There are several Ceph daemons in a storage cluster:
Ceph OSDs (Object Storage Daemons) store most of the data in Ceph. Usually each OSD is backed by a single storage device. This can be a traditional hard disk (HDD) or a solid state disk (SSD). OSDs can also be backed by a combination of devices: for example, a HDD for most data and an SSD (or partition of an SSD) for some metadata. The number of OSDs in a cluster is usually a function of the amount of data to be stored, the size of each storage device, and the level and type of redundancy specified (replication or erasure coding).
Ceph Monitor daemons manage critical cluster state. This includes cluster membership and authentication information. Small clusters require only a few gigabytes of storage to hold the monitor database. In large clusters, however, the monitor database can reach sizes of tens of gigabytes to hundreds of gigabytes.
Ceph Manager daemons run alongside monitor daemons, providing additional monitoring and providing interfaces to external monitoring and management systems.
OSD Back Ends
There are two ways that OSDs manage the data they store. As of the Luminous 12.2.z release, the default (and recommended) back end is BlueStore. Prior to the Luminous release, the default (and only) back end was Filestore.
BlueStore is a special-purpose storage back end designed specifically for managing data on disk for Ceph OSD workloads. BlueStore’s design is based on a decade of experience of supporting and managing Filestore OSDs.
Key BlueStore features include:
Direct management of storage devices. BlueStore consumes raw block devices or partitions. This avoids intervening layers of abstraction (such as local file systems like XFS) that can limit performance or add complexity.
Metadata management with RocksDB. RocksDB’s key/value database is embedded in order to manage internal metadata, including the mapping of object names to block locations on disk.
Full data and metadata checksumming. By default, all data and metadata written to BlueStore is protected by one or more checksums. No data or metadata is read from disk or returned to the user without being verified.
Inline compression. Data can be optionally compressed before being written to disk.
Multi-device metadata tiering. BlueStore allows its internal journal (write-ahead log) to be written to a separate, high-speed device (like an SSD, NVMe, or NVDIMM) for increased performance. If a significant amount of faster storage is available, internal metadata can be stored on the faster device.
Efficient copy-on-write. RBD and CephFS snapshots rely on a copy-on-write clone mechanism that is implemented efficiently in BlueStore. This results in efficient I/O both for regular snapshots and for erasure-coded pools (which rely on cloning to implement efficient two-phase commits).
FileStore is the legacy approach to storing objects in Ceph. It relies on a standard file system (normally XFS) in combination with a key/value database (traditionally LevelDB, now RocksDB) for some metadata.
FileStore is well-tested and widely used in production. However, it suffers from many performance deficiencies due to its overall design and its reliance on a traditional file system for object data storage.
Although FileStore is capable of functioning on most POSIX-compatible file systems (including btrfs and ext4), we recommend that only the XFS file system be used with Ceph. Both btrfs and ext4 have known bugs and deficiencies and their use may lead to data loss. By default, all Ceph provisioning tools use XFS.
For more information, see Filestore Config Reference.