Chapter 2. Crush admin overview The Controlled Replication Under Scalable Hashing (CRUSH) algorithm determines how to store and retrieve data by computing data storage locations. Any sufficiently advanced technology is indistinguishable from magic. -- Arthur C. Clarke 2.1. Crush introduction The CRUSH map for your storage cluster describes your device locations within CRUSH hierarchies and a rule for each hierarchy that determines how Ceph stores data. The CRUSH map contains at least one hierarchy of nodes and leaves. The nodes of a hierarchy, called "buckets" in Ceph, are any aggregation of storage locations as defined by their type. For example, rows, racks, chassis, hosts, and devices. Each leaf of the hierarchy consists essentially of one of the storage devices in the list of storage devices. A leaf is always contained in one node or "bucket." A CRUSH map also has a list of rules that determine how CRUSH stores and retrieves data. Note Storage devices are added to the CRUSH map when adding an OSD to the cluster. The CRUSH algorithm distributes data objects among storage devices according to a per-device weight value, approximating a uniform probability distribution. CRUSH distributes objects and their replicas or erasure-coding chunks according to the hierarchical cluster map an administrator defines. The CRUSH map represents the available storage devices and the logical buckets that contain them for the rule, and by extension each pool that uses the rule. To map placement groups to OSDs across failure domains or performance domains, a CRUSH map defines a hierarchical list of bucket types; that is, under types in the generated CRUSH map. The purpose of creating a bucket hierarchy is to segregate the leaf nodes by their failure domains or performance domains or both. Failure domains include hosts, chassis, racks, power distribution units, pods, rows, rooms, and data centers. Performance domains include failure domains and OSDs of a particular configuration. For example, SSDs, SAS drives with SSD journals, SATA drives, and so on. Devices have the notion of a class , such as hdd , ssd and nvme to more rapidly build CRUSH hierarchies with a class of devices. With the exception of the leaf nodes representing OSDs, the rest of the hierarchy is arbitrary, and you can define it according to your own needs if the default types do not suit your requirements. We recommend adapting your CRUSH map bucket types to your organization's hardware naming conventions and using instance names that reflect the physical hardware names. Your naming practice can make it easier to administer the cluster and troubleshoot problems when an OSD or other hardware malfunctions and the administrator needs remote or physical access to the host or other hardware. In the following example, the bucket hierarchy has four leaf buckets ( osd 1-4 ), two node buckets ( host 1-2 ) and one rack node ( rack 1 ). Since leaf nodes reflect storage devices declared under the devices list at the beginning of the CRUSH map, there is no need to declare them as bucket instances. The second lowest bucket type in the hierarchy usually aggregates the devices; that is, it is usually the computer containing the storage media, and uses whatever term administrators prefer to describe it, such as "node", "computer", "server," "host", "machine", and so on. In high density environments, it is increasingly common to see multiple hosts/nodes per card and per chassis. Make sure to account for card and chassis failure too, for example, the need to pull a card or chassis if a node fails can result in bringing down numerous hosts/nodes and their OSDs. When declaring a bucket instance, specify its type, give it a unique name as a string, assign it an optional unique ID expressed as a negative integer, specify a weight relative to the total capacity or capability of its items, specify the bucket algorithm such as straw2 , and the hash that is usually 0 reflecting hash algorithm rjenkins1 . A bucket can have one or more items. The items can consist of node buckets or leaves. Items can have a weight that reflects the relative weight of the item. 2.1.1. Dynamic data placement Ceph Clients and Ceph OSDs both use the CRUSH map and the CRUSH algorithm. Ceph Clients: By distributing CRUSH maps to Ceph clients, CRUSH empowers Ceph clients to communicate with OSDs directly. This means that Ceph clients avoid a centralized object look-up table that could act as a single point of failure, a performance bottleneck, a connection limitation at a centralized look-up server and a physical limit to the storage cluster's scalability. Ceph OSDs: By distributing CRUSH maps to Ceph OSDs, Ceph empowers OSDs to handle replication, backfilling and recovery. This means that the Ceph OSDs handle storage of object replicas (or coding chunks) on behalf of the Ceph client. It also means that Ceph OSDs know enough about the cluster to re-balance the cluster (backfilling) and recover from failures dynamically. 2.1.2. CRUSH failure domain Having multiple object replicas or M erasure coding chunks helps prevent data loss, but it is not sufficient to address high availability. By reflecting the underlying physical organization of the Ceph Storage Cluster, CRUSH can model-and thereby address-potential sources of correlated device failures. By encoding the cluster's topology into the cluster map, CRUSH placement policies can separate object replicas or erasure coding chunks across different failure domains while still maintaining the desired pseudo-random distribution. For example, to address the possibility of concurrent failures, it might be desirable to ensure that data replicas or erasure coding chunks are on devices using different shelves, racks, power supplies, controllers or physical locations. This helps to prevent data loss and allows the cluster to operate in a degraded state. 