowing the optimizer to correct for a subtle skew of data toward devices with small weights relative to their peers (and effect that is usually only apparently in very large clusters but which can cause balancing problems).

When weight sets are in use, the weights associated with each node in the hierarchy is visible as a separate column (labeled either (compat) or the pool name) from the command:

ceph osd crush tree

When both compat and per-pool weight sets are in use, data placement for a particular pool will use its own per-pool weight set if present. If not, it will use the compat weight set if present. If neither are present, it will use the normal CRUSH weights.

Although weight sets can be set up and manipulated by hand, it is recommended that the ceph-mgr balancer module be enabled to do so automatically when running Luminous or later releases.

Modifying the CRUSH map¶

Add/Move an OSD¶

To add or move an OSD in the CRUSH map of a running cluster:

ceph osd crush set {name} {weight} root={root} [{bucket-type}={bucket-name} ...]

Where:

name

Description: The full name of the OSD.
Type: String
Required: Yes
Example: osd.0

weight

Description: The CRUSH weight for the OSD, normally its size measure in terabytes (TB).
Type: Double
Required: Yes
Example: 2.0

root

Description: The root node of the tree in which the OSD resides (normally default)
Type: Key/value pair.
Required: Yes
Example: root=default

bucket-type

Description: You may specify the OSD’s location in the CRUSH hierarchy.
Type: Key/value pairs.
Required: No
Example: datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1

The following example adds osd.0 to the hierarchy, or moves the OSD from a previous location:

ceph osd crush set osd.0 1.0 root=default datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1

Adjust OSD weight¶

To adjust an OSD’s CRUSH weight in the CRUSH map of a running cluster, execute the following:

ceph osd crush reweight {name} {weight}

Where:

name

Description: The full name of the OSD.
Type: String
Required: Yes
Example: osd.0

weight

Description: The CRUSH weight for the OSD.
Type: Double
Required: Yes
Example: 2.0

Remove an OSD¶

To remove an OSD from the CRUSH map of a running cluster, execute the following:

ceph osd crush remove {name}

Where:

name

Description: The full name of the OSD.
Type: String
Required: Yes
Example: osd.0

Add a Bucket¶

To add a bucket in the CRUSH map of a running cluster, execute the ceph osd crush add-bucket command:

ceph osd crush add-bucket {bucket-name} {bucket-type}

Where:

bucket-name

Description: The full name of the bucket.
Type: String
Required: Yes
Example: rack12

bucket-type

Description: The type of the bucket. The type must already exist in the hierarchy.
Type: String
Required: Yes
Example: rack

The following example adds the rack12 bucket to the hierarchy:

ceph osd crush add-bucket rack12 rack

Move a Bucket¶

To move a bucket to a different location or position in the CRUSH map hierarchy, execute the following:

ceph osd crush move {bucket-name} {bucket-type}={bucket-name}, [...]

Where:

bucket-name

Description: The name of the bucket to move/reposition.
Type: String
Required: Yes
Example: foo-bar-1

bucket-type

Description: You may specify the bucket’s location in the CRUSH hierarchy.
Type: Key/value pairs.
Required: No
Example: datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1

Remove a Bucket¶

To remove a bucket from the CRUSH hierarchy, execute the following:

ceph osd crush remove {bucket-name}

Note

A bucket must be empty before removing it from the CRUSH hierarchy.

Where:

bucket-name

Description: The name of the bucket that you’d like to remove.
Type: String
Required: Yes
Example: rack12

The following example removes the rack12 bucket from the hierarchy:

ceph osd crush remove rack12

Creating a compat weight set¶

To create a compat weight set:

ceph osd crush weight-set create-compat

Weights for the compat weight set can be adjusted with:

ceph osd crush weight-set reweight-compat {name} {weight}

The compat weight set can be destroyed with:

ceph osd crush weight-set rm-compat

Creating per-pool weight sets¶

To create a weight set for a specific pool:

ceph osd crush weight-set create {pool-name} {mode}

Note

Per-pool weight sets require that all servers and daemons run Luminous v12.2.z or later.

