Hello list, I'm currently trying to understand what I can do with Ceph to optimize it for a cold-storage (write-once, read-very-rarely) like scenario, trying to compare cost against LTO-6 tape. There is a single main objective: - minimal cost/GB/month of operations (including power, DC) To achieve this, I can break it down to: - Use best cost/GB HDD * SMR today - Minimal cost/3.5"-slot - Minimal power-utilization/drive While staying within what is available today, I don't imagine going to power-down individual disk slots using IPMI etc, as some vendors allow. Now, putting Ceph on this, drives will be on, but it would be very useful to be able to spin-down drives that aren't used. It then seems to me that I want to do a few things with Ceph: - Have only a subest of the cluster 'active' for writes at any point in time - Yet, still have the entire cluster online and available for reads - Minimize concurrent OSD operations in a node that uses RAM, e.g. - Scrubbing, minimal number of OSDs active (ideally max 1) - In general, minimize concurrent "active" OSDs as per above - Minimize risk that any type of re-balancing of data occurs at all - E.g. use a "high" number of EC parity chunks Assuming e.g. 16 drives/host and 10TB drives, at ~100MB/s read and nearly full cluster, deep scrubbing the host would take 18.5 days. This means roughly 2 deep scrubs per month. Using EC pool, I wouldn't be very worried about errors, so perhaps that's ok (calculable), but they need to be repaired obviously. Mathematically, I can use an increase of parity chunks to lengthen the interval between deep scrubs. Is there anyone on the list who can provide some thoughts on the higher-order goal of "Minimizing concurrently active OSDs in a node"? I imagine I need to steer writes towards a subset of the system - but I have no idea how to implement it - using multiple separate clusters eg. each OSD on a node participate in unique clusters could perhaps help. Any feedback appreciated. It does appear a hot topic (pun intended). Best, Martin
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