2.1.3. CRUSH performance domain Ceph can support multiple hierarchies to separate one type of hardware performance profile from another type of hardware performance profile. For example, CRUSH can create one hierarchy for hard disk drives and another hierarchy for SSDs. Performance domains- hierarchies that take the performance profile of the underlying hardware into consideration- are increasingly popular due to the need to support different performance characteristics. Operationally, these are just CRUSH maps with more than one root type bucket. Use case examples include: Object Storage: Ceph hosts that serve as an object storage back end for S3 and Swift interfaces might take advantage of less expensive storage media such as SATA drives that might not be suitable for VMs- reducing the cost per gigabyte for object storage, while separating more economical storage hosts from more performing ones intended for storing volumes and images on cloud platforms. HTTP tends to be the bottleneck in object storage systems. Cold Storage : Systems designed for cold storage- infrequently accessed data, or data retrieval with relaxed performance requirements- might take advantage of less expensive storage media and erasure coding. However, erasure coding might require a bit of additional RAM and CPU, and thus differ in RAM and CPU requirements from a host used for object storage or VMs. SSD-backed Pools: SSDs are expensive, but they provide significant advantages over hard disk drives. SSDs have no seek time and they provide high total throughput. In addition to using SSDs for journaling, a cluster can support SSD-backed pools. Common use cases include high performance SSD pools. For example, it is possible to map the .rgw.buckets.index pool for the Ceph Object Gateway to SSDs instead of SATA drives. A CRUSH map supports the notion of a device class . Ceph can discover aspects of a storage device and automatically assign a class such as hdd , ssd or nvme . However, CRUSH is not limited to these defaults. For example, CRUSH hierarchies might also be used to separate different types of workloads. For example, an SSD might be used for a journal or write-ahead log, a bucket index or for raw object storage. CRUSH can support different device classes, such as ssd-bucket-index or ssd-object-storage so Ceph does not use the same storage media for different workloads- making performance more predictable and consistent. Behind the scenes, Ceph generates a crush root for each device-class. These roots should only be modified by setting or changing device classes on OSDs. You can view the generated roots using the following command: Example 2.2. CRUSH hierarchy The CRUSH map is a directed acyclic graph, so it can accommodate multiple hierarchies, for example, performance domains. The easiest way to create and modify a CRUSH hierarchy is with the Ceph CLI; however, you can also decompile a CRUSH map, edit it, recompile it, and activate it. When declaring a bucket instance with the Ceph CLI, you must specify its type and give it a unique string name. Ceph automatically assigns a bucket ID, sets the algorithm to straw2 , sets the hash to 0 reflecting rjenkins1 and sets a weight. When modifying a decompiled CRUSH map, assign the bucket a unique ID expressed as a negative integer (optional), specify a weight relative to the total capacity/capability of its item(s), specify the bucket algorithm (usually straw2 ), and the hash (usually 0 , reflecting hash algorithm rjenkins1 ). A bucket can have one or more items. The items can consist of node buckets (for example, racks, rows, hosts) or leaves (for example, an OSD disk). Items can have a weight that reflects the relative weight of the item. When modifying a decompiled CRUSH map, you can declare a node bucket with the following syntax: For example, using the diagram above, we would define two host buckets and one rack bucket. The OSDs are declared as items within the host buckets: Note In the foregoing example, note that the rack bucket does not contain any OSDs. Rather it contains lower level host buckets, and includes the sum total of their weight in the item entry. 2.2.1. CRUSH location A CRUSH location is the position of an OSD in terms of the CRUSH map's hierarchy. When you express a CRUSH location on the command line interface, a CRUSH location specifier takes the form of a list of name/value pairs describing the OSD's position. For example, if an OSD is in a particular row, rack, chassis and host, and is part of the default CRUSH tree, its crush location could be described as: Note: 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 . You might edit the CRUSH map to change the types to suit your needs. You do not need to specify all the buckets/keys. For example, by default, Ceph automatically sets a ceph-osd daemon's location to be root=default host={HOSTNAME} (based on the output from hostname -s ). 2.2.2. Adding a bucket To add a bucket instance to your CRUSH hierarchy, specify the bucket name and its type. Bucket names must be unique in the CRUSH map. If you plan to use multiple hierarchies, for example, for different hardware performance profiles, consider naming buckets based on their type of hardware or use case. For example, you could create a hierarchy for solid state drives ( ssd ), a hierarchy for SAS disks with SSD journals ( hdd-journal ), and another hierarchy for SATA drives ( hdd ): The Ceph CLI outputs: Important Using colons (:) in bucket names is not supported. Add an instance of each bucket type you need for your hierarchy. The following example demonstrates adding buckets for a row with a rack of SSD hosts and a rack of hosts for object storage. Once you have completed these steps, view your tree. Notice that the hierarchy remains flat. You must move your buckets into a hierarchical position after you add them to the CRUSH map. 2.2.3. Moving a bucket When you create your initial cluster, Ceph has a default CRUSH map with a root bucket named default and your initial OSD hosts appear under the default bucket. When you add a bucket instance to your CRUSH map, it appears in the CRUSH hierarchy, but it does not necessarily appear under a particular bucket. To move a bucket instance to a particular location in your CRUSH hierarchy, specify the bucket name and its type. For example: Once you have completed these steps, you can view your tree. Note You can also use ceph osd crush create-or-move to create a location while moving an OSD. 2.2.4. Removing a bucket To remove a bucket instance from your CRUSH hierarchy, specify the bucket name. For example: Or: Note The bucket must be empty in order to remove it. If you are removing higher level buckets (for example, a root like default ), check to see if a pool uses a CRUSH rule that selects that bucket. If so, you need to modify your CRUSH rules; otherwise, peering fails. 