BlueStore Config Reference


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 used as a whole, occupying the full device that is managed directly by BlueStore. This primary device is normally identified by a block symlink in the data directory.

The data directory is a tmpfs mount which gets populated (at boot time, or when ceph-volume activates it) with all the common OSD files that hold information about the OSD, like: its identifier, which cluster it belongs to, and its private keyring.

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

  • A WAL device (identified as block.wal in the data directory) can be used for BlueStore’s internal journal or write-ahead log. 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 (identified as block.db in the data directory) 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 fit).

A single-device BlueStore OSD can be provisioned with:

ceph-volume lvm prepare --bluestore --data <device>

To specify a WAL device and/or DB device,

ceph-volume lvm prepare --bluestore --data <device> --block.wal <wal-device> --block.db <db-device>


–data can be a Logical Volume using the vg/lv notation. Other devices can be existing logical volumes or GPT partitions

Provisioning strategies

Although there are multiple ways to deploy a Bluestore OSD (unlike Filestore which had 1) here are two common use cases that should help clarify the initial deployment strategy:

block (data) only

If all the devices are the same type, for example all are spinning drives, and there are no fast devices to combine these, it makes sense to just deploy with block only and not try to separate block.db or block.wal. The lvm call for a single /dev/sda device would look like:

ceph-volume lvm create --bluestore --data /dev/sda

If logical volumes have already been created for each device (1 LV using 100% of the device), then the lvm call for an lv named ceph-vg/block-lv would look like:

ceph-volume lvm create --bluestore --data ceph-vg/block-lv

block and block.db

If there is a mix of fast and slow devices (spinning and solid state), it is recommended to place block.db on the faster device while block (data) lives on the slower (spinning drive). Sizing for block.db should be as large as possible to avoid performance penalties otherwise. The ceph-volume tool is currently not able to create these automatically, so the volume groups and logical volumes need to be created manually.

For the below example, lets assume 4 spinning drives (sda, sdb, sdc, and sdd) and 1 solid state drive (sdx). First create the volume groups:

$ vgcreate ceph-block-0 /dev/sda
$ vgcreate ceph-block-1 /dev/sdb
$ vgcreate ceph-block-2 /dev/sdc
$ vgcreate ceph-block-3 /dev/sdd

Now create the logical volumes for block:

$ lvcreate -l 100%FREE -n block-0 ceph-block-0
$ lvcreate -l 100%FREE -n block-1 ceph-block-1
$ lvcreate -l 100%FREE -n block-2 ceph-block-2
$ lvcreate -l 100%FREE -n block-3 ceph-block-3

We are creating 4 OSDs for the four slow spinning devices, so assuming a 200GB SSD in /dev/sdx we will create 4 logical volumes, each of 50GB:

$ vgcreate ceph-db-0 /dev/sdx
$ lvcreate -L 50GB -n db-0 ceph-db-0
$ lvcreate -L 50GB -n db-1 ceph-db-0
$ lvcreate -L 50GB -n db-2 ceph-db-0
$ lvcreate -L 50GB -n db-3 ceph-db-0

Finally, create the 4 OSDs with ceph-volume:

$ ceph-volume lvm create --bluestore --data ceph-block-0/block-0 --block.db ceph-db-0/db-0
$ ceph-volume lvm create --bluestore --data ceph-block-1/block-1 --block.db ceph-db-0/db-1
$ ceph-volume lvm create --bluestore --data ceph-block-2/block-2 --block.db ceph-db-0/db-2
$ ceph-volume lvm create --bluestore --data ceph-block-3/block-3 --block.db ceph-db-0/db-3

These operations should end up creating 4 OSDs, with block on the slower spinning drives and a 50GB logical volume for each coming from the solid state drive.


When using a mixed spinning and solid drive setup it is important to make a large-enough block.db logical volume for Bluestore. Generally, block.db should have as large as possible logical volumes.

It is recommended that the block.db size isn’t smaller than 4% of block. For example, if the block size is 1TB, then block.db shouldn’t be less than 40GB.

If not using a mix of fast and slow devices, it isn’t required to create separate logical volumes for block.db (or block.wal). Bluestore will automatically manage these within the space of block.

Automatic Cache Sizing

Bluestore can be configured to automatically resize it’s caches when tc_malloc is configured as the memory allocator and the bluestore_cache_autotune setting is enabled. This option is currently enabled by default. Bluestore will attempt to keep OSD heap memory usage under a designated target size via the osd_memory_target configuration option. This is a best effort algorithm and caches will not shrink smaller than the amount specified by osd_memory_cache_min. Cache ratios will be chosen based on a hierarchy of priorities. If priority information is not availabe, the bluestore_cache_meta_ratio and bluestore_cache_kv_ratio options are used as fallbacks.


