BlueStore devices
BlueStore manages either one, two, or three storage devices in the backend. The backend devices include primary, write-ahead-log (WAL), and database (DB).
In the simplest case, BlueStore consumes a single primary storage device. The storage device is partitioned into two parts that contain:
- OSD metadata
- A small partition that contains basic metadata for the OSD. This data directory includes information about the OSD, such as its identifier, which cluster it belongs to, and its private keyring.
- Data
- A large partition occupying the rest of the device that is managed directly by BlueStore and that contains all of the OSD data. This primary device is identified by a block symbolic link in the data directory.
You can also use two additional devices:
- A WAL (write-ahead-log) device
- A device that stores BlueStore internal journal or write-ahead log. It is identified by the
block.walsymbolic link in the data directory. Consider using a WAL device only if the device is faster than the primary device. For example, when the WAL device uses an SSD disk and the primary device uses an HDD disk. - A DB device
- A device that stores BlueStore internal metadata. The embedded RocksDB database puts as much metadata as it can on the DB device instead of on the primary device to improve performance. If the DB device is full, it starts adding metadata to the primary device. Consider using a DB device only if the device is faster than the primary device.
Warning: If you have only less than a gigabyte storage available on fast devices, IBM recommends using it as a WAL device. If you have more fast devices available, consider using it as a DB device. The BlueStore journal is always placed on the fastest device, so using a DB device provides the same benefit that the WAL device provides while also allowing for storing additional metadata.