2.2.5. CRUSH Bucket algorithms When you create buckets using the Ceph CLI, Ceph sets the algorithm to straw2 by default. Ceph supports four bucket algorithms, each representing a tradeoff between performance and reorganization efficiency. If you are unsure of which bucket type to use, we recommend using a straw2 bucket. The bucket algorithms are: Uniform: Uniform buckets aggregate devices with exactly the same weight. For example, when firms commission or decommission hardware, they typically do so with many machines that have exactly the same physical configuration (for example, bulk purchases). When storage devices have exactly the same weight, you can use the uniform bucket type, which allows CRUSH to map replicas into uniform buckets in constant time. With non-uniform weights, you should use another bucket algorithm. List : List buckets aggregate their content as linked lists. Based on the RUSH (Replication Under Scalable Hashing) P algorithm, a list is a natural and intuitive choice for an expanding cluster : either an object is relocated to the newest device with some appropriate probability, or it remains on the older devices as before. The result is optimal data migration when items are added to the bucket. Items removed from the middle or tail of the list, however, can result in a significant amount of unnecessary movement, making list buckets most suitable for circumstances in which they never, or very rarely shrink . Tree : Tree buckets use a binary search tree. They are more efficient than listing buckets when a bucket contains a larger set of items. Based on the RUSH (Replication Under Scalable Hashing) R algorithm, tree buckets reduce the placement time to zero (log n ), making them suitable for managing much larger sets of devices or nested buckets. Straw2 (default): List and Tree buckets use a divide and conquer strategy in a way that either gives certain items precedence, for example, those at the beginning of a list or obviates the need to consider entire subtrees of items at all. That improves the performance of the replica placement process, but can also introduce suboptimal reorganization behavior when the contents of a bucket change due an addition, removal, or re-weighting of an item. The straw2 bucket type allows all items to fairly "compete" against each other for replica placement through a process analogous to a draw of straws. 2.3. Ceph OSDs in CRUSH Once you have a CRUSH hierarchy for the OSDs, add OSDs to the CRUSH hierarchy. You can also move or remove OSDs from an existing hierarchy. The Ceph CLI usage has the following values: id Description The numeric ID of the OSD. Type Integer Required Yes Example 0 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 root Description The name of the root bucket of the hierarchy or tree in which the OSD resides. Type Key-value pair. Required Yes Example root=default , root=replicated_rule , and so on bucket-type Description One or more name-value pairs, where the name is the bucket type and the value is the bucket's name. You can specify a CRUSH location for an OSD in the CRUSH hierarchy. Type Key-value pairs. Required No Example datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1 2.3.1. Viewing OSDs in CRUSH The ceph osd crush tree command prints CRUSH buckets and items in a tree view. Use this command to determine a list of OSDs in a particular bucket. It will print output similar to ceph osd tree . To return additional details, execute the following: The command returns an output similar to the following: 2.3.2. Adding an OSD to CRUSH Adding a Ceph OSD to a CRUSH hierarchy is the final step before you might start an OSD (rendering it up and in ) and Ceph assigns placement groups to the OSD. You must prepare a Ceph OSD before you add it to the CRUSH hierarchy. Deployment utilities, such as the Ceph Orchestrator, can perform this step for you. For example creating a Ceph OSD on a single node: Syntax The CRUSH hierarchy is notional, so the ceph osd crush add command allows you to add OSDs to the CRUSH hierarchy wherever you wish. The location you specify should reflect its actual location. If you specify at least one bucket, the command places the OSD into the most specific bucket you specify, and it moves that bucket underneath any other buckets you specify. To add an OSD to a CRUSH hierarchy: Syntax Important If you specify only the root bucket, the command attaches the OSD directly to the root. However, CRUSH rules expect OSDs to be inside of hosts or chassis, and host or chassis should be inside of other buckets reflecting your cluster topology. The following example adds osd.0 to the hierarchy: Note You can also use ceph osd crush set or ceph osd crush create-or-move to add an OSD to the CRUSH hierarchy. 2.3.3. Moving an OSD within a CRUSH Hierarchy If the storage cluster topology changes, you can move an OSD in the CRUSH hierarchy to reflect its actual location. Important Moving an OSD in the CRUSH hierarchy means that Ceph will recompute which placement groups get assigned to the OSD, potentially resulting in significant redistribution of data. To move an OSD within the CRUSH hierarchy: Syntax Note You can also use ceph osd crush create-or-move to move an OSD within the CRUSH hierarchy. 2.3.4. Removing an OSD from a CRUSH Hierarchy Removing an OSD from a CRUSH hierarchy is the first step when you want to remove an OSD from your cluster. When you remove the OSD from the CRUSH map, CRUSH recomputes which OSDs get the placement groups and data re-balances accordingly. See Adding/Removing OSDs for additional details. To remove an OSD from the CRUSH map of a running cluster, execute the following: Syntax 2.4. Device class Ceph's CRUSH map provides extraordinary flexibility in controlling data placement. This is one of Ceph's greatest strengths. Early Ceph deployments used hard disk drives almost exclusively. Today, Ceph clusters are frequently built with multiple types of storage devices: HDD, SSD, NVMe, or even various classes of the foregoing. For example, it is common in Ceph Object Gateway deployments to have storage policies where clients can store data on slower HDDs and other storage policies for storing data on fast SSDs. Ceph Object Gateway deployments might even have a pool backed by fast SSDs for bucket indices. Additionally, OSD nodes also frequently have SSDs used exclusively for journals or write-ahead logs that do NOT appear in the CRUSH map. These complex hardware scenarios historically required manually editing the CRUSH map, which can be time-consuming and tedious. It is not required to have different CRUSH hierarchies for different classes of storage devices. CRUSH rules work in terms of the CRUSH hierarchy. However, if different classes of storage devices reside in the same hosts, the process becomes more complicated- requiring users to create multiple CRUSH hierarchies for each class of device, and then disable the osd crush update on start option that automates much of the CRUSH hierarchy management. Device classes eliminate this tediousness by telling the CRUSH rule what class of device to use, dramatically simplifying CRUSH management tasks. Note The ceph osd tree command has a column reflecting a device class. 