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Date:   Thu, 11 Aug 2022 15:17:46 +0200
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Subject: Re: [PATCH v9 13/13] dm: add power-of-2 target for zoned devices
 with non power-of-2 zone sizes
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To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        <Johannes.Thumshirn@wdc.com>, <snitzer@kernel.org>,
        <axboe@kernel.dk>, <agk@redhat.com>, <hch@lst.de>
CC:     <dm-devel@redhat.com>, <matias.bjorling@wdc.com>,
        <gost.dev@samsung.com>, <linux-kernel@vger.kernel.org>,
        <pankydev8@gmail.com>, <jaegeuk@kernel.org>, <hare@suse.de>,
        <linux-block@vger.kernel.org>, <linux-nvme@lists.infradead.org>,
        <bvanassche@acm.org>, Damien Le Moal <damien.lemoal@wdc.com>
From:   Pankaj Raghav <p.raghav@samsung.com>
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>> +==========
>> +dm-po2zone
>> +==========
>> +The dm-po2zone device mapper target exposes a zoned block device with a
>> +non-power-of-2(npo2) zone number of sectors as a power-of-2(po2) zone number
> 
> s/zone number of sectors/number of sectors per zone (zone size)
> 
>> +of sectors.
>> +The filesystems that support zoned block devices such as F2FS and BTRFS
>> +assume po2 zone size sectors as the kernel has traditionally only supported
>> +those devices. However, as the kernel now supports zoned block devices with
>> +npo2 zone size sectors, the dm-po2zone target can be used by the filesystems
>> +before adding native support.
> 
> filesystems will not "use" the target. The user has to set up the target first
> and the filesystem will run on top of it. Detail, but rewording this make it
> clear that this is not an automatic thing magically happening.
> 
> Not that your patch series is lacking f2fs and btrfs patches to check for the
> power of 2 zone size of the zoned device. Unless these checks are already in place ?
> 
btrfs already had these checks in place and f2fs patch to allow only po2
zone size was already merged by Jaegeuk.

7f262f737502 ("f2fs: ensure only power of 2 zone sizes are allowed")
>> +
>> +Partial mapping of the underlying device is not supported by this target.
>> +
>> +Algorithm
>> +=========
>> +The device mapper target maps the underlying device's zone size to the
>> +zone capacity and changes the zone size to the nearest po2 number of sectors.
>> +The gap between the zone capacity and the zone size is emulated in the target.
>> +E.g., a zoned block device with a zone size (and capacity) of 3M will have an
>> +equivalent target layout with mapping as follows:
>> +
>> +::
>> +
>> +  0M           3M  4M        6M 8M
>> +  |             |  |          |  |
>> +  +x------------+--+x---------+--+x-------  Target
>> +  |x            |  |x         |  |x
>> +   x               x             x
>> +   x               x             x
>> +   x              x             x
>> +   x             x             x
>> +  |x            |x            |x
>> +  +x------------+x------------+x----------  Device
>> +  |             |             |
>> +  0M           3M            6M
>> +