Re: [dm-devel] Please further explain Linux's "zoned storage" roadmap [was: Re: [PATCH v14 00/13] support zoned block devices with non-power-of-2 zone sizes]

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Sep 21 2022 at  7:55P -0400,
Damien Le Moal <damien.lemoal@opensource.wdc.com> wrote:

> On 9/22/22 02:27, Mike Snitzer wrote:
> > On Tue, Sep 20 2022 at  5:11P -0400,
> > Pankaj Raghav <p.raghav@samsung.com> wrote:
> > 
> >> - Background and Motivation:
> >>
> >> The zone storage implementation in Linux, introduced since v4.10, first
> >> targetted SMR drives which have a power of 2 (po2) zone size alignment
> >> requirement. The po2 zone size was further imposed implicitly by the
> >> block layer's blk_queue_chunk_sectors(), used to prevent IO merging
> >> across chunks beyond the specified size, since v3.16 through commit
> >> 762380ad9322 ("block: add notion of a chunk size for request merging").
> >> But this same general block layer po2 requirement for blk_queue_chunk_sectors()
> >> was removed on v5.10 through commit 07d098e6bbad ("block: allow 'chunk_sectors'
> >> to be non-power-of-2").
> >>
> >> NAND, which is the media used in newer zoned storage devices, does not
> >> naturally align to po2. In these devices, zone capacity(cap) is not the
> >> same as the po2 zone size. When the zone cap != zone size, then unmapped
> >> LBAs are introduced to cover the space between the zone cap and zone size.
> >> po2 requirement does not make sense for these type of zone storage devices.
> >> This patch series aims to remove these unmapped LBAs for zoned devices when
> >> zone cap is npo2. This is done by relaxing the po2 zone size constraint
> >> in the kernel and allowing zoned device with npo2 zone sizes if zone cap
> >> == zone size.
> >>
> >> Removing the po2 requirement from zone storage should be possible
> >> now provided that no userspace regression and no performance regressions are
> >> introduced. Stop-gap patches have been already merged into f2fs-tools to
> >> proactively not allow npo2 zone sizes until proper support is added [1].
> >>
> >> There were two efforts previously to add support to npo2 devices: 1) via
> >> device level emulation [2] but that was rejected with a final conclusion
> >> to add support for non po2 zoned device in the complete stack[3] 2)
> >> adding support to the complete stack by removing the constraint in the
> >> block layer and NVMe layer with support to btrfs, zonefs, etc which was
> >> rejected with a conclusion to add a dm target for FS support [0]
> >> to reduce the regression impact.
> >>
> >> This series adds support to npo2 zoned devices in the block and nvme
> >> layer and a new **dm target** is added: dm-po2zoned-target. This new
> >> target will be initially used for filesystems such as btrfs and
> >> f2fs until native npo2 zone support is added.
> > 
> > As this patchset nears the point of being "ready for merge" and DM's
> > "zoned" oriented targets are multiplying, I need to understand: where
> > are we collectively going?  How long are we expecting to support the
> > "stop-gap zoned storage" layers we've constructed?
> > 
> > I know https://zonedstorage.io/docs/introduction exists... but it
> > _seems_ stale given the emergence of ZNS and new permutations of zoned
> > hardware. Maybe that isn't quite fair (it does cover A LOT!) but I'm
> > still left wanting (e.g. "bring it all home for me!")...
> > 
> > Damien, as the most "zoned storage" oriented engineer I know, can you
> > please kick things off by shedding light on where Linux is now, and
> > where it's going, for "zoned storage"?
> 
> Let me first start with what we have seen so far with deployments in the
> field.

<snip>

Thanks for all your insights on zoned storage, very appreciated!

> > In addition, it was my understanding that WDC had yet another zoned DM
> > target called "dm-zap" that is for ZNS based devices... It's all a bit
> > messy in my head (that's on me for not keeping up, but I think we need
> > a recap!)
> 
> Since the ZNS specification does not define conventional zones, dm-zoned
> cannot be used as a standalone DM target (read: single block device) with
> NVMe zoned block devices. Furthermore, due to its block mapping scheme,
> dm-zoned does not support devices with zones that have a capacity lower
> than the zone size. So ZNS is really a big *no* for dm-zoned. dm-zap is a
> prototype and in a nutshell is the equivalent of dm-zoned for ZNS. dm-zap
> can deal with the smaller zone capacity and does not require conventional
> zones. We are not trying to push for dm-zap to be merged for now as we are
> still evaluating its potential use cases. We also have a different but
> functionally equivalent approach implemented as a block device driver that
> we are evaluating internally.
> 
> Given the above mentioned usage pattern we have seen so far for zoned
> storage, it is not yet clear if something like dm-zap for ZNS is needed
> beside some niche use cases.

OK, good to know.  I do think dm-zoned should be trained to _not_
allow use with ZNS NVMe devices (maybe that is in place and I just
missed it?).  Because there is some confusion with at least one
customer that is asserting dm-zoned is somehow enabling them to use
ZNS NVMe devices!

Maybe they somehow don't _need_ conventional zones (writes are handled
by some other layer? and dm-zoned access is confined to read only)!?
And might they also be using ZNS NVMe devices to do _not_ have a
zone capacity lower than the zone size?

Or maybe they are mistaken and we should ask more specific questions
of them?

> > So please help me, and others, become more informed as quickly as
> > possible! ;)
> 
> I hope the above helps. If you want me to develop further any of the
> points above, feel free to let me know.

You've been extremely helpful, thanks!

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