2.4.1. Setting a device class To set a device class for an OSD, execute the following: Syntax Example Note Ceph might assign a class to a device automatically. However, class names are simply arbitrary strings. There is no requirement to adhere to hdd , ssd or nvme . In the foregoing example, a device class named bucket-index might indicate an SSD device that a Ceph Object Gateway pool uses exclusively bucket index workloads. To change a device class that was already set, use ceph osd crush rm-device-class first. 2.4.2. Removing a device class To remove a device class for an OSD, execute the following: Syntax Example 2.4.3. Renaming a device class To rename a device class for all OSDs that use that class, execute the following: Syntax Example 2.4.4. Listing a device class To list device classes in the CRUSH map, execute the following: Syntax The output will look something like this: Example 2.4.5. Listing OSDs of a device class To list all OSDs that belong to a particular class, execute the following: Syntax Example The output is simply a list of OSD numbers. For example: 2.4.6. Listing CRUSH Rules by Class To list all crush rules that reference the same class, execute the following: Syntax Example 2.5. CRUSH weights The CRUSH algorithm assigns a weight value in terabytes (by convention) per OSD device with the objective of approximating a uniform probability distribution for write requests that assign new data objects to PGs and PGs to OSDs. For this reason, as a best practice, we recommend creating CRUSH hierarchies with devices of the same type and size, and assigning the same weight. We also recommend using devices with the same I/O and throughput characteristics so that you will also have uniform performance characteristics in your CRUSH hierarchy, even though performance characteristics do not affect data distribution. Since using uniform hardware is not always practical, you might incorporate OSD devices of different sizes and use a relative weight so that Ceph will distribute more data to larger devices and less data to smaller devices. 2.5.1. Setting CRUSH weights of OSDs To set an OSD CRUSH weight in Terabytes within the CRUSH map, execute the following command Where: name Description The full name of the OSD. Type String Required Yes Example osd.0 weight Description The CRUSH weight for the OSD. This should be the size of the OSD in Terabytes, where 1.0 is 1 Terabyte. Type Double Required Yes Example 2.0 This setting is used when creating an OSD or adjusting the CRUSH weight immediately after adding the OSD. It usually does not change over the life of the OSD. 2.5.2. Setting a Bucket's OSD Weights Using ceph osd crush reweight can be time-consuming. You can set (or reset) all Ceph OSD weights under a bucket (row, rack, node, and so on) by executing: Syntax Where, name is the name of the CRUSH bucket. 2.5.3. Set an OSD's in Weight For the purposes of ceph osd in and ceph osd out , an OSD is either in the cluster or out of the cluster. That is how a monitor records an OSD's status. However, even though an OSD is in the cluster, it might be experiencing a malfunction such that you do not want to rely on it as much until you fix it (for example, replace a storage drive, change out a controller, and so on). You can increase or decrease the in weight of a particular OSD (that is, without changing its weight in Terabytes) by executing: Syntax Where: id is the OSD number. weight is a range from 0.0-1.0, where 0 is not in the cluster (that is, it does not have any PGs assigned to it) and 1.0 is in the cluster (that is, the OSD receives the same number of PGs as other OSDs). 2.5.4. Setting the OSDs weight by utilization CRUSH is designed to approximate a uniform probability distribution for write requests that assign new data objects PGs and PGs to OSDs. However, a cluster might become imbalanced anyway. This can happen for a number of reasons. For example: Multiple Pools: You can assign multiple pools to a CRUSH hierarchy, but the pools might have different numbers of placement groups, size (number of replicas to store), and object size characteristics. Custom Clients: Ceph clients such as block device, object gateway and filesystem share data from their clients and stripe the data as objects across the cluster as uniform-sized smaller RADOS objects. So except for the foregoing scenario, CRUSH usually achieves its goal. However, there is another case where a cluster can become imbalanced: namely, using librados to store data without normalizing the size of objects. This scenario can lead to imbalanced clusters (for example, storing 100 1 MB objects and 10 4 MB objects will make a few OSDs have more data than the others). Probability: A uniform distribution will result in some OSDs with more PGs and some with less. For clusters with a large number of OSDs, the statistical outliers will be further out. You can reweight OSDs by utilization by executing the following: Syntax Example Where: threshold is a percentage of utilization such that OSDs facing higher data storage loads will receive a lower weight and thus fewer PGs assigned to them. The default value is 120 , reflecting 120%. Any value from 100+ is a valid threshold. Optional. weight_change_amount is the amount to change the weight. Valid values are greater than 0.0 - 1.0 . The default value is 0.05 . Optional. number_of_OSDs is the maximum number of OSDs to reweight. For large clusters, limiting the number of OSDs to reweight prevents significant rebalancing. Optional. no-increasing is off by default. Increasing the osd weight is allowed when using the reweight-by-utilization or test-reweight-by-utilization commands. If this option is used with these commands, it prevents the OSD weight from increasing, even if the OSD is underutilized. Optional. Important Executing reweight-by-utilization is recommended and somewhat inevitable for large clusters. Utilization rates might change over time, and as your cluster size or hardware changes, the weightings might need to be updated to reflect changing utilization. If you elect to reweight by utilization, you might need to re-run this command as utilization, hardware or cluster size change. Executing this or other weight commands that assign a weight will override the weight assigned by this command (for example, osd reweight-by-utilization , osd crush weight , osd weight , in or out ). 