Using crushtool to Manage CRUSH Maps

crushtool is a command-line utility used to create, compile, and decompile CRUSH maps. These maps define how data is distributed across an object storage cluster (e.g., Ceph). Below are the key steps to use it effectively.

Creating a New CRUSH Map

   crushtool --create osdmap --output osdmap

   crushtool --add-osd 1 --output osdmap
   crushtool --add-osd 2 --output osdmap

1. Generate a new map using the default configuration:
2. Add OSDs to the map (replace 1, 2, etc., with your OSD IDs):

Compiling the Map

Convert the human-readable map into a binary format for cluster use:

crushtool --compile osdmap --output osdmap.bin

Decompiling a Binary Map

Convert a binary map back to text for editing:

crushtool --decompile osdmap.bin --output osdmap

Modifying the Map

Edit the text-based map file (e.g., osdmap) to adjust rules, such as adding a new rule or changing replication settings. Use a text editor to modify the map structure.

Applying the Map to the Cluster

Replace the current map with your compiled version:

ceph osd setcrushmap -i osdmap.bin

Verifying the Map

Check the updated map status:

ceph osd tree

Key Notes

• Always back up the current map before making changes.
• Ensure the map syntax is valid to avoid cluster instability.
• Use crushtool only for advanced cluster configuration; most users should rely on ceph commands for routine operations.