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84 lines
3.7 KiB
ReStructuredText
84 lines
3.7 KiB
ReStructuredText
=================
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Storage Devices
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=================
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There are two Ceph daemons that store data on disk:
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* **Ceph OSDs** (or Object Storage Daemons) are where most of the
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data is stored in Ceph. Generally speaking, each OSD is backed by
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a single storage device, like a traditional hard disk (HDD) or
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solid state disk (SSD). OSDs can also be backed by a combination
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of devices, like a HDD for most data and an SSD (or partition of an
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SSD) for some metadata. The number of OSDs in a cluster is
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generally a function of how much data will be stored, how big each
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storage device will be, and the level and type of redundancy
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(replication or erasure coding).
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* **Ceph Monitor** daemons manage critical cluster state like cluster
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membership and authentication information. For smaller clusters a
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few gigabytes is all that is needed, although for larger clusters
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the monitor database can reach tens or possibly hundreds of
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gigabytes.
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OSD Backends
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============
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There are two ways that OSDs can manage the data they store. Starting
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with the Luminous 12.2.z release, the new default (and recommended) backend is
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*BlueStore*. Prior to Luminous, the default (and only option) was
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*FileStore*.
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BlueStore
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---------
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BlueStore is a special-purpose storage backend designed specifically
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for managing data on disk for Ceph OSD workloads. It is motivated by
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experience supporting and managing OSDs using FileStore over the
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last ten years. Key BlueStore features include:
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* Direct management of storage devices. BlueStore consumes raw block
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devices or partitions. This avoids any intervening layers of
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abstraction (such as local file systems like XFS) that may limit
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performance or add complexity.
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* Metadata management with RocksDB. We embed RocksDB's key/value database
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in order to manage internal metadata, such as the mapping from object
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names to block locations on disk.
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* Full data and metadata checksumming. By default all data and
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metadata written to BlueStore is protected by one or more
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checksums. No data or metadata will be read from disk or returned
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to the user without being verified.
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* Inline compression. Data written may be optionally compressed
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before being written to disk.
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* Multi-device metadata tiering. BlueStore allows its internal
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journal (write-ahead log) to be written to a separate, high-speed
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device (like an SSD, NVMe, or NVDIMM) to increased performance. If
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a significant amount of faster storage is available, internal
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metadata can also be stored on the faster device.
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* Efficient copy-on-write. RBD and CephFS snapshots rely on a
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copy-on-write *clone* mechanism that is implemented efficiently in
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BlueStore. This results in efficient IO both for regular snapshots
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and for erasure coded pools (which rely on cloning to implement
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efficient two-phase commits).
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For more information, see :doc:`bluestore-config-ref` and :doc:`/rados/operations/bluestore-migration`.
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FileStore
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---------
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FileStore is the legacy approach to storing objects in Ceph. It
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relies on a standard file system (normally XFS) in combination with a
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key/value database (traditionally LevelDB, now RocksDB) for some
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metadata.
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FileStore is well-tested and widely used in production but suffers
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from many performance deficiencies due to its overall design and
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reliance on a traditional file system for storing object data.
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Although FileStore is generally capable of functioning on most
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POSIX-compatible file systems (including btrfs and ext4), we only
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recommend that XFS be used. Both btrfs and ext4 have known bugs and
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deficiencies and their use may lead to data loss. By default all Ceph
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provisioning tools will use XFS.
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For more information, see :doc:`filestore-config-ref`.
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