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diff --git a/doc/rados/configuration/storage-devices.rst b/doc/rados/configuration/storage-devices.rst new file mode 100644 index 00000000..262778d0 --- /dev/null +++ b/doc/rados/configuration/storage-devices.rst @@ -0,0 +1,83 @@ +================= + Storage Devices +================= + +There are two Ceph daemons that store data on disk: + +* **Ceph OSDs** (or Object Storage Daemons) are where most of the + data is stored in Ceph. Generally speaking, each OSD is backed by + a single storage device, like a traditional hard disk (HDD) or + solid state disk (SSD). OSDs can also be backed by a combination + of devices, like a HDD for most data and an SSD (or partition of an + SSD) for some metadata. The number of OSDs in a cluster is + generally a function of how much data will be stored, how big each + storage device will be, and the level and type of redundancy + (replication or erasure coding). +* **Ceph Monitor** daemons manage critical cluster state like cluster + membership and authentication information. For smaller clusters a + few gigabytes is all that is needed, although for larger clusters + the monitor database can reach tens or possibly hundreds of + gigabytes. + + +OSD Backends +============ + +There are two ways that OSDs can manage the data they store. Starting +with the Luminous 12.2.z release, the new default (and recommended) backend is +*BlueStore*. Prior to Luminous, the default (and only option) was +*FileStore*. + +BlueStore +--------- + +BlueStore is a special-purpose storage backend designed specifically +for managing data on disk for Ceph OSD workloads. It is motivated by +experience supporting and managing OSDs using FileStore over the +last ten years. Key BlueStore features include: + +* Direct management of storage devices. BlueStore consumes raw block + devices or partitions. This avoids any intervening layers of + abstraction (such as local file systems like XFS) that may limit + performance or add complexity. +* Metadata management with RocksDB. We embed RocksDB's key/value database + in order to manage internal metadata, such as the mapping from 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 will be read from disk or returned + to the user without being verified. +* Inline compression. Data written may 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) to increased performance. If + a significant amount of faster storage is available, internal + metadata can also 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 IO both for regular snapshots + and for erasure coded pools (which rely on cloning to implement + efficient two-phase commits). + +For more information, see :doc:`bluestore-config-ref` and :doc:`/rados/operations/bluestore-migration`. + +FileStore +--------- + +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 but suffers +from many performance deficiencies due to its overall design and +reliance on a traditional file system for storing object data. + +Although FileStore is generally capable of functioning on most +POSIX-compatible file systems (including btrfs and ext4), we only +recommend that XFS be used. Both btrfs and ext4 have known bugs and +deficiencies and their use may lead to data loss. By default all Ceph +provisioning tools will use XFS. + +For more information, see :doc:`filestore-config-ref`. |