2.5.5. Setting an OSD's Weight by PG distribution In CRUSH hierarchies with a smaller number of OSDs, it's possible for some OSDs to get more PGs than other OSDs, resulting in a higher load. You can reweight OSDs by PG distribution to address this situation by executing the following: Syntax Where: poolname is the name of the pool. Ceph will examine how the pool assigns PGs to OSDs and reweight the OSDs according to this pool's PG distribution. Note that multiple pools could be assigned to the same CRUSH hierarchy. Reweighting OSDs according to one pool's distribution could have unintended effects for other pools assigned to the same CRUSH hierarchy if they do not have the same size (number of replicas) and PGs. 2.5.6. Recalculating a CRUSH Tree's weights CRUSH tree buckets should be the sum of their leaf weights. If you manually edit the CRUSH map weights, you should execute the following to ensure that the CRUSH bucket tree accurately reflects the sum of the leaf OSDs under the bucket. Syntax 2.6. Primary affinity When a Ceph Client reads or writes data, it always contacts the primary OSD in the acting set. For set [2, 3, 4] , osd.2 is the primary. Sometimes an OSD is not well suited to act as a primary compared to other OSDs (for example, it has a slow disk or a slow controller). To prevent performance bottlenecks (especially on read operations) while maximizing utilization of your hardware, you can set a Ceph OSD's primary affinity so that CRUSH is less likely to use the OSD as a primary in an acting set. : Syntax Primary affinity is 1 by default ( that is, an OSD might act as a primary). You might set the OSD primary range from 0-1 , where 0 means that the OSD might NOT be used as a primary and 1 means that an OSD might be used as a primary. When the weight is < 1 , it is less likely that CRUSH will select the Ceph OSD Daemon to act as a primary. 2.7. CRUSH rules CRUSH rules define how a Ceph client selects buckets and the primary OSD within them to store objects, and how the primary OSD selects buckets and the secondary OSDs to store replicas or coding chunks. For example, you might create a rule that selects a pair of target OSDs backed by SSDs for two object replicas, and another rule that selects three target OSDs backed by SAS drives in different data centers for three replicas. A rule takes the following form: id Description A unique whole number for identifying the rule. Purpose A component of the rule mask. Type Integer Required Yes Default 0 type Description Describes a rule for either a storage drive replicated or erasure coded. Purpose A component of the rule mask. Type String Required Yes Default replicated Valid Values Currently only replicated min_size Description If a pool makes fewer replicas than this number, CRUSH will not select this rule. Type Integer Purpose A component of the rule mask. Required Yes Default 1 max_size Description If a pool makes more replicas than this number, CRUSH will not select this rule. Type Integer Purpose A component of the rule mask. Required Yes Default 10 step take <bucket-name> [class <class-name>] Description Takes a bucket name, and begins iterating down the tree. Purpose A component of the rule. Required Yes Example step take data step take data class ssd step choose firstn <num> type <bucket-type> Description Selects the number of buckets of the given type. The number is usually the number of replicas in the pool (that is, pool size). If <num> == 0 , choose pool-num-replicas buckets (all available). If <num> > 0 && < pool-num-replicas , choose that many buckets. If <num> < 0 , it means pool-num-replicas - {num} . Purpose A component of the rule. Prerequisite Follow step take or step choose . Example step choose firstn 1 type row step chooseleaf firstn <num> type <bucket-type> Description Selects a set of buckets of {bucket-type} and chooses a leaf node from the subtree of each bucket in the set of buckets. The number of buckets in the set is usually the number of replicas in the pool (that is, pool size). If <num> == 0 , choose pool-num-replicas buckets (all available). If <num> > 0 && < pool-num-replicas , choose that many buckets. If <num> < 0 , it means pool-num-replicas - <num> . Purpose A component of the rule. Usage removes the need to select a device using two steps. Prerequisite Follows step take or step choose . Example step chooseleaf firstn 0 type row step emit Description Outputs the current value and empties the stack. Typically used at the end of a rule, but might also be used to pick from different trees in the same rule. Purpose A component of the rule. Prerequisite Follows step choose . Example step emit firstn versus indep Description Controls the replacement strategy CRUSH uses when OSDs are marked down in the CRUSH map. If this rule is to be used with replicated pools it should be firstn and if it is for erasure-coded pools it should be indep . Example You have a PG stored on OSDs 1, 2, 3, 4, 5 in which 3 goes down.. In the first scenario, with the firstn mode, CRUSH adjusts its calculation to select 1 and 2, then selects 3 but discovers it is down, so it retries and selects 4 and 5, and then goes on to select a new OSD 6. The final CRUSH mapping change is from 1, 2, 3, 4, 5 to 1, 2, 4, 5, 6 . In the second scenario, with indep mode on an erasure-coded pool, CRUSH attempts to select the failed OSD 3, tries again and picks out 6, for a final transformation from 1, 2, 3, 4, 5 to 1, 2, 6, 4, 5 . Important A given CRUSH rule can be assigned to multiple pools, but it is not possible for a single pool to have multiple CRUSH rules. 