Jonmichael, > are there capabilities of REQ_OP_WRITE_ZEROES for detection of NVMe > DLFEAT in the identify namespace information? The purpose of this > capability is for operating systems to detect it, precisely for use > cases like we have identified where deterministic read zero is > required to save a tremendous amount of time and NAND endurance. I don't believe DEAC/DLFEAT are currently wired up in the NVMe driver but it would be trivial to match what SCSI does in that department. The intent of the REQ_OP_WRITE_ZEROES interface is to provide the choice between deallocate semantics (think discard) and allocate semantics (think write same) for zeroing. See the BLKDEV_ZERO_NOUNMAP flag for more info. The important distinction between REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES is that the latter is a data integrity operation that produces deterministic results. I.e. guarantees that all blocks will return zeroes on subsequent reads. Whereas REQ_OP_DISCARD is a hint that can and often will skip portions of the request sent. It was a mistake to conflate deallocation and zeroing in our initial implementation of discards in Linux. We have painstakingly removed that and now provide two distinct interfaces: REQ_OP_DISCARD tells a device that a block range is no longer in use, we don't care about block contents for future reads. Whereas REQ_OP_WRITE_ZEROES aims to provide an optimal interface for clearing block ranges given the reported characteristics of a given device. Note that I am careful about using REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES terminology to describe the block layer primitives for deallocating and zeroing block ranges here. At the bottom of the stack, a REQ_OP_WRITE_ZEROES operation could very well end up issuing what people would think of as a "discard" operation (DSM TRIM, WRITE SAME w/UNMAP) assuming the device has been identified as doing the right thing. Anything operating at the block device level should be using the REQ_OP_DISCARD/REQ_OP_WRITE_ZEROES primitives (or their corresponding ioctls or fallocate flags). And if there is a need to address how those primitives are translated into commands for a given device, then we should handle that in the relevant device driver. -- Martin K. Petersen Oracle Linux Engineering
