On Wed, Oct 23, 2019 at 04:09:50PM +0300, Boaz Harrosh wrote: > On 22/10/2019 14:21, Boaz Harrosh wrote: > > On 20/10/2019 18:59, ira.weiny@xxxxxxxxx wrote: > Please explain the use case behind your model? No application changes needed to control whether they use DAX or not. It allows the admin to control the application behaviour completely, so they can turn off DAX if necessary. Applications are unaware of constraints that may prevent DAX from being used, and so admins need a mechanism to prevent DAX aware application from actually using DAX if the capability is present. e.g. given how slow some PMEM devices are when it comes to writing data, especially under extremely high concurrency, DAX is not necessarily a performance win for every application. Admins need a guaranteed method of turning off DAX in these situations - apps may not provide such a knob, or even be aware of a thing called DAX... e.g. the data set being accessed by the application is mapped and modified by RDMA applications, so those files must not be accessed using DAX by any application because DAX+RDMA are currently incompatible. Hence you can have RDMA on pmem devices co-exist within the same filesystem as other applications using DAX to access the pmem... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
