ceph/doc/rados/configuration/bluestore-config-ref.rst

==========================
BlueStore Config Reference
==========================

Devices
=======

BlueStore manages either one, two, or (in certain cases) three storage
devices.

In the simplest case, BlueStore consumes a single (primary) storage
device.  The storage device is normally partitioned into two parts:

#. A small partition is formatted with XFS and contains basic metadata
   for the OSD.  This *data directory* includes information about the
   OSD (its identifier, which cluster it belongs to, and its private
   keyring.

#. The rest of the device is normally a large partition occupying the
   rest of the device that is managed directly by BlueStore contains
   all of the actual data.  This *primary device* is normally identifed
   by a ``block`` symlink in data directory.

It is also possible to deploy BlueStore across two additional devices:

* A *WAL device* can be used for BlueStore's internal journal or
  write-ahead log.  It is identified by the ``block.wal`` symlink in
  the data directory.  It is only useful to use a WAL device if the
  device is faster than the primary device (e.g., when it is on an SSD
  and the primary device is an HDD).
* A *DB device* can be used for storing BlueStore's internal metadata.
  BlueStore (or rather, the embedded RocksDB) will put as much
  metadata as it can on the DB device to improve performance.  If the
  DB device fills up, metadata will spill back onto the primary device
  (where it would have been otherwise).  Again, it is only helpful to
  provision a DB device if it is faster than the primary device.

If there is only a small amount of fast storage available (e.g., less
than a gigabyte), we recommend using it as a WAL device.  If there is
more, provisioning a DB device makes more sense.  The BlueStore
journal will always be placed on the fastest device available, so
using a DB device will provide the same benefit that the WAL device
would while *also* allowing additional metadata to be stored there (if
it will fix).

A single-device BlueStore OSD can be provisioned with::

  ceph-disk prepare --bluestore <device>

To specify a WAL device and/or DB device, ::

  ceph-disk prepare --bluestore <device> --block.wal <wal-device> --block-db <db-device>

Cache size
==========

The amount of memory consumed by each OSD for BlueStore's cache is
determined by the ``bluestore_cache_size`` configuration option.  If
that config option is not set (i.e., remains at 0), there is a
different default value that is used depending on whether an HDD or
SSD is used for the primary device (set by the
``bluestore_cache_size_ssd`` and ``bluestore_cache_size_hdd`` config
options).

BlueStore and the rest of the Ceph OSD does the best it can currently
to stick to the budgeted memory.  Note that on top of the configured
cache size, there is also memory consumed by the OSD itself, and
generally some overhead due to memory fragmentation and other
allocator overhead.

The configured cache memory budget can be used in a few different ways:

* Key/Value metadata (i.e., RocksDB's internal cache)
* BlueStore metadata
* BlueStore data (i.e., recently read or written object data)

Cache memory usage is governed by the following options:
``bluestore_cache_meta_ratio``, ``bluestore_cache_kv_ratio``, and
``bluestore_cache_kv_max``.  The fraction of the cache devoted to data
is 1.0 minus the meta and kv ratios.  The memory devoted to kv
metadata (the RocksDB cache) is capped by ``bluestore_cache_kv_max``
since our testing indicates there are diminishing returns beyond a
certain point.

``bluestore_cache_size``

:Description: The amount of memory BlueStore will use for its cache.  If zero, ``bluestore_cache_size_hdd`` or ``bluestore_cache_size_ssd`` will be used instead.
:Type: Integer
:Required: Yes
:Default: ``0``

``bluestore_cache_size_hdd``

:Description: The default amount of memory BlueStore will use for its cache when backed by an HDD.
:Type: Integer
:Required: Yes
:Default: ``1 * 1024 * 1024 * 1024`` (1 GB)

``bluestore_cache_size_ssd``

:Description: The default amount of memory BlueStore will use for its cache when backed by an SSD.
:Type: Integer
:Required: Yes
:Default: ``3 * 1024 * 1024 * 1024`` (3 GB)

``bluestore_cache_meta_ratio``

:Description: The ratio of cache devoted to metadata.
:Type: Floating point
:Required: Yes
:Default: ``.01``

``bluestore_cache_kv_ratio``

:Description: The ratio of cache devoted to key/value data (rocksdb).
:Type: Floating point
:Required: Yes
:Default: ``.99``

``bluestore_cache_kv_max``

:Description: The maximum amount of cache devoted to key/value data (rocksdb).
:Type: Floating point
:Required: Yes
:Default: ``512 * 1024*1024`` (512 MB)


Checksums
=========

BlueStore checksums all metadata and data written to disk.  Metadata
checksumming is handled by RocksDB and uses `crc32c`. Data
checksumming is done by BlueStore and can make use of `crc32c`,
`xxhash32`, or `xxhash64`.  The default is `crc32c` and should be
suitable for most purposes.

Full data checksumming does increase the amount of metadata that
BlueStore must store and manage.  When possible, e.g., when clients
hint that data is written and read sequentially, BlueStore will
checksum larger blocks, but in many cases it must store a checksum
value (usually 4 bytes) for every 4 kilobyte block of data.

