write caching

[doomcubed]

On a Mac, in what circumstances does write caching apply, and in what circumstances is the write cache flushed?

For example:

1. Does write caching apply whenever any writing is involved? Or only when a file is edited? Does write caching apply when a file is being copied?

2. Is the write cache flushed only when shutting down? Does it happen on putting the computer to sleep? Does it happen when logging out of a user account? Does it happen at other times, e.g. when there's not much happening on the computer?

[Turboladdade]

I believe it is when a user logs out (and/or power off). Typically one doesn't have to think much about this very obscure setting, especially with SSDs. Are you experience a particular problem you are trying to solve?

[doomcubed]

Thanks for reply, Turboladdade. No, I've not been having a problem. I'm just trying to understand the subject.

I believe your answer (re logging out / shutting down) is right as far as it goes, but I strongly suspect that there's more to it. Surely the computer is not going to wait hours on some occasions before flushing the cache.

And the other point was: does write caching apply whenever **anything** is written to the device in question? Or only when a file on that device is edited? What about when a file is copied from the computer to an external device? Is it really copied straight away, as the progress window would have us believe? Or is that a snare and a delusion?

Surely somebody out there in Internetland must know.

[doomcubed]

Whoops, must have hit the button twice.

[Turboladdade]

I couldn't say for sure, this is a very specific technical question that I think you might find the answer in Apple's private developer forums (but you need a dev account to access those).

[doomcubed]

Thanks again, Turboladdade, but I'd rather cut off my leg and eat it than have any kind of Apple account. Furthermore, I don't believe I'd get a straight answer out of Apple.