Re: [PATCH 5/5] nvme: support for zoned namespaces

[Damien Le Moal <Damien.LeMoal@wdc.com>]

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 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.

> We can agree that a number of initial customers will use these devices
> raw, using the in-kernel I/O path, but without a FS on top.
> 
> Thoughts?
> 
>> and note that
>> this emulation would require the drive to be operated with mq-deadline to enable
>> zone write locking for preserving write command order. While on a HDD the
>> performance penalty is minimal, it will likely be significant on a SSD.
> 
> Exactly my concern. I do not want ZNS SSDs to be impacted by this type
> of design decision at the driver level.

But your proposed FS level approach would end up doing the exact same thing with
the same limitation and so the same potential performance impact. The block
layer generic approach has the advantage that we do not bother the higher levels
with the implementation of in-order request dispatch guarantees. File systems
are complex enough. The less complexity is required for zone support, the better.

> 
> Thanks,
> Javier
> 


-- 
Damien Le Moal
Western Digital Research