crushtool
crushtool is a utility in Ceph used to interact with CRUSH (Controlled Replication Under Scalable Hashing) maps. CRUSH is the algorithm Ceph uses to determine how data is distributed and stored across the cluster. The crushtool provides various commands to create, decompile, compile, and analyze CRUSH maps, which control how Ceph places data onto OSDs based on failure domains, weights, and replication rules.
Common crushtool Commands and Subcommands
crushtool Commands and Subcommands1. decompile
Decompiles a binary CRUSH map into a human-readable text format.
Example:
This command takes the binary CRUSH map /etc/ceph/crushmap and decompiles it into the human-readable file crushmap.txt.
2. compile
Compiles a text-format CRUSH map into a binary format that Ceph uses.
Example:
This command compiles the human-readable crushmap.txt into the binary CRUSH map /etc/ceph/crushmap.
3. show
Displays the contents of a compiled CRUSH map in a more readable format directly in the terminal without decompiling it into a separate file.
Example:
This command prints the contents of the binary CRUSH map in a readable format.
4. analyze
Analyzes how data will be distributed according to a given CRUSH map. You can use it to simulate data placement and failure domains.
Example:
This command simulates data placement using the CRUSH map's rule 0, distributing replicas across OSDs with a replication factor of 3.
5. test
Tests how a specific CRUSH map distributes data for a particular bucket or item, allowing administrators to verify placement behavior.
Example:
This command tests the distribution of data according to rule 0 with 3 replicas and displays the placement mappings for the current CRUSH map.
6. reweight
Reweights the CRUSH map’s OSDs or buckets based on a new weight, adjusting the distribution of data.
Example:
This command reweights osd.0 to 0.8 in the CRUSH map.
7. add-bucket
Adds a new bucket to the CRUSH map. Buckets define failure domains such as racks, hosts, and OSDs.
Example:
This command adds a new bucket named rack0 of type rack to the CRUSH map.
8. set-item-class
Assigns a class to an item (such as an OSD) in the CRUSH map. Ceph uses classes to differentiate between storage types (e.g., SSD, HDD).
Example:
This command sets osd.1 to belong to the ssd class.
9. rm-bucket
Removes a bucket from the CRUSH map.
Example:
This command removes the bucket rack0 from the CRUSH map.
10. rm-item
Removes an item (such as an OSD) from the CRUSH map.
Example:
This command removes osd.0 from the CRUSH map.
11. move
Moves a bucket to a different location in the hierarchy of the CRUSH map.
Example:
This command moves the bucket host1 to rack1.
12. tree
Prints the CRUSH map hierarchy in a tree-like format.
Example:
This command displays the hierarchy of the CRUSH map, showing how OSDs, hosts, racks, etc., are organized.
Typical Use Cases:
Customizing Data Placement: Ceph admins use
crushtoolto modify CRUSH maps for fine-tuned control over how data is replicated and distributed across failure domains (e.g., racks, hosts, OSDs).Simulating Data Distribution: With commands like
analyzeandtest, admins can simulate how CRUSH maps will distribute data across the cluster, ensuring even utilization and high availability.Reweighting for Performance: Using
reweight, admins can adjust the weights of OSDs or other CRUSH components to ensure even distribution of data, optimizing performance and capacity usage.Failure Domain Management: By adding and removing buckets, Ceph administrators can redefine failure domains like hosts and racks, ensuring the cluster's resilience against hardware failures.
Storage Tiering with Classes: Assigning items to classes (e.g., SSDs or HDDs) allows for storage tiering, where Ceph can manage fast storage devices (SSDs) differently from slower ones (HDDs).
Conclusion
The crushtool is an essential utility for managing and customizing CRUSH maps in a Ceph cluster. It offers various commands for creating, compiling, testing, and analyzing CRUSH maps, giving administrators the ability to define and optimize how Ceph distributes and replicates data across the cluster. Through crushtool, failure domains, replication rules, and storage tiers can be efficiently controlled, providing resilience and performance to the Ceph cluster.
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