On Tue, May 19, 2020 at 06:02:32PM +0800, Anand Jain wrote: > On 16/5/20 3:58 am, David Sterba wrote: > > On Thu, Apr 30, 2020 at 05:02:27PM +0800, Anand Jain wrote: > >> I am not sure if this will be integrated in 5.8 and worth the time to > >> rebase. Kindly suggest. > > > > The preparatory work is ok, but the actual mirror selection policy > > addresses a usecase that I think is not the one most users are > > interested in. Devices of vastly different performance capabilities like > > rotational disks vs nvme vs ssd vs network block devices in one > > filesystem are not something commonly found. > > > > What we really need is a saner balancing mechanism than pid-based, that > > is also going to be used any time there are more devices from the same > > speed class for the fast devices too. > > There are two things here, the read_policy framework in the preparatory > patches and a new balancing or read_policy, device. > > > So, no the patchset is not on track for a merge without the improved > > default balancing. > > It can be worked on top of the preparatory read_policy framework? Yes. > This patchset does not change any default read_policy (or balancing) > which is pid as of now. Working on a default read_policy/balancing > was out of the scope of this patchset. > > > The preferred device for reads can be one of the > > policies, I understand the usecase and have not problem with that > > although wouldn't probably have use for it. > > For us, read_policy:device helps to reproduce raid1 data corruption > https://patchwork.kernel.org/patch/11475417/ > And xfstests btrfs/14[0-3] can be improved so that the reads directly > go the device of the choice, instead of waiting for the odd/even pid. > > Common configuration won't need this, advance configurations assembled > with heterogeneous devices where read performance is more critical than > write will find read_policy:device useful. Yes that's the usecase and the possibility to make more targeted tests is also good, but that still means the feature is half-baked and missing the main part. If it was out of scope, ok fair, but I don't want to merge it at that state. It would be embarassing to announce mirror selection followed by "ah no it's useless for anything than this special usecase".