Where:

pool-name

Description: The name of a RADOS pool
Type: String
Required: Yes
Example: rbd

mode

Description: Either flat or positional. A flat weight set has a single weight for each device or bucket. A positional weight set has a potentially different weight for each position in the resulting placement mapping. For example, if a pool has a replica count of 3, then a positional weight set will have three weights for each device and bucket.
Type: String
Required: Yes
Example: flat

To adjust the weight of an item in a weight set:

ceph osd crush weight-set reweight {pool-name} {item-name} {weight [...]}

To list existing weight sets:

ceph osd crush weight-set ls

To remove a weight set:

ceph osd crush weight-set rm {pool-name}

Creating a rule for a replicated pool¶

For a replicated pool, the primary decision when creating the CRUSH rule is what the failure domain is going to be. For example, if a failure domain of host is selected, then CRUSH will ensure that each replica of the data is stored on a unique host. If rack is selected, then each replica will be stored in a different rack. What failure domain you choose primarily depends on the size and topology of your cluster.

In most cases the entire cluster hierarchy is nested beneath a root node named default. If you have customized your hierarchy, you may want to create a rule nested at some other node in the hierarchy. It doesn’t matter what type is associated with that node (it doesn’t have to be a root node).

It is also possible to create a rule that restricts data placement to a specific class of device. By default, Ceph OSDs automatically classify themselves as either hdd or ssd, depending on the underlying type of device being used. These classes can also be customized.

To create a replicated rule:

ceph osd crush rule create-replicated {name} {root} {failure-domain-type} [{class}]

Where:

name

Description: The name of the rule
Type: String
Required: Yes
Example: rbd-rule

root

Description: The name of the node under which data should be placed.
Type: String
Required: Yes
Example: default

failure-domain-type

Description: The type of CRUSH nodes across which we should separate replicas.
Type: String
Required: Yes
Example: rack

class

Description: The device class on which data should be placed.
Type: String
Required: No
Example: ssd

Creating a rule for an erasure coded pool¶

For an erasure-coded (EC) pool, the same basic decisions need to be made: what is the failure domain, which node in the hierarchy will data be placed under (usually default), and will placement be restricted to a specific device class. Erasure code pools are created a bit differently, however, because they need to be constructed carefully based on the erasure code being used. For this reason, you must include this information in the erasure code profile. A CRUSH rule will then be created from that either explicitly or automatically when the profile is used to create a pool.

The erasure code profiles can be listed with:

ceph osd erasure-code-profile ls

An existing profile can be viewed with:

ceph osd erasure-code-profile get {profile-name}

Normally profiles should never be modified; instead, a new profile should be created and used when creating a new pool or creating a new rule for an existing pool.

An erasure code profile consists of a set of key=value pairs. Most of these control the behavior of the erasure code that is encoding data in the pool. Those that begin with crush-, however, affect the CRUSH rule that is created.

The erasure code profile properties of interest are:

crush-root: the name of the CRUSH node under which to place data [default: default].

crush-failure-domain: the CRUSH bucket type across which to distribute erasure-coded shards [default: host].

crush-device-class: the device class on which to place data [default: none, meaning all devices are used].

k and m (and, for the lrc plugin, l): these determine the number of erasure code shards, affecting the resulting CRUSH rule.

Once a profile is defined, you can create a CRUSH rule with:

ceph osd crush rule create-erasure {name} {profile-name}

Deleting rules¶

Rules that are not in use by pools can be deleted with:

ceph osd crush rule rm {rule-name}

Tunables¶

Over time, we have made (and continue to make) improvements to the CRUSH algorithm used to calculate the placement of data. In order to support the change in behavior, we have introduced a series of tunable options that control whether the legacy or improved variation of the algorithm is used.

