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

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Date:   Wed, 4 May 2022 23:14:40 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Pankaj Raghav <p.raghav@...sung.com>
Cc:     Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "jaegeuk@...nel.org" <jaegeuk@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "snitzer@...nel.org" <snitzer@...nel.org>,
        "hch@....de" <hch@....de>, "mcgrof@...nel.org" <mcgrof@...nel.org>,
        Naohiro Aota <Naohiro.Aota@....com>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        "damien.lemoal@...nsource.wdc.com" <damien.lemoal@...nsource.wdc.com>,
        "dsterba@...e.com" <dsterba@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
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        "chao@...nel.org" <chao@...nel.org>,
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        <linux-f2fs-devel@...ts.sourceforge.net>,
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        Matias Bjørling <Matias.Bjorling@....com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH 00/16] support non power of 2 zoned devices

On Tue, May 03, 2022 at 11:12:04AM +0200, Pankaj Raghav wrote:
> Hi Johannes,
> On 2022-05-03 00:07, Johannes Thumshirn wrote:
> >> 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 commit: `btrfs: zoned: relax the alignment constraint for zoned
> devices` makes sure the zone size is BTRFS_STRIPE_LEN aligned (64K). So
> even the npo2 zoned device should be aligned to `fs_info->sectorsize`,
> which is typically 4k.
> 
> This was one of the comment that came from David Sterba:
> https://lore.kernel.org/all/20220315142740.GU12643@twin.jikos.cz/
> where he suggested to have some sane alignment for the zone sizes.

My idea of 'sane' value would be 1M, that we have 4K for sectors is
because of the 1:1 mapping to pages, but RAM sizes are on a different
scale than storage devices. The 4K is absolute minimum but if the page
size is taken as a basic constraint, ARM has 64K and there are some 256K
arches.