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One of the perks of being distributed, I guess.

The kind of failure that a system can tolerate with strict fsync but can't tolerate with lazy fsync (i.e. the software 'confirms' a write to its caller but then crashes) is probably not the kind of failure you'd expect to encounter on a majority of your nodes all at the same time.

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It is if they’re in the same physical datacenter. Usually the way this is done is to wait for at least M replicas to fsync, but only require the data to be in memory for the rest. It smooths out the tail latencies, which are quite high for SSDs.

You can push the safety envelope a bit further and wait for your data to only be in memory in N separate fault domains. Yes, your favorite ultra-reliable cloud service may be doing this.

> It smooths out the tail latencies, which are quite high for SSDs.

I'm sorry, tail latencies are high for SSDs? In my experience, the tail latencies are much higher for traditional rotating media (tens of seconds, vs 10s of milliseconds for SSDs).

They’re higher relative to median latencies for each. A high end SSD’s P99/median is higher than a high end HDD. That’s the relevant metric for request hedging.

It's approximately a factor of 1000x for both.

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