On Tue, Jan 29, 2019 at 11:07:02AM +0100, Jan Kara wrote: > On Mon 28-01-19 16:24:24, Keith Busch wrote: > > On Mon, Jan 28, 2019 at 04:47:09AM -0800, Jan Kara wrote: > > > On Fri 25-01-19 09:23:53, Keith Busch wrote: > > > > On Wed, Jan 09, 2019 at 09:00:57PM +0530, Kanchan Joshi wrote: > > > > > Towards supporing write-hints/streams for filesystem journal. > > > > > > > > > > Here is the v1 patch for background - > > > > > https://marc.info/?l=linux-fsdevel&m=154444637519020&w=2 > > > > > > > > > > Changes since v1: > > > > > - introduce four more hints for in-kernel use, as recommended by Dave chinner > > > > > & Jens axboe. This isolates kernel-mode hints from user-mode ones. > > > > > > > > The nvme driver disables streams if the controller doesn't support > > > > BLK_MAX_WRITE_HINT number of streams, so this series breaks the feature > > > > for controllers that only support up to 4. > > > > > > Right. Do you know if there are such controllers? Or are you just afraid > > > that there could be? > > > > I've asked around, and the concensus I received is all currently support > > at least 8, but they couldn't say if that would be true for potential > > lower budget products. Can we implement a reasonable fallback to use > > what's available? > > OK, thanks for input. So probably we should just map kernel stream IDs to 0 > if the device doesn't support them. But that probably means we need to > propagate number of available streams up from NVME into the block layer so > that this can be handled reasonably seamlessly. Jens, Kanchan? Yeah, that's basically what I said we needed to do when this was last discussed. i.e. that the block layer needed to know how many streams the hardware had and map the 4 "kernel internal" hints appropriately to what he device supports. e.g. if the device only supports 4 hints, then it needs to map the kernel hints either to zero. If it supports less than 8 streams, then they need otbe mapped into the hints above index 5. If there are N streams, then they need to be mapped to the hints {N-3,N} And, to top it all off, there needs to be guards so that if we want to grow the userspace hints to more than 4 hints, they don't crash into ranges the kernel is already reserving because of limited device range support. Nothing is ever simple.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
