On 2020/06/17 16:02, Javier González wrote: > On 17.06.2020 06:47, Damien Le Moal wrote: >> On 2020/06/17 15:10, Javier González wrote: >>> On 17.06.2020 00:14, Damien Le Moal wrote: >>>> On 2020/06/17 0:02, Javier González wrote: >>>>> On 16.06.2020 14:42, Damien Le Moal wrote: >>>>>> On 2020/06/16 23:16, Javier González wrote: >>>>>>> On 16.06.2020 12:35, Damien Le Moal wrote: >>>>>>>> On 2020/06/16 21:24, Javier González wrote: >>>>>>>>> On 16.06.2020 14:06, Matias Bjørling wrote: >>>>>>>>>> On 16/06/2020 14.00, Javier González wrote: >>>>>>>>>>> On 16.06.2020 13:18, Matias Bjørling wrote: >>>>>>>>>>>> On 16/06/2020 12.41, Javier González wrote: >>>>>>>>>>>>> On 16.06.2020 08:34, Keith Busch wrote: >>>>>>>>>>>>>> Add support for NVM Express Zoned Namespaces (ZNS) Command Set defined >>>>>>>>>>>>>> in NVM Express TP4053. Zoned namespaces are discovered based on their >>>>>>>>>>>>>> Command Set Identifier reported in the namespaces Namespace >>>>>>>>>>>>>> Identification Descriptor list. A successfully discovered Zoned >>>>>>>>>>>>>> Namespace will be registered with the block layer as a host managed >>>>>>>>>>>>>> zoned block device with Zone Append command support. A namespace that >>>>>>>>>>>>>> does not support append is not supported by the driver. >>>>>>>>>>>>> >>>>>>>>>>>>> Why are we enforcing the append command? Append is optional on the >>>>>>>>>>>>> current ZNS specification, so we should not make this mandatory in the >>>>>>>>>>>>> implementation. See specifics below. >>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> There is already general support in the kernel for the zone append >>>>>>>>>>>> command. Feel free to submit patches to emulate the support. It is >>>>>>>>>>>> outside the scope of this patchset. >>>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> It is fine that the kernel supports append, but the ZNS specification >>>>>>>>>>> does not impose the implementation for append, so the driver should not >>>>>>>>>>> do that either. >>>>>>>>>>> >>>>>>>>>>> ZNS SSDs that choose to leave append as a non-implemented optional >>>>>>>>>>> command should not rely on emulated SW support, specially when >>>>>>>>>>> traditional writes work very fine for a large part of current ZNS use >>>>>>>>>>> cases. >>>>>>>>>>> >>>>>>>>>>> Please, remove this virtual constraint. >>>>>>>>>> >>>>>>>>>> The Zone Append command is mandatory for zoned block devices. Please >>>>>>>>>> see https://lwn.net/Articles/818709/ for the background. >>>>>>>>> >>>>>>>>> I do not see anywhere in the block layer that append is mandatory for >>>>>>>>> zoned devices. Append is emulated on ZBC, but beyond that there is no >>>>>>>>> mandatory bits. Please explain. >>>>>>>> >>>>>>>> This is to allow a single write IO path for all types of zoned block device for >>>>>>>> higher layers, e.g file systems. The on-going re-work of btrfs zone support for >>>>>>>> instance now relies 100% on zone append being supported. That significantly >>>>>>>> simplifies the file system support and more importantly remove the need for >>>>>>>> locking around block allocation and BIO issuing, allowing to preserve a fully >>>>>>>> asynchronous write path that can include workqueues for efficient CPU usage of >>>>>>>> things like encryption and compression. Without zone append, file system would >>>>>>>> either (1) have to reject these drives that do not support zone append, or (2) >>>>>>>> implement 2 different write IO path (slower regular write and zone append). None >>>>>>>> of these options are ideal, to say the least. >>>>>>>> >>>>>>>> So the approach is: mandate zone append support for ZNS devices. To allow other >>>>>>>> ZNS drives, an emulation similar to SCSI can be implemented, with that emulation >>>>>>>> ideally combined to work for both types of drives if possible. >>>>>>> >>>>>>> Enforcing QD=1 becomes a problem on devices with large zones. In >>>>>>> a ZNS device that has smaller zones this should not be a problem. >>>>>> >>>>>> Let's be precise: this is not running the drive at QD=1, it is "at most one >>>>>> write *request* per zone". If the FS is simultaneously using multiple block >>>>>> groups mapped to different zones, you will get a total write QD > 1, and as many >>>>>> reads as you want. >>>>>> >>>>>>> Would you agree that it is possible to have a write path that relies on >>>>>>> QD=1, where the FS / application has the responsibility for enforcing >>>>>>> this? Down the road this QD can be increased if the device is able to >>>>>>> buffer the writes. >>>>>> >>>>>> Doing QD=1 per zone for writes at the FS layer, that is, at the BIO layer does >>>>>> not work. This is because BIOs can be as large as the FS wants them to be. Such >>>>>> large BIO will be split into multiple requests in the block layer, resulting in >>>>>> more than one write per zone. That is why the zone write locking is at the >>>>>> scheduler level, between BIO split and request dispatch. That avoids the >>>>>> multiple requests