BlueStore caching
The BlueStore cache is a collection of buffers that, depending on configuration, can be populated with data as the OSD daemon does reading from or writing to the disk.
By default in IBM Storage Ceph, BlueStore will cache on reads, but not writes. This is because the bluestore_default_buffered_write option is set to false to avoid potential overhead associated with cache eviction.
If the bluestore_default_buffered_write option is set to true, data is written to the buffer first, and then committed to disk. Afterward, a write acknowledgment is sent to the client, allowing subsequent reads faster access to the data already in cache, until that data is evicted.
Read-heavy workloads will not see an immediate benefit from BlueStore caching. As more reading is done, the cache will grow over time and subsequent reads will see an improvement in performance. How fast the cache populates depends on the BlueStore block and database disk type, and the client’s workload requirements.