2.7.1. Listing CRUSH rules To list CRUSH rules from the command line, execute the following: Syntax 2.7.2. Dumping CRUSH rules To dump the contents of a specific CRUSH rule, execute the following: Syntax 2.7.3. Adding CRUSH rules To add a CRUSH rule, you must specify a rule name, the root node of the hierarchy you wish to use, the type of bucket you want to replicate across (for example, 'rack', 'row', and so on and the mode for choosing the bucket. Syntax Ceph creates a rule with chooseleaf and one bucket of the type you specify. Example Create the following rule: 2.7.4. Creating CRUSH rules for replicated pools To create a CRUSH rule for a replicated pool, execute the following: Syntax Where: <name> : The name of the rule. <root> : The root of the CRUSH hierarchy. <failure-domain> : The failure domain. For example: host or rack . <class> : The storage device class. For example: hdd or ssd . Example 2.7.5. Creating CRUSH rules for erasure coded pools To add a CRUSH rule for use with an erasure coded pool, you might specify a rule name and an erasure code profile. Syntax Example Additional Resources See Erasure code profiles for more details. 2.7.6. Removing CRUSH rules To remove a rule, execute the following and specify the CRUSH rule name: Syntax 2.8. CRUSH tunables overview The Ceph project has grown exponentially with many changes and many new features. Beginning with the first commercially supported major release of Ceph, v0.48 (Argonaut), Ceph provides the ability to adjust certain parameters of the CRUSH algorithm, that is, the settings are not frozen in the source code. A few important points to consider: Adjusting CRUSH values might 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 receive an updated map. However, already-connected clients are effectively grandfathered in, and will misbehave if they do not support the new feature. Make sure when you upgrade your Ceph Storage Cluster daemons that you also update your Ceph clients. If the CRUSH tunables are set to non-legacy values and then later changed back to the legacy 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. 2.8.1. CRUSH tuning Before you tune CRUSH, you should ensure that all Ceph clients and all Ceph daemons use the same version. If you have recently upgraded, ensure that you have restarted daemons and reconnected clients. The simplest way to adjust the CRUSH tunables is by changing to a known profile. Those are: legacy : The legacy behavior from v0.47 (pre-Argonaut) and earlier. argonaut : The legacy values supported by v0.48 (Argonaut) release. bobtail : The values supported by the v0.56 (Bobtail) release. firefly : The values supported by the v0.80 (Firefly) release. hammer : The values supported by the v0.94 (Hammer) release. jewel : The values supported by the v10.0.2 (Jewel) release. optimal : The current best values. default : The current default values for a new cluster. You can select a profile on a running cluster with the command: Syntax Note This might result in some data movement. Generally, you should set the CRUSH tunables after you upgrade, or if you receive a warning. Starting with version v0.74, Ceph issues a health warning if the CRUSH tunables are not set to their optimal values, the optimal values are the default as of v0.73. You can remove the warning by adjusting 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 might affect performance. You can enable optimal tunables with: If things go poorly (for example, 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 to an earlier profile: Syntax For example, to restore the pre-v0.48 (Argonaut) values, execute: Example 2.8.2. CRUSH tuning, the hard way If you can ensure that all clients are running recent code, you can adjust the tunables by extracting the CRUSH map, modifying the values, and reinjecting it into the cluster. Extract the latest CRUSH map: Adjust tunables. These values appear to offer the best behavior for both large and small clusters we tested with. You will need to additionally specify the --enable-unsafe-tunables argument to crushtool for this to work. Please use this option with extreme care.: Reinject modified map: 2.8.3. CRUSH legacy values For reference, the legacy values for the CRUSH tunables can be set with: Again, the special --enable-unsafe-tunables option is required. Further, as noted above, be careful running old versions of the ceph-osd daemon after reverting to legacy values as the feature bit is not perfectly enforced. 2.9. Edit a CRUSH map Generally, modifying your CRUSH map at runtime with the Ceph CLI is more convenient than editing the CRUSH map manually. However, there are times when you might choose to edit it, such as changing the default bucket types, or using a bucket algorithm other than straw2 . To edit an existing CRUSH map: Getting the CRUSH map . Decompiling the CRUSH map . Edit at least one of the devices, and buckets and rules. Compile the CRUSH map Setting a CRUSH map . To activate a CRUSH Map rule for a specific pool, identify the common rule number and specify that rule number for the pool when creating the pool. 2.9.1. Getting the CRUSH map To get the CRUSH map for your cluster, execute the following: Syntax Ceph will output (-o) a compiled CRUSH map to the file name you specified. Since the CRUSH map is in a compiled form, you must decompile it first before you can edit it. 2.9.2. Decompiling the CRUSH map To decompile a CRUSH map, execute the following: Syntax Ceph decompiles (-d) the compiled CRUSH map and send the output (-o) to the file name you specified. 2.9.3. Setting a CRUSH map To set the CRUSH map for your cluster, execute the following: Syntax Ceph inputs the compiled CRUSH map of the file name you specified as the CRUSH map for the cluster. 2.9.4. Compiling the CRUSH map To compile a CRUSH map, execute the following: Syntax Ceph will store a compiled CRUSH map to the file name you specified. 2.10. CRUSH storage strategies examples If you want to have most pools default to OSDs backed by large hard drives, but have some pools mapped to OSDs backed by fast solid-state drives (SSDs). CRUSH can handle these scenarios easily. Use device classes. The process is simple to add a class to each device. Syntax Example Then, create rules to use the devices. Syntax Example Finally, set pools to use the rules. Syntax Example There is no need to manually edit the CRUSH map, because one hierarchy can serve multiple classes of devices.