In order to use newer tunables, both clients and servers must support the new version of CRUSH. For this reason, we have created profiles that are named after the Ceph version in which they were introduced. For example, the firefly tunables are first supported by the Firefly release, and will not work with older (e.g., Dumpling) clients. Once a given set of tunables are changed from the legacy default behavior, the ceph-mon and ceph-osd will prevent older clients who do not support the new CRUSH features from connecting to the cluster.

argonaut (legacy)¶

The legacy CRUSH behavior used by Argonaut and older releases works fine for most clusters, provided there are not many OSDs that have been marked out.

bobtail (CRUSH_TUNABLES2)¶

The bobtail tunable profile fixes a few key misbehaviors:

For hierarchies with a small number of devices in the leaf buckets, some PGs map to fewer than the desired number of replicas. This commonly happens for hierarchies with “host” nodes with a small number (1-3) of OSDs nested beneath each one.

For large clusters, some small percentages of PGs map to fewer than the desired number of OSDs. This is more prevalent when there are mutiple hierarchy layers in use (e.g., row, rack, host, osd).

When some OSDs are marked out, the data tends to get redistributed to nearby OSDs instead of across the entire hierarchy.

The new tunables are:

choose_local_tries: Number of local retries. Legacy value is 2, optimal value is 0.

choose_local_fallback_tries: Legacy value is 5, optimal value is 0.

choose_total_tries: Total number of attempts to choose an item. Legacy value was 19, subsequent testing indicates that a value of 50 is more appropriate for typical clusters. For extremely large clusters, a larger value might be necessary.

chooseleaf_descend_once: Whether a recursive chooseleaf attempt will retry, or only try once and allow the original placement to retry. Legacy default is 0, optimal value is 1.

Migration impact:

Moving from argonaut to bobtail tunables triggers a moderate amount of data movement. Use caution on a cluster that is already populated with data.

firefly (CRUSH_TUNABLES3)¶

The firefly tunable profile fixes a problem with chooseleaf CRUSH rule behavior that tends to result in PG mappings with too few results when too many OSDs have been marked out.

The new tunable is:

chooseleaf_vary_r: Whether a recursive chooseleaf attempt will start with a non-zero value of r, based on how many attempts the parent has already made. Legacy default is 0, but with this value CRUSH is sometimes unable to find a mapping. The optimal value (in terms of computational cost and correctness) is 1.

Migration impact:

For existing clusters that house lots of data, changing from 0 to 1 will cause a lot of data to move; a value of 4 or 5 will allow CRUSH to still find a valid mapping but will cause less data to move.

straw_calc_version tunable (introduced with Firefly too)¶

There were some problems with the internal weights calculated and stored in the CRUSH map for straw algorithm buckets. Specifically, when there were items with a CRUSH weight of 0, or both a mix of different and unique weights, CRUSH would distribute data incorrectly (i.e., not in proportion to the weights).

The new tunable is:

straw_calc_version: A value of 0 preserves the old, broken internal weight calculation; a value of 1 fixes the behavior.

Migration impact:

Moving to straw_calc_version 1 and then adjusting a straw bucket (by adding, removing, or reweighting an item, or by using the reweight-all command) can trigger a small to moderate amount of data movement if the cluster has hit one of the problematic conditions.

This tunable option is special because it has absolutely no impact concerning the required kernel version in the client side.

hammer (CRUSH_V4)¶

The hammer tunable profile does not affect the mapping of existing CRUSH maps simply by changing the profile. However:

There is a new bucket algorithm (straw2) supported. The new straw2 bucket algorithm fixes several limitations in the original straw. Specifically, the old straw buckets would change some mappings that should have changed when a weight was adjusted, while straw2 achieves the original goal of only changing mappings to or from the bucket item whose weight has changed.

straw2 is the default for any newly created buckets.

Migration impact:

Changing a bucket type from straw to straw2 will result in a reasonably small amount of data movement, depending on how much the bucket item weights vary from each other. When the weights are all the same no data will move, and when item weights vary significantly there will be more movement.

jewel (CRUSH_TUNABLES5)¶

The jewel tunable profile improves the overall behavior of CRUSH such that significantly fewer mappings change when an OSD is marked out of the cluster. This results in significantly less data movement.

