Re: [PATCH 00/16] support non power of 2 zoned devices

[Johannes Thumshirn]

On 27/04/2022 09:03, Pankaj Raghav 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, and so the po2 requirement
> does not make sense for those type of zone storage devices.
> 
> Removing the po2 requirement from zone storage should therefore 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 [0].
> Additional kernel stop-gap patches are provided in this series for dm-zoned.
> Support for npo2 zonefs and btrfs support is addressed in this series.
> 
> There was an effort previously [1] to add support to non po2 devices via
> device level emulation but that was rejected with a final conclusion
> to add support for non po2 zoned device in the complete stack[2].

Hey Pankaj,

One thing I'm concerned with this patches is, once we have npo2 zones (or to be precise 
not fs_info->sectorsize aligned zones) we have to check on every allocation if we still 
have at least have fs_info->sectorsize bytes left in a zone. If not we need to 
explicitly finish the zone, otherwise we'll run out of max active zones. 

This is a problem for zoned btrfs at the moment already but it'll be even worse
with npo2, because we're never implicitly finishing zones.

See also 
https://lore.kernel.org/linux-btrfs/42758829d8696a471a27f7aaeab5468f60b1565d.1651157034.git.naohiro.aota@xxxxxxx

Thanks,
	Johannes