fragments of a large BIO to be reordered and fail. That is >>>>>> mandatory as the block layer itself can occasionally reorder requests and lower >>>>>> levels such as AHCI HW is also notoriously good at reversing sequential >>>>>> requests. For NVMe with multi-queue, the IO issuing process getting rescheduled >>>>>> on a different CPU can result in sequential IOs being in different queues, with >>>>>> the likely result of an out-of-order execution. All cases are avoided with zone >>>>>> write locking and at most one write request dispatch per zone as recommended by >>>>>> the ZNS specifications (ZBC and ZAC standards for SMR HDDs are silent on this). >>>>>> >>>>> >>>>> I understand. I agree that the current FSs supporting ZNS follow this >>>>> approach and it makes sense that there is a common interface that >>>>> simplifies the FS implementation. See the comment below on the part I >>>>> believe we see things differently. >>>>> >>>>> >>>>>>> I would be OK with some FS implementations to rely on append and impose >>>>>>> the constraint that append has to be supported (and it would be our job >>>>>>> to change that), but I would like to avoid the driver rejecting >>>>>>> initializing the device because current FS implementations have >>>>>>> implemented this logic. >>>>>> >>>>>> What is the difference between the driver rejecting drives and the FS rejecting >>>>>> the same drives ? That has the same end result to me: an entire class of devices >>>>>> cannot be used as desired by the user. Implementing zone append emulation avoids >>>>>> the rejection entirely while still allowing the FS to have a single write IO >>>>>> path, thus simplifying the code. >>>>> >>>>> The difference is that users that use a raw ZNS device submitting I/O >>>>> through the kernel would still be able to use these devices. The result >>>>> would be that the ZNS SSD is recognized and initialized, but the FS >>>>> format fails. >>>> >>>> I understand your point of view. Raw ZNS block device access by an application >>>> is of course a fine use case. SMR also has plenty of these. >>>> >>>> My point is that enabling this regular write/raw device use case should not >>>> prevent using btrfs or other kernel components that require zone append. >>>> Implementing zone append emulation in the NVMe/ZNS driver for devices without >>>> native support for the command enables *all* use cases without impacting the use >>>> case you are interested in. >>>> >>>> This approach is, in my opinion, far better. No one is left out and the user >>>> gains a flexible system with different setup capabilities. The user wins here. >>> >>> So, do you see a path where we enable the following: >>> >>> 1. We add the emulation layer to the NVMe driver for enabling FSs >>> that currently support zoned devices >>> 2. We add a path from user-space (e.g., uring) to enable passthru >>> commands to the NVMe driver to enable a raw ZNS path from the >>> application. This path does not require the device to support >>> append. An initial limitation is that I/Os must be of < 127 bio >>> segments (same as append) to avod bio splits >>> 3. As per above, the NVMe driver allows ZNS drives without append >>> support to be initialized and the check moves to the FS >>> formatting. >>> >>> 2 and 3. is something we have on our end. We need to rebase on top of >>> the patches you guys submitted. 1. is something we can help with after >>> that. >>> >>> Does the above make sense to you? >> >> Doing (1) first will give you a regular nvme namespace block device that you can >> use to send passthrough commands with ioctl(). So (1) gives you (2). >> >> However, I do not understand what io-uring has to do with passthrough. io-uring >> being a block layer functionality, I do not think you can use it to send >> passthrough commands to the driver. I amy be wrong though, but my understanding >> is that for NVMe, passthrough is either ioctl() to device file or the entire >> driver in user space with SPDK. > > We would like to have an async() passthru I/O path and it seems possible > to do through uring. As mentioned on the other email, the goal is to > have the I/O go through the block layer for better integration, but this > work is still ongoing. See other thread. Indeed. I do not think that is special to ZNS at all. >> As for (3), I do not understand your point. If you have (1), then an FS >> requiring zone append will work. > > In order to enable (2), we need the device to come up first. At the > moment the NVMe driver rejects ZNS devices without append support, so > either ioctl() or the uring path will not work. I repeat again here: if you implement zone append emulation, there is no reason to reject devices that do not have zone append native support. Zone append emulation gives you the block device, you can do ioctl(), implement the new async passthrough and file systems requiring zone append work too. All problems solved. -- Damien Le Moal Western Digital Research
