I'm a cofounder at MemSQL. The explanation around durability and in-memory databases is more nuanced than what is assumed with disk-based systems. First, it's

I'm a cofounder at MemSQL. The explanation around durability and in-memory databases is more nuanced than what is assumed with disk-based systems. First, it's worth mentioning that you use in-memory databases when you have performance requirements that far exceed physical capabilities of hard disks or even SSDs. The workload will have a high degree of contention, high concurrency, etc. Second, you're probably working with non-human-generated data. If you're not a bank that needs to guarantee a debit and credit went through (and that's slow human-generated data anyway), then you're looking at in-memory technology because you can guarantee every read will hit memory. For writes, you basically have to use more machines to guarantee durability. Any in-memory database worth its salt will write a transaction log to disk, so in the event of a power failure, it will read back from disk into memory. You can increase the probability of 0 data loss, even in high contention and concurrent workloads, if you run the dataset across a set of machines, which both multiplies the number of disks capable of writing a sequential log as well as storing an extra copy in memory for high availability.