Commands:
ceph osd crush tree --show-shadow ID CLASS WEIGHT TYPE NAME -24 ssd 4.54849 root default~ssd -19 ssd 0.90970 host ceph01~ssd 8 ssd 0.90970 osd.8 -20 ssd 0.90970 host ceph02~ssd 7 ssd 0.90970 osd.7 -21 ssd 0.90970 host ceph03~ssd 3 ssd 0.90970 osd.3 -22 ssd 0.90970 host ceph04~ssd 5 ssd 0.90970 osd.5 -23 ssd 0.90970 host ceph05~ssd 6 ssd 0.90970 osd.6 -2 hdd 50.94173 root default~hdd -4 hdd 7.27739 host ceph01~hdd 10 hdd 7.27739 osd.10 -12 hdd 14.55478 host ceph02~hdd 0 hdd 7.27739 osd.0 12 hdd 7.27739 osd.12 -6 hdd 14.55478 host ceph03~hdd 4 hdd 7.27739 osd.4 11 hdd 7.27739 osd.11 -10 hdd 7.27739 host ceph04~hdd 1 hdd 7.27739 osd.1 -8 hdd 7.27739 host ceph05~hdd 2 hdd 7.27739 osd.2 -1 55.49022 root default -3 8.18709 host ceph01 10 hdd 7.27739 osd.10 8 ssd 0.90970 osd.8 -11 15.46448 host ceph02 0 hdd 7.27739 osd.0 12 hdd 7.27739 osd.12 7 ssd 0.90970 osd.7 -5 15.46448 host ceph03 4 hdd 7.27739 osd.4 11 hdd 7.27739 osd.11 3 ssd 0.90970 osd.3 -9 8.18709 host ceph04 1 hdd 7.27739 osd.1 5 ssd 0.90970 osd.5 -7 8.18709 host ceph05 2 hdd 7.27739 osd.2 6 ssd 0.90970 osd.6
[bucket-type] [bucket-name] { id [a unique negative numeric ID] weight [the relative capacity/capability of the item(s)] alg [the bucket type: uniform | list | tree | straw2 ] hash [the hash type: 0 by default] item [item-name] weight [weight] }
host node1 { id -1 alg straw2 hash 0 item osd.0 weight 1.00 item osd.1 weight 1.00 } host node2 { id -2 alg straw2 hash 0 item osd.2 weight 1.00 item osd.3 weight 1.00 } rack rack1 { id -3 alg straw2 hash 0 item node1 weight 2.00 item node2 weight 2.00 }
root=default row=a rack=a2 chassis=a2a host=a2a1
ceph osd crush add-bucket {name} {type}
ceph osd crush add-bucket ssd-root root ceph osd crush add-bucket hdd-journal-root root ceph osd crush add-bucket hdd-root root
added bucket ssd-root type root to crush map added bucket hdd-journal-root type root to crush map added bucket hdd-root type root to crush map
ceph osd crush add-bucket ssd-row1 row ceph osd crush add-bucket ssd-row1-rack1 rack ceph osd crush add-bucket ssd-row1-rack1-host1 host ceph osd crush add-bucket ssd-row1-rack1-host2 host ceph osd crush add-bucket hdd-row1 row ceph osd crush add-bucket hdd-row1-rack2 rack ceph osd crush add-bucket hdd-row1-rack1-host1 host ceph osd crush add-bucket hdd-row1-rack1-host2 host ceph osd crush add-bucket hdd-row1-rack1-host3 host ceph osd crush add-bucket hdd-row1-rack1-host4 host
ceph osd tree
ceph osd crush move ssd-row1 root=ssd-root ceph osd crush move ssd-row1-rack1 row=ssd-row1 ceph osd crush move ssd-row1-rack1-host1 rack=ssd-row1-rack1 ceph osd crush move ssd-row1-rack1-host2 rack=ssd-row1-rack1
ceph osd tree
ceph osd crush remove {bucket-name}
ceph osd crush rm {bucket-name}
ceph osd crush tree -f json-pretty
[ { "id": -2, "name": "ssd", "type": "root", "type_id": 10, "items": [ { "id": -6, "name": "dell-per630-11-ssd", "type": "host", "type_id": 1, "items": [ { "id": 6, "name": "osd.6", "type": "osd", "type_id": 0, "crush_weight": 0.099991, "depth": 2 } ] }, { "id": -7, "name": "dell-per630-12-ssd", "type": "host", "type_id": 1, "items": [ { "id": 7, "name": "osd.7", "type": "osd", "type_id": 0, "crush_weight": 0.099991, "depth": 2 } ] }, { "id": -8, "name": "dell-per630-13-ssd", "type": "host", "type_id": 1, "items": [ { "id": 8, "name": "osd.8", "type": "osd", "type_id": 0, "crush_weight": 0.099991, "depth": 2 } ] } ] }, { "id": -1, "name": "default", "type": "root", "type_id": 10, "items": [ { "id": -3, "name": "dell-per630-11", "type": "host", "type_id": 1, "items": [ { "id": 0, "name": "osd.0", "type": "osd", "type_id": 0, "crush_weight": 0.449997, "depth": 2 }, { "id": 3, "name": "osd.3", "type": "osd", "type_id": 0, "crush_weight": 0.289993, "depth": 2 } ] }, { "id": -4, "name": "dell-per630-12", "type": "host", "type_id": 1, "items": [ { "id": 1, "name": "osd.1", "type": "osd", "type_id": 0, "crush_weight": 0.449997, "depth": 2 }, { "id": 4, "name": "osd.4", "type": "osd", "type_id": 0, "crush_weight": 0.289993, "depth": 2 } ] }, { "id": -5, "name": "dell-per630-13", "type": "host", "type_id": 1, "items": [ { "id": 2, "name": "osd.2", "type": "osd", "type_id": 0, "crush_weight": 0.449997, "depth": 2 }, { "id": 5, "name": "osd.5", "type": "osd", "type_id": 0, "crush_weight": 0.289993, "depth": 2 } ] } ] } ]
ceph orch daemon add osd HOST :_DEVICE_,[ DEVICE ]
ceph osd crush add ID_OR_NAME WEIGHT [ BUCKET_TYPE = BUCKET_NAME ...]
