Comment by marcan_42
4 years ago
We're probably going to have a knob to defer flushes (but still do them, unlike Apple, after a max timeout) that will be on by default on laptops, and make sure panics flush the cache if we can. Also apparently we need to do something for the power button too, as I just tested how macOS handles that. There is a warning before the system shuts down but we need to listen to it. Same with critical battery states.
Then I misunderstood. Do you mean that Apple doesn't implement ANY timeout? So they only flush when the cache is full or when a shutdown routine has started?
They flush the cache when something requests the cache be flushed; I don't know if there is a timeout, because presumably it's not difficult for some random process to issue a FULLFSYNC and flush everything prior as a side-effect (the flush is global). But I've seen at least 5-10 seconds of data loss from drive cache loss on the Mac Mini, so if they do do deferred flushes the timeout is longer than that.
WTF, that is worse than I thought then. That's the dirtiest hack I've read, it's of very low quality for a company like Apple. That I'd expect for a OnePlus device, not for a full fledged Macbook.
When do off-the-shelf NVMe controllers flush their internal DRAM buffer? I presume that happened after a timeout, even if the OS does not issue a NVMe flush command.
Does Apple implement the NVMe spec on their controller, i.e. do they indicate "Volatile Write Cache"?