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OSD
Object Storage Daemon (OSD) is an actual storage media or partition within media such as HDD/SSD that stores the actual cluster data. OSDs are responsible for data replication, recovery, and rebalancing. Each OSD provides monitored information to Ceph Monitors to check for a heartbeat. A Ceph cluster requires a minimum of two OSDs to be in the active+clean state. A Ceph cluster regularly provides feedback on a cluster status. An active+clean state expresses an error- or warning-free cluster. See the Placement Group section for the other states that a Ceph cluster can achieve. As of Proxmox Version 3.2, OSDs can be managed through the Proxmox GUI.
In Ceph, I/O writes are first written to an OSD Journal before they is transferred to an actual OSD. Journals are simply smaller partitions that accept smaller data at a time while the backend OSDs catch up with the writings. By putting Journals on faster access disk drives such as SSD, we can increase a Ceph operation significantly because user data is written to a Journal at a higher speed. At the same time, the Journal sends short bursts of data to OSDs, giving them time to catch up. Journals for multiple OSDs can be stored in one SSD per node. Alternatively, OSDs can be divided into multiple SSDs. For a small cluster of up to eight OSDs per node, using an SSD improves performance. However, while working with a larger cluster with a higher number OSDs per node, collocating the Journal with the same OSDs, rather than SSD, increases performance. The combined write speed for all the OSDs outperforms the speed of one or two SSDs as a Journal.
The important thing about Journal to remember is that the loss of a Journal partition causes OSD data loss. Thus, when using a single SSD to store all OSD Journals, we recommend that you add the SSD to the RAID configuration.
Note that as of the Ceph release codenamed Firefly, Journal is not needed any more. However, since these changes are too new, we recommend that you wait for several months before implementing the new release of Ceph in a production environment. By this time, the Ceph developer will have taken care of any major known or unknown bug in the new release. In this chapter, we are going to create a Ceph cluster using Journal. The installation process is the same as that for Ceph Firefly.