The new tunable is:

chooseleaf_stable: Whether a recursive chooseleaf attempt will use a better value for an inner loop that greatly reduces the number of mapping changes when an OSD is marked out. The legacy value is 0, while the new value of 1 uses the new approach.

Migration impact:

Changing this value on an existing cluster will result in a very large amount of data movement as almost every PG mapping is likely to change.

Which client versions support CRUSH_TUNABLES¶

argonaut series, v0.48.1 or later

v0.49 or later

Linux kernel version v3.6 or later (for the file system and RBD kernel clients)

Which client versions support CRUSH_TUNABLES2¶

v0.55 or later, including bobtail series (v0.56.x)

Linux kernel version v3.9 or later (for the file system and RBD kernel clients)

Which client versions support CRUSH_TUNABLES3¶

v0.78 (firefly) or later

Linux kernel version v3.15 or later (for the file system and RBD kernel clients)

Which client versions support CRUSH_V4¶

v0.94 (hammer) or later

Linux kernel version v4.1 or later (for the file system and RBD kernel clients)

Which client versions support CRUSH_TUNABLES5¶

v10.0.2 (jewel) or later

Linux kernel version v4.5 or later (for the file system and RBD kernel clients)

Warning when tunables are non-optimal¶

Starting with version v0.74, Ceph will issue a health warning if the current CRUSH tunables don’t include all the optimal values from the default profile (see below for the meaning of the default profile). To make this warning go away, you have two options:

Adjust the tunables on the existing cluster. Note that this will result in some data movement (possibly as much as 10%). This is the preferred route, but should be taken with care on a production cluster where the data movement may affect performance. You can enable optimal tunables with:
```
ceph osd crush tunables optimal
```
If things go poorly (e.g., too much load) and not very much progress has been made, or there is a client compatibility problem (old kernel CephFS or RBD clients, or pre-Bobtail librados clients), you can switch back with:
```
ceph osd crush tunables legacy
```
You can make the warning go away without making any changes to CRUSH by adding the following option to your ceph.conf [mon] section:
```
mon warn on legacy crush tunables = false
```
For the change to take effect, you will need to restart the monitors, or apply the option to running monitors with:
```
ceph tell mon.\* config set mon_warn_on_legacy_crush_tunables false
```

A few important points¶

Adjusting these values will result in the shift of some PGs between storage nodes. If the Ceph cluster is already storing a lot of data, be prepared for some fraction of the data to move.

The ceph-osd and ceph-mon daemons will start requiring the feature bits of new connections as soon as they get the updated map. However, already-connected clients are effectively grandfathered in, and will misbehave if they do not support the new feature.

If the CRUSH tunables are set to non-legacy values and then later changed back to the default values, ceph-osd daemons will not be required to support the feature. However, the OSD peering process requires examining and understanding old maps. Therefore, you should not run old versions of the ceph-osd daemon if the cluster has previously used non-legacy CRUSH values, even if the latest version of the map has been switched back to using the legacy defaults.

Tuning CRUSH¶

The simplest way to adjust CRUSH tunables is by applying them in matched sets known as profiles. As of the Octopus release these are:

legacy: the legacy behavior from argonaut and earlier.

argonaut: the legacy values supported by the original argonaut release

bobtail: the values supported by the bobtail release

firefly: the values supported by the firefly release

hammer: the values supported by the hammer release

jewel: the values supported by the jewel release

optimal: the best (i.e. optimal) values of the current version of Ceph

default: the default values of a new cluster installed from scratch. These values, which depend on the current version of Ceph, are hardcoded and are generally a mix of optimal and legacy values. These values generally match the optimal profile of the previous LTS release, or the most recent release for which we generally expect most users to have up-to-date clients for.

