Application Scenarios

The smart write cache optimizes the Ceph I/O processes when bcache devices are used as Ceph OSDs. It is valid only for the OSDs that use the BlueStore storage engine. Given that OSDs in the Ceph cluster can be added or modified in actual applications, the tool needs to obtain the OSD information in the cluster. Therefore, ensure that the OSD deployment path is the official default path /var/lib/ceph/osd. If any other path is used, the optimization will not be performed.

Ceph storage has the following application scenarios:

• Balanced: The cluster uses solid-state drives (SSDs) and hard disk drives (HDDs) to deploy OSDs. SSDs are used to store OSD metadata (DB) and log data (WAL), and HDDs are used to store real data (data).
• All-flash: The cluster uses SSDs to deploy OSDs. The data, DB, and WAL of the OSDs are stored in the SSDs.
• Cold storage: The cluster uses HDDs to deploy OSDs. The data, DB, and WAL of the OSDs are stored in the HDDs.
• Bcache: SSDs and HDDs are bound as a bcache partition. SSDs are used as the cache of HDDs. The cluster uses the bcache partition to deploy the OSDs. The data in OSDs is stored in the bcache partition, and the DB and WAL are stored in an independent NVMe partition.

Currently, the smart write cache is available only for the bcache scenario.

The kernel page size of CentOS 7.6 for aarch64 is 64 KB by default. The smart write cache must be run in the 4 KB page size environment.

For convenience, this document provides the following two methods:

• If the environment page size is 64 KB, recompile and replace the entire kernel. You can compile and install the kernel and bcache at the same time. For details, see Compiling and Installing the Kernel.
• If the environment page size is 4 KB (using the getconf PAGESIZE command to query), you only need to install and replace bcache.ko. For details, see Compiling and Installing the Bcache.