On Tue, May 31, 2022 at 12:42:49PM -0700, Eric Wheeler wrote: > On Sat, 28 May 2022, Keith Busch wrote: > > On Sat, May 28, 2022 at 12:57:26PM +0000, Adriano Silva wrote: > > > Dear Christoph, > > > > > > > Once you do that, the block layer ignores all flushes and FUA bits, so > > > > yes it is going to be a lot faster. But also completely unsafe because > > > > it does not provide any data durability guarantees. > > > > > > Sorry, but wouldn't it be the other way around? Or did I really not > > > understand your answer? > > > > > > Sorry, I don't know anything about kernel code, but wouldn't it be the > > > other way around? > > > > > > It's just that, I may not be understanding. And it's likely that I'm > > > not, because you understand more about this, I'm new to this subject. > > > I know very little about it, or almost nothing. > > > > > > But it's just that I've read the opposite about it. > > > > > > Isn't "write through" to provide more secure writes? > > > > > > I also see that "write back" would be meant to be faster. No? > > > > The sysfs "write_cache" attribute just controls what the kernel does. It > > doesn't change any hardware settings. > > > > In "write back" mode, a sync write will have FUA set, which will generally be > > slower than a write without FUA. In "write through" mode, the kernel doesn't > > set FUA so the data may not be durable after the completion if the controller > > is using a volatile write cache. > > Something seems wrong here: Typically on a RAID controller LUN > configuration "writeback" means that the non-volatile cache is active so > "write back caching" is enabled. > > According to https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt: > > "When read, this file will display whether the device has write > back caching enabled or not. It will return "write back" for the former > case, and "write through" for the latter." > > If my text mailer would underline then I would underline this from the > documentation: "whether the device has write back caching enabled or not" Maybe this is confusing because we let the user change the kernel's behavior regardless of how the storage device is configured? > Is there a good explanation for why the kernel setting is exactly > _opposite_ of the controller setting? By default, the drivers should have the correct setting reported for their devices, not the opposite. The user can override the sysfs attribute to the opposite setting though, so it's not necessarily an accurate report of what the device has actually enabled.