You can apply a profile to a running cluster with the command:

ceph osd crush tunables {PROFILE}

Note that this may result in data movement, potentially quite a bit. Study release notes and documentation carefully before changing the profile on a running cluster, and consider throttling recovery/backfill parameters to limit the impact of a bolus of backfill.

Primary Affinity¶

When a Ceph Client reads or writes data, it first contacts the primary OSD in each affected PG’s acting set. By default, the first OSD in the acting set is the primary. For example, in the acting set [2, 3, 4], osd.2 is listed first and thus is the primary (aka lead) OSD. Sometimes we know that an OSD is less well suited to act as the lead than are other OSDs (e.g., it has a slow drive or a slow controller). To prevent performance bottlenecks (especially on read operations) while maximizing utilization of your hardware, you can influence the selection of primary OSDs by adjusting primary affinity values, or by crafting a CRUSH rule that selects preferred OSDs first.

Tuning primary OSD selection is mainly useful for replicated pools, because by default read operations are served from the primary OSD for each PG. For erasure coded (EC) pools, a way to speed up read operations is to enable fast read as described in Pool settings.

A common scenario for primary affinity is when a cluster contains a mix of drive sizes, for example older racks with 1.9 TB SATA SSDS and newer racks with 3.84TB SATA SSDs. On average the latter will be assigned double the number of PGs and thus will serve double the number of write and read operations, thus they’ll be busier than the former. A rough assignment of primary affinity inversely proportional to OSD size won’t be 100% optimal, but it can readily achieve a 15% improvement in overall read throughput by utilizing SATA interface bandwidth and CPU cycles more evenly.

By default, all ceph OSDs have primary affinity of 1, which indicates that any OSD may act as a primary with equal probability.

You can reduce a Ceph OSD’s primary affinity so that CRUSH is less likely to choose the OSD as primary in a PG’s acting set.:

ceph osd primary-affinity <osd-id> <weight>

You may set an OSD’s primary affinity to a real number in the range [0-1], where 0 indicates that the OSD may NOT be used as a primary and 1 indicates that an OSD may be used as a primary. When the weight is between these extremes, it is less likely that CRUSH will select that OSD as a primary. The process for selecting the lead OSD is more nuanced than a simple probability based on relative affinity values, but measurable results can be achieved even with first-order approximations of desirable values.

Custom CRUSH Rules¶

There are occasional clusters that balance cost and performance by mixing SSDs and HDDs in the same replicated pool. By setting the primary affinity of HDD OSDs to 0 one can direct operations to the SSD in each acting set. An alternative is to define a CRUSH rule that always selects an SSD OSD as the first OSD, then selects HDDs for the remaining OSDs. Thus, each PG’s acting set will contain exactly one SSD OSD as the primary with the balance on HDDs.

For example, the CRUSH rule below:

rule mixed_replicated_rule {
        id 11
        type replicated
        min_size 1
        max_size 10
        step take default class ssd
        step chooseleaf firstn 1 type host
        step emit
        step take default class hdd
        step chooseleaf firstn 0 type host
        step emit
}

chooses an SSD as the first OSD. Note that for an N-times replicated pool this rule selects N+1 OSDs to guarantee that N copies are on different hosts, because the first SSD OSD might be co-located with any of the N HDD OSDs.

This extra storage requirement can be avoided by placing SSDs and HDDs in different hosts with the tradeoff that hosts with SSDs will receive all client requests. You may thus consider faster CPU(s) for SSD hosts and more modest ones for HDD nodes, since the latter will normally only service recovery operations. Here the CRUSH roots ssd_hosts and hdd_hosts strictly must not contain the same servers:

rule mixed_replicated_rule_two {
       id 1
       type replicated
       min_size 1
       max_size 10
       step take ssd_hosts class ssd
       step chooseleaf firstn 1 type host
       step emit
       step take hdd_hosts class hdd
       step chooseleaf firstn -1 type host
       step emit
}

Note also that on failure of an SSD, requests to a PG will be served temporarily from a (slower) HDD OSD until the PG’s data has been replicated onto the replacement primary SSD OSD.