On Tue, Aug 01, 2017 at 12:42:18PM +1000, Dave Chinner wrote: > I've outlined other use cases in previous discussions. To repeat > myself, every so often we get someone with, say, a new high > speed camera that want to dma the camera frames direct to the > storage because they can't push 500,000 frames/s through the CPU > to storage. Hence they want to bypass the OS and DMA the data direct > to the storage. To do this they need a mechanism to freeze and unfreeze > the block map of the file so that nothing modifies the block map > while the camera hardware is dumping data direct to the storage. > Immutable extent maps provide the functionality they need to > implement this safely. And we have such a mechanism already: it's called the iolock during I/O, and dirct I/O. I've worked on plenty such schemes and the proper way works perfectly fine. Just because people ask for stupid ways to archives that doesn't mean they understand what they are doing. > There's also other similar use cases for RDMA targets on PMEM > (regardless of whether DAX is enabled or not), and I've come across > a couple of requests for mechanisms to allow fabric based nvme > storage to do direct data transfers between storage devices, too. > All of these use cases can be safely implemented if there is a > mechanism to mark extent maps as immutable for the duration of > the operation they need to perform. As someone who spent most of them time on the last 2 years in this area: we have a massive problem discoverability and addressing (lack of struct page) for p2p devices. We have absolutely no problem with the direct I/O model with them. > DAX isn't the driver of that functionality, it's the other use cases > that need it, and why the proposed "only remove flag if len == 0" > API is a non-starter.... The other "use" cases are even more bullshit than the DAX one.
