Hi, Consider a ceph cluster with one IO intensive pool (e.g. VM storage) plus a few not-so IO intensive. I'm thinking of whether it makes sense to use the available SSDs in the cluster nodes (1 SSD for 4 HDDs) as part of a writeback cache pool in front of the IO intensive pool, instead of using them as journal SSDs? With this method, the OSD journals would be co-located on the HDDs or the SSD:HDD ratio could be reduced from 1:4 to something like 1:10. The write operations would still hit SSDs first (though latency would increase compared to writing to dedicated SSD partitions local to the server), and as far as I understand the cache flush operations are happening in a coalesced fashion. Plus a definite advantage would be that besides functioning as a 'write log' (aka. journal), the SSDs would be serving as a read cache for hot data.
What do you think? Cheers, Benjamin
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