ceph osd crush add osd.0 1.0 root=default datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1
ceph osd crush set ID_OR_NAME WEIGHT root= POOL_NAME [ BUCKET_TYPE = BUCKET_NAME ...]
ceph osd crush remove NAME
ceph osd crush set-device-class CLASS OSD_ID [ OSD_ID ..]
ceph osd crush set-device-class hdd osd.0 osd.1 ceph osd crush set-device-class ssd osd.2 osd.3 ceph osd crush set-device-class bucket-index osd.4
ceph osd crush rm-device-class CLASS OSD_ID [ OSD_ID ..]
ceph osd crush rm-device-class hdd osd.0 osd.1 ceph osd crush rm-device-class ssd osd.2 osd.3 ceph osd crush rm-device-class bucket-index osd.4
ceph osd crush class rename OLD_NAME NEW_NAME
ceph osd crush class rename hdd sas15k
ceph osd crush class ls
hdd, ssd, bucket-index
ceph osd crush class ls-osd CLASS
ceph osd crush class ls-osd hdd
0 1 2 3 4 5 6
ceph osd crush rule ls-by-class CLASS
ceph osd crush rule ls-by-class hdd
ceph osd crush reweight _NAME_ _WEIGHT_
osd crush reweight-subtree NAME
ceph osd reweight ID WEIGHT
ceph osd reweight-by-utilization [THRESHOLD_] [ WEIGHT_CHANGE_AMOUNT ] [ NUMBER_OF_OSDS ] [--no-increasing]
ceph osd test-reweight-by-utilization 110 .5 4 --no-increasing
osd reweight-by-pg POOL_NAME
osd crush reweight-all
ceph osd primary-affinity OSD_ID WEIGHT
rule <rulename> { id <unique number> type [replicated | erasure] min_size <min-size> max_size <max-size> step take <bucket-type> [class <class-name>] step [choose|chooseleaf] [firstn|indep] <N> <bucket-type> step emit }
ceph osd crush rule list ceph osd crush rule ls
ceph osd crush rule dump NAME
ceph osd crush rule create-simple RUENAME ROOT BUCKET_NAME FIRSTN_OR_INDEP
ceph osd crush rule create-simple deleteme default host firstn
{ "id": 1, "rule_name": "deleteme", "type": 1, "min_size": 1, "max_size": 10, "steps": [ { "op": "take", "item": -1, "item_name": "default"}, { "op": "chooseleaf_firstn", "num": 0, "type": "host"}, { "op": "emit"}]}
ceph osd crush rule create-replicated NAME ROOT FAILURE_DOMAIN CLASS
ceph osd crush rule create-replicated fast default host ssd
ceph osd crush rule create-erasure RULE_NAME PROFILE_NAME
ceph osd crush rule create-erasure default default
ceph osd crush rule rm NAME
ceph osd crush tunables PROFILE
ceph osd crush tunables optimal
ceph osd crush tunables PROFILE
ceph osd crush tunables legacy
ceph osd getcrushmap -o /tmp/crush
crushtool -i /tmp/crush --set-choose-local-tries 0 --set-choose-local-fallback-tries 0 --set-choose-total-tries 50 -o /tmp/crush.new
ceph osd setcrushmap -i /tmp/crush.new
crushtool -i /tmp/crush --set-choose-local-tries 2 --set-choose-local-fallback-tries 5 --set-choose-total-tries 19 --set-chooseleaf-descend-once 0 --set-chooseleaf-vary-r 0 -o /tmp/crush.legacy
ceph osd getcrushmap -o COMPILED_CRUSHMAP_FILENAME
crushtool -d COMPILED_CRUSHMAP_FILENAME -o DECOMPILED_CRUSHMAP_FILENAME
ceph osd setcrushmap -i COMPILED_CRUSHMAP_FILENAME
crushtool -c DECOMPILED_CRUSHMAP_FILENAME -o COMPILED_CRUSHMAP_FILENAME
ceph osd crush set-device-class CLASS OSD_ID [ OSD_ID ]
ceph osd crush set-device-class hdd osd.0 osd.1 osd.4 osd.5 ceph osd crush set-device-class ssd osd.2 osd.3 osd.6 osd.7
ceph osd crush rule create-replicated RULENAME ROOT FAILURE_DOMAIN_TYPE DEVICE_CLASS
ceph osd crush rule create-replicated cold default host hdd ceph osd crush rule create-replicated hot default host ssd
ceph osd pool set POOL_NAME crush_rule RULENAME
ceph osd pool set cold crush_rule hdd ceph osd pool set hot crush_rule ssd
device 0 osd.0 class hdd device 1 osd.1 class hdd device 2 osd.2 class ssd device 3 osd.3 class ssd device 4 osd.4 class hdd device 5 osd.5 class hdd device 6 osd.6 class ssd device 7 osd.7 class ssd host ceph-osd-server-1 { id -1 alg straw2 hash 0 item osd.0 weight 1.00 item osd.1 weight 1.00 item osd.2 weight 1.00 item osd.3 weight 1.00 } host ceph-osd-server-2 { id -2 alg straw2 hash 0 item osd.4 weight 1.00 item osd.5 weight 1.00 item osd.6 weight 1.00 item osd.7 weight 1.00 } root default { id -3 alg straw2 hash 0 item ceph-osd-server-1 weight 4.00 item ceph-osd-server-2 weight 4.00 } rule cold { ruleset 0 type replicated min_size 2 max_size 11 step take default class hdd step chooseleaf firstn 0 type host step emit } rule hot { ruleset 1 type replicated min_size 2 max_size 11 step take default class ssd step chooseleaf firstn 0 type host step emit }