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From:   Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
To:     Pankaj Raghav <p.raghav@samsung.com>,
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        Naohiro Aota <Naohiro.Aota@wdc.com>,
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Subject: Re: [PATCH 00/16] support non power of 2 zoned devices
Thread-Topic: [PATCH 00/16] support non power of 2 zoned devices
Thread-Index: AQHYWlBbVAcTcxyrHkCrWR4NhJ2aaA==
Date:   Mon, 2 May 2022 22:07:09 +0000
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 <20220427160255.300418-1-p.raghav@samsung.com>
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On 27/04/2022 09:03, Pankaj Raghav wrote:=0A=
> - Background and Motivation:=0A=
> =0A=
> The zone storage implementation in Linux, introduced since v4.10, first=
=0A=
> targetted SMR drives which have a power of 2 (po2) zone size alignment=0A=
> requirement. The po2 zone size was further imposed implicitly by the=0A=
> block layer's blk_queue_chunk_sectors(), used to prevent IO merging=0A=
> across chunks beyond the specified size, since v3.16 through commit=0A=
> 762380ad9322 ("block: add notion of a chunk size for request merging").=
=0A=
> But this same general block layer po2 requirement for blk_queue_chunk_sec=
tors()=0A=
> was removed on v5.10 through commit 07d098e6bbad ("block: allow 'chunk_se=
ctors'=0A=
> to be non-power-of-2"). NAND, which is the media used in newer zoned stor=
age=0A=
> devices, does not naturally align to po2, and so the po2 requirement=0A=
> does not make sense for those type of zone storage devices.=0A=
> =0A=
> Removing the po2 requirement from zone storage should therefore be possib=
le=0A=
> now provided that no userspace regression and no performance regressions =
are=0A=
> introduced. Stop-gap patches have been already merged into f2fs-tools to=
=0A=
> proactively not allow npo2 zone sizes until proper support is added [0].=
=0A=
> Additional kernel stop-gap patches are provided in this series for dm-zon=
ed.=0A=
> Support for npo2 zonefs and btrfs support is addressed in this series.=0A=
> =0A=
> There was an effort previously [1] to add support to non po2 devices via=
=0A=
> device level emulation but that was rejected with a final conclusion=0A=
> to add support for non po2 zoned device in the complete stack[2].=0A=
=0A=
Hey Pankaj,=0A=
=0A=
One thing I'm concerned with this patches is, once we have npo2 zones (or t=
o be precise =0A=
not fs_info->sectorsize aligned zones) we have to check on every allocation=
 if we still =0A=
have at least have fs_info->sectorsize bytes left in a zone. If not we need=
 to =0A=
explicitly finish the zone, otherwise we'll run out of max active zones. =
=0A=
=0A=
This is a problem for zoned btrfs at the moment already but it'll be even w=
orse=0A=
with npo2, because we're never implicitly finishing zones.=0A=
=0A=
See also =0A=
https://lore.kernel.org/linux-btrfs/42758829d8696a471a27f7aaeab5468f60b1565=
d.1651157034.git.naohiro.aota@wdc.com=0A=
=0A=
Thanks,=0A=
	Johannes=0A=