On 2020/08/19 18:27, Kanchan Joshi wrote: > On Tue, Aug 18, 2020 at 12:46 PM Christoph Hellwig <h...@lst.de> wrote: >> >> On Tue, Aug 18, 2020 at 10:59:35AM +0530, Kanchan Joshi wrote: >>> Set elevator feature ELEVATOR_F_ZBD_SEQ_WRITE required for ZNS. >> >> No, it is not. > > Are you saying MQ-Deadline (write-lock) is not needed for writes on ZNS? > I see that null-block zoned and SCSI-ZBC both set this requirement. I > wonder how it became different for NVMe.
It is not required for an NVMe ZNS drive that has zone append native support. zonefs and upcoming btrfs do not use regular writes, removing the requirement for zone write locking. In the context of your patch series, ELEVATOR_F_ZBD_SEQ_WRITE should be set only and only if the drive does not have native zone append support. And even in that case, since for an emulated zone append the zone write lock is taken and released by the emulation driver itself, ELEVATOR_F_ZBD_SEQ_WRITE is required only if the user will also be issuing regular writes at high QD. And that is trivially controllable by the user by simply setting the drive elevator to mq-deadline. Conclusion: setting ELEVATOR_F_ZBD_SEQ_WRITE is not needed. -- Damien Le Moal Western Digital Research
