On Tue, Mar 15, 2016 at 05:51:17PM -0700, Chris Mason wrote: > On Tue, Mar 15, 2016 at 07:30:14PM -0500, Eric Sandeen wrote: > > On 3/15/16 7:06 PM, Linus Torvalds wrote: > > > On Tue, Mar 15, 2016 at 4:52 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > >> > > > >> > It is pretty clear that the onus is on the patch submitter to > > >> > provide justification for inclusion, not for the reviewer/Maintainer > > >> > to have to prove that the solution is unworkable. > > > I agree, but quite frankly, performance is a good justification. > > > > > > So if Ted can give performance numbers, that's justification enough. > > > We've certainly taken changes with less. > > > > I've been away from ext4 for a while, so I'm really not on top of the > > mechanics of the underlying problem at the moment. > > > > But I would say that in addition to numbers showing that ext4 has trouble > > with unwritten extent conversion, we should have an explanation of > > why it can't be solved in a way that doesn't open up these concerns. > > > > XFS certainly has different mechanisms, but is the demonstrated workload > > problematic on XFS (or btrfs) as well? If not, can ext4 adopt any of the > > solutions that make the workload perform better on other filesystems? > > When I've benchmarked this in the past, doing small random buffered writes > into an preallocated extent was dramatically (3x or more) slower on xfs > than doing them into a fully written extent. That was two years ago, > but I can redo it. So I re-ran some benchmarks, with 4K O_DIRECT random ios on nvme (4.5 kernel). This is O_DIRECT without O_SYNC. I don't think xfs will do commits for each IO into the prealloc file? O_SYNC makes it much slower, so hopefully I've got this right. The test runs for 60 seconds, and I used an iodepth of 4: prealloc file: 32,000 iops overwrite: 121,000 iops If I bump the iodepth up to 512: prealloc file: 33,000 iops overwrite: 279,000 iops For streaming writes, XFS converts prealloc to written much better when the IO isn't random. You can start seeing the difference at 16K sequential O_DIRECT writes, but really its not a huge impact. The worst case is 4K: prealloc file: 227MB/s overwrite: 340MB/s I can't think of sequential workloads where this will matter, since they will either end up with bigger IO or the performance impact won't get noticed. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
