Ceph determines the placement of Placement Groups (PGs) based on the CRUSH algorithm. CRUSH (Controlled Replication Under Scalable Hashing) is a deterministic data distribution algorithm used by Ceph to calculate the placement of data across Object Storage Daemons (OSDs) in the cluster. This algorithm helps maintain data distribution and balance in the Ceph cluster.
When a virtual machine (VM) is created in Proxmox using Ceph as the storage backend, the following happens:
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CRUSH Map: Ceph uses a CRUSH map that defines the hierarchy of the cluster, including the storage devices, OSDs, hosts, and zones. The CRUSH map is typically managed by the Ceph administrator and defines the rules for data placement.
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Pool Configuration: When you create a storage pool for the virtual machines in Proxmox, you specify the replication factor and other pool settings. The replication factor determines the number of OSDs that will store copies of the data for each PG.
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PG Creation: When a VM is created in Proxmox, it will be stored as objects in the Ceph pool. Ceph automatically calculates the PG ID based on the pool and the hash of the object name.
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CRUSH Placement: Once the PG ID is determined, the CRUSH algorithm is used to calculate the placement of the PG across the available OSDs. The CRUSH algorithm takes into account the CRUSH map, the pool settings, and the state of OSDs to ensure data is distributed efficiently and that replicas are placed on different physical devices to achieve redundancy and fault tolerance.
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Data Placement: The data of the virtual machine, which is stored as objects, will be placed in the PG as calculated by the CRUSH algorithm. The OSDs responsible for the PG will store the data and handle read and write requests for the VM's objects.
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Monitoring and Rebalancing: Ceph continuously monitors the cluster, and if the CRUSH map or OSD states change (e.g., OSD failure, addition of new OSDs), it will automatically rebalance data to maintain distribution and data redundancy according to the CRUSH rules.
The CRUSH algorithm ensures that data is distributed across the cluster in a scalable and deterministic manner, and it avoids data hotspots by distributing the load evenly across available OSDs. This approach provides excellent fault tolerance, scalability, and performance for distributed storage in the Ceph cluster.
Overall, Ceph's ability to automatically manage data placement through the CRUSH algorithm simplifies the administration of storage and helps ensure the resilience and performance of the storage infrastructure for virtual machines in Proxmox.
