Comment by seiji

In days of old from ACID was dreamt of, computers were built head to tail by giant engineering firms largely with three letter names. In those days, when you wrote to disk, it was on disk. If you handed it to a subsystem to write to disk, you could believe the answer because your people built that subsystem and it had better do what you wanted.

Now, computers are built with subsystems from a half dozen different companies, none of which can be bothered to fully document their product's behavior. Product life cycles are so fast it isn't worth working out what a device actually does, because by the time you do you either won't be able to buy it anymore, or someone's new iteration will be 20% cheaper/faster/bigger and you will get killed in the market if you don't switch to it.

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PS: We were also happy to have a whole MIP of Vax equivalent (or identical) processing power, and I was slightly reknowned for my ability to code cleverly enough to get 42 disk IO/second out of our main disk drive, so I'd rather not go back, think you very much. I'll get along as best I can with the miraculous and inexpensive rubbish we build our systems from today.

Well it means the system tries the best it can do make the value durable. It will do a sync after every write perhaps. If the disk lies, well there is not much sqlite can do. If disk has a capacitor and battery maybe it can still lie that it wrote but keeps it in its internal buffer and if someone pulls the cord it has enough energy to flush everything it "lied about" to disk. Oh but what if you shoot a proton beam through it and blow away its capacitor and battery, well then you are out of luck.

As someone mentioned, this is contrast to say how MongoDB was shipping not too long ago. They had turned of any acknowledgement for writes. So doing a db write was more like a throw over the fence and pray operation. So it didn't even try to be durable, by design.

Sqlite guarentees Durability by waiting and checking the disk to make sure it is stored correctly before reporting a success. This default behavior is rather slow and can be turned off.

This is in stark contrast to something like MongoDB, which barely parses the request before reporting a success, and doesn't make any guarantees of when or even if it will ever save the data (though it usually does).