On Sat, 30 Mar 2019 at 13:00, Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote: > I'm purposing to measure the execution time of flush/fsync, not write. > > And if flush takes 0ms, it means it doesn't really write cached data > onto disk. > That is correct. The controller ignores your flush requests on the virtual disk by design. When the data hits the controller it is considered "stored" - the physical disk(s) storing the virtual disk is an implementation detail. The performance characteristics of these controllers are needed to make big arrays work in a useful manner. My controller is connected to 4 HP 2600 enclosures with 12 drives each. Waiting for a flush on a single disk before continuing work on the remaining 47 disks would be catastrophic for performance. Regards
