On Wed, Feb 06, 2019 at 03:30:27PM -0800, Dan Williams wrote: > On Wed, Feb 6, 2019 at 3:21 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > > > On Wed, Feb 06, 2019 at 02:44:45PM -0800, Dan Williams wrote: > > > > > > Do they need to stick with xfs? > > > > > > Can you clarify the motivation for that question? This problem exists > > > for any filesystem that implements an mmap that where the physical > > > page backing the mapping is identical to the physical storage location > > > for the file data. > > > > .. and needs to dynamicaly change that mapping. Which is not really > > something inherent to the general idea of a filesystem. A file system > > that had *strictly static* block assignments would work fine. > > > > Not all filesystem even implement hole punch. > > > > Not all filesystem implement reflink. > > > > ftruncate doesn't *have* to instantly return the free blocks to > > allocation pool. > > > > ie this is not a DAX & RDMA issue but a XFS & RDMA issue. > > > > Replacing XFS is probably not be reasonable, but I wonder if a XFS-- > > operating mode could exist that had enough features removed to be > > safe? > > You're describing the current situation, i.e. Linux already implements > this, it's called Device-DAX and some users of RDMA find it > insufficient. The choices are to continue to tell them "no", or say > "yes, but you need to submit to lease coordination". Device-DAX is not what I'm imagining when I say XFS--. I mean more like XFS with all features that require rellocation of blocks disabled. Forbidding hold punch, reflink, cow, etc, doesn't devolve back to device-dax. Jason
