Messages in this thread | | | From | Kanchan Joshi <> | Date | Wed, 19 Aug 2020 16:01:54 +0530 | Subject | Re: [PATCH 1/2] nvme: set io-scheduler requirement for ZNS |
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On Wed, Aug 19, 2020 at 3:08 PM Damien Le Moal <Damien.LeMoal@wdc.com> wrote: > > On 2020/08/19 18:27, Kanchan Joshi wrote: > > On Tue, Aug 18, 2020 at 12:46 PM Christoph Hellwig <hch@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.
I understand that if a particular user (zonefs, btrfs etc) is not sending regular-write and sending append instead, write-lock is not required. But if that particular user or some other user (say F2FS) sends regular write(s), write-lock is needed. Above block-layer, both the opcodes REQ_OP_WRITE and REQ_OP_ZONE_APPEND are available to be used by users. And I thought write-lock is taken or not is a per-opcode thing and not per-user (FS, MD/DM, user-space etc.), is not that correct? And MQ-deadline can cater to both the opcodes, while other schedulers cannot serve REQ_OP_WRITE well for zoned-device.
> 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.
Sure I can keep it that way, once I get it right. If it is really not required for native-append drive, it should not be here at the place where I added.
> 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.
Are we saying applications should switch schedulers based on the write QD (use any-scheduler for QD1 and mq-deadline for QD-N). Even if it does that, it does not know what other applications would be doing. That seems hard-to-get-right and possible only in a tightly-controlled environment.
-- Joshi
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