Hi, Since a few versions, Ceph has deprecated caching tiers with no alternative mentioned. Thus I've been looking and somewhat settled on using dm-cache for two reasons: it's LVM and ceph-volume mentions it's supported and "transparent":
For ceph-volume, the use of dm-cache is transparent, there is no difference for the tool, and it treats dm-cache like a plain logical volume.
However, what does this really mean? Is it that I can just slap an lvm cache on the LVs created by ceph-volume and call it a day? Is this the canonical replacement to Ceph's native cache tiers? Is the tuning of the cache settings somehow managed by Ceph or is it fully on the user? And, related, can the caching be declared in the orch OSD spec? As for the context: I have a cluster with *very* bad SSDs (consumer DRAM-less BX500) and I'm trying to get it somewhat OK until the market for flash improves. As the problems are due to GC pauses from the drive caused by the sustained IO, I'm hoping that putting a cache tier (with smaller faster enterprise SSDs) in front of the drives will change the IO behavior enough for the drives to perform their GC without impacting the cluster. I already have the good smaller drives that have WAL+DB for the slow drives, but they have enough free space to cache the block. Thanks!