It is possible to use a smaller checksum value by truncating the
checksum to two or one byte, reducing the metadata overhead.  The
trade-off is that the probability that a random error will not be
detected is higher with a smaller checksum, going from about one if
four billion with a 32-bit (4 byte) checksum to one is 65,536 for a
16-bit (2 byte) checksum or one in 256 for an 8-bit (1 byte) checksum.
The smaller checksum values can be used by selecting `crc32c_16` or
`crc32c_8` as the checksum algorithm.

The *checksum algorithm* can be set either via a per-pool
``csum_type`` property or the global config option.  For example, ::

  ceph osd pool set <pool-name> csum_type <algorithm>

``bluestore_csum_type``

:Description: The default checksum algorithm to use.
:Type: String
:Required: Yes
:Valid Settings: ``none``, ``crc32c``, ``crc32c_16``, ``crc32c_8``, ``xxhash32``, ``xxhash64``
:Default: ``crc32c``


Inline Compression
==================

BlueStore supports inline compression using `snappy`, `zlib`, or
`lz4`. Please note that the `lz4` compression plugin is not
distributed in the official release.

Whether data in BlueStore is compressed is determined by a combination
of the *compression mode* and any hints associated with a write
operation.  The modes are:

* **none**: Never compress data.
* **passive**: Do not compress data unless the write operation as a
  *compressible* hint set.
* **aggressive**: Compress data unless the write operation as an
  *incompressible* hint set.
* **force**: Try to compress data no matter what.

For more information about the *compressible* and *incompressible* IO
hints, see :c:func:`rados_set_alloc_hint`.

Note that regardless of the mode, if the size of the data chunk is not
reduced sufficiently it will not be used and the original
(uncompressed) data will be stored.  For example, if the ``bluestore
compression required ratio`` is set to ``.7`` then the compressed data
must be 70% of the size of the original (or smaller).

The *compression mode*, *compression algorithm*, *compression required
ratio*, *min blob size*, and *max blob size* can be set either via a
per-pool property or a global config option.  Pool properties can be
set with::

  ceph osd pool set <pool-name> compression_algorithm <algorithm>
  ceph osd pool set <pool-name> compression_mode <mode>
  ceph osd pool set <pool-name> compression_required_ratio <ratio>
  ceph osd pool set <pool-name> compression_min_blob_size <size>
  ceph osd pool set <pool-name> compression_max_blob_size <size>

``bluestore compression algorithm``

:Description: The default compressor to use (if any) if the per-pool property
              ``compression_algorithm`` is not set. Note that zstd is *not*
              recommended for bluestore due to high CPU overhead when
              compressing small amounts of data.
:Type: String
:Required: No
:Valid Settings: ``lz4``, ``snappy``, ``zlib``, ``zstd``
:Default: ``snappy``

``bluestore compression mode``

:Description: The default policy for using compression if the per-pool property
              ``compression_mode`` is not set. ``none`` means never use
              compression. ``passive`` means use compression when
              :c:func:`clients hint <rados_set_alloc_hint>` that data is
              compressible.  ``aggressive`` means use compression unless
              clients hint that data is not compressible.  ``force`` means use
              compression under all circumstances even if the clients hint that
              the data is not compressible.
:Type: String
:Required: No
:Valid Settings: ``none``, ``passive``, ``aggressive``, ``force``
:Default: ``none``

``bluestore compression required ratio``

:Description: The ratio of the size of the data chunk after
              compression relative to the original size must be at
              least this small in order to store the compressed
              version.

:Type: Floating point
:Required: No
:Default: .875

``bluestore compression min blob size``

:Description: Chunks smaller than this are never compressed.
              The per-pool property ``compression_min_blob_size`` overrides
              this setting.

:Type: Unsigned Integer
:Required: No
:Default: 0

``bluestore compression min blob size hdd``

:Description: Default value of ``bluestore compression min blob size``
              for rotational media.

:Type: Unsigned Integer
:Required: No
:Default: 128K

``bluestore compression min blob size ssd``

:Description: Default value of ``bluestore compression min blob size``
              for non-rotational (solid state) media.

:Type: Unsigned Integer
:Required: No
:Default: 8K

``bluestore compression max blob size``

:Description: Chunks larger than this are broken into smaller blobs sizing
              ``bluestore compression max blob size`` before being compressed.
              The per-pool property ``compression_max_blob_size`` overrides
              this setting.

:Type: Unsigned Integer
:Required: No
:Default: 0

``bluestore compression max blob size hdd``

:Description: Default value of ``bluestore compression max blob size``
              for rotational media.

:Type: Unsigned Integer
:Required: No
:Default: 512K

``bluestore compression max blob size ssd``

:Description: Default value of ``bluestore compression max blob size``
              for non-rotational (solid state) media.

:Type: Unsigned Integer
:Required: No
:Default: 64K