Description:Automatically tune the ratios assigned to different bluestore caches while respecting minimum values.


Description:When tcmalloc is available and cache autotuning is enabled, try to keep this many bytes mapped in memory. Note: This may not exactly match the RSS memory usage of the process. While the total amount of heap memory mapped by the process should generally stay close to this target, there is no guarantee that the kernel will actually reclaim memory that has been unmapped. During initial developement, it was found that some kernels result in the OSD’s RSS Memory exceeding the mapped memory by up to 20%. It is hypothesised however, that the kernel generally may be more aggressive about reclaiming unmapped memory when there is a high amount of memory pressure. Your mileage may vary.
Type:Unsigned Integer


Description:The chunk size in bytes to allocate to caches when cache autotune is enabled. When the autotuner assigns memory to different caches, it will allocate memory in chunks. This is done to avoid evictions when there are minor fluctuations in the heap size or autotuned cache ratios.
Type:Unsigned Integer


Description:The number of seconds to wait between rebalances when cache autotune is enabled. This setting changes how quickly the ratios of the difference caches are recomputed. Note: Setting the interval too small can result in high CPU usage and lower performance.


Description:When tcmalloc and cache autotuning is enabled, estimate the minimum amount of memory in bytes the OSD will need. This is used to help the autotuner estimate the expected aggregate memory consumption of the caches.
Type:Unsigned Interger


Description:When tcmalloc and cache autotuning is enabled, estimate the percent of memory fragmentation. This is used to help the autotuner estimate the expected aggregate memory consumption of the caches.


Description:When tcmalloc and cache autotuning is enabled, set the minimum amount of memory used for caches. Note: Setting this value too low can result in significant cache thrashing.
Type:Unsigned Integer


Description:When tcmalloc and cache autotuning is enabled, wait this many seconds between resizing caches. This setting changes the total amount of memory available for bluestore to use for caching. Note: Setting the interval too small can result in memory allocator thrashing and lower performance.

Manual Cache Sizing

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.


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:Unsigned Integer


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


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


Description:The ratio of cache devoted to metadata.
Type:Floating point


Description:The ratio of cache devoted to key/value data (rocksdb).
Type:Floating point


Description:The maximum amount of cache devoted to key/value data (rocksdb).
Type:Unsigned Integer
Default:512 * 1024*1024 (512 MB)


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 in four billion with a 32-bit (4 byte) checksum to one in 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>


Description:The default checksum algorithm to use.
Valid Settings:none, crc32c, crc32c_16, crc32c_8, xxhash32, xxhash64

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 has a compressible hint set.
  • aggressive: Compress data unless the write operation has an incompressible hint set.
  • force: Try to compress data no matter what.

For more information about the compressible and incompressible IO hints, see 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.
Valid Settings:lz4, snappy, zlib, zstd

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 clients 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.
Valid Settings:none, passive, aggressive, force

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

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

bluestore compression min blob size hdd

Description:Default value of bluestore compression min blob size for rotational media.
Type:Unsigned Integer

bluestore compression min blob size ssd

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

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

bluestore compression max blob size hdd

Description:Default value of bluestore compression max blob size for rotational media.
Type:Unsigned Integer

bluestore compression max blob size ssd

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

SPDK Usage

If you want to use SPDK driver for NVME SSD, you need to ready your system. Please refer to SPDK document for more details.

SPDK offers a script to configure the device automatically. Users can run the script as root:

$ sudo src/spdk/scripts/

Then you need to specify NVMe device’s device selector here with “spdk:” prefix for bluestore_block_path.

For example, users can find the device selector of an Intel PCIe SSD with:

$ lspci -mm -n -D -d 8086:0953

The device selector always has the form of DDDD:BB:DD.FF or DDDD.BB.DD.FF.

and then set:

bluestore block path = spdk:0000:01:00.0

Where 0000:01:00.0 is the device selector found in the output of lspci command above.

If you want to run multiple SPDK instances per node, you must specify the amount of dpdk memory size in MB each instance will use, to make sure each instance uses its own dpdk memory

In most cases, we only need one device to serve as data, db, db wal purposes. We need to make sure configurations below to make sure all IOs issued under SPDK.:

bluestore_block_db_path = ""
bluestore_block_db_size = 0
bluestore_block_wal_path = ""
bluestore_block_wal_size = 0

Otherwise, the current implementation will setup symbol file to kernel filesystem location and uses kernel driver to issue DB/WAL IO.