Dave, >> > WRITE SAME also has an ANCHOR flag which provides a use case we >> > currently don't have fallocate plumbing for: Allocating blocks without >> > caring about their contents. I.e. the blocks described by the I/O are >> > locked down to prevent ENOSPC for future writes. > > So WRITE_SAME (0) with an ANCHOR flag does not return zeroes on > subsequent reads? i.e. it is effectively > fallocate(FALLOC_FL_NO_HIDE_STALE) preallocation semantics? The answer is that it depends. It can return zeroes or a device-specific initialization pattern (oh joy). > For many use cases cases we actually want zeroed space to be > guaranteed so we don't expose stale data from previous device use into > the new user's visibility - can that be done with WRITE_SAME and the > ANCHOR flag? That's just a regular zeroout. We have: Allocate and zero: FALLOC_FL_ZERO_RANGE Deallocate and zero: FALLOC_FL_PUNCH_HOLE Deallocate: FALLOC_FL_PUNCH_HOLE | FALLOC_FL_NO_HIDE_STALE but are missing: Allocate: FALLOC_FL_ZERO_RANGE | FALLOC_FL_NO_HIDE_STALE The devices that implement anchor semantics are few and far between. I have yet to see one. -- Martin K. Petersen Oracle Linux Engineering
