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We are trying to assess if we are going to see a data loss if an SSD that is hosting journals for few OSDs crashes. In our configuration, each SSD is partitioned into 5 chunks and each chunk is mapped as
a journal drive for one OSD. What I understand from the Ceph documentation: “Consistency: Ceph OSD Daemons
require a filesystem interface that guarantees atomic compound operations. Ceph OSD Daemons write a description of the operation to the journal and apply the operation to the filesystem. This enables atomic updates to an object (for example, placement group
metadata). Every few seconds–between filestore max sync interval and filestore min sync interval–the
Ceph OSD Daemon stops writes and synchronizes the journal with the filesystem, allowing Ceph OSD Daemons to trim operations from the journal and reuse the space. On failure, Ceph OSD Daemons replay the journal starting after the last synchronization operation.”
So, my question is what happens if an SSD fails – am I going to lose all the data that has not been written/synchronized to OSD? In my case, am I going to lose data for all the 5 OSDs which can be
bad? This is of concern to us. What are the options to prevent any data loss at all? Is it better to have the journals on the same hard drive, i.e., to have one journal per OSD and host it on the same hard drive? Of course, performance will not be as good
as having an SSD for OSD journal. In this case, I am thinking I will not lose data as there are secondary OSDs where data is replicated (we are using triple replication). Any thoughts? What other solutions people have adopted for data reliability and consistency
to address the case I am mentioning?
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