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Date:   Wed, 28 Sep 2022 10:22:36 -0400
From:   Mike Snitzer <snitzer@redhat.com>
To:     Pankaj Raghav <p.raghav@samsung.com>
Cc:     snitzer@kernel.org, axboe@kernel.dk, agk@redhat.com, hch@lst.de,
        damien.lemoal@opensource.wdc.com,
        Damien Le Moal <damien.lemoal@wdc.com>, bvanassche@acm.org,
        pankydev8@gmail.com, gost.dev@samsung.com,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, dm-devel@redhat.com,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        jaegeuk@kernel.org, matias.bjorling@wdc.com
Subject: Re: [PATCH v15 13/13] dm: add power-of-2 target for zoned devices
 with non power-of-2 zone sizes
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References: <20220923173618.6899-1-p.raghav@samsung.com>
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 <20220923173618.6899-14-p.raghav@samsung.com>
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On Fri, Sep 23 2022 at  1:36P -0400,
Pankaj Raghav <p.raghav@samsung.com> wrote:

> Only zoned devices with power-of-2(po2) number of sectors per zone(zone
> size) were supported in linux but now non power-of-2(npo2) zone sizes
> support has been added to the block layer.
> 
> Filesystems such as F2FS and btrfs have support for zoned devices with
> po2 zone size assumption. Before adding native support for npo2 zone
> sizes, it was suggested to create a dm target for npo2 zone size device to
> appear as a po2 zone size target so that file systems can initially
> work without any explicit changes.
> 
> The design of this target is very simple: remap the device zone size to
> the zone capacity and change the zone size to be the nearest power of 2
> value.
> 
> For e.g., a device with a zone size/capacity of 3M will have an equivalent
> target layout as follows:
> 
> Device layout :-
> zone capacity = 3M
> zone size = 3M
> 
> |--------------|-------------|
> 0             3M            6M
> 
> Target layout :-
> zone capacity=3M
> zone size = 4M
> 
> |--------------|---|--------------|---|
> 0             3M  4M             7M  8M
> 
> The area between target's zone capacity and zone size will be emulated
> in the target.
> The read IOs that fall in the emulated gap area will return 0 filled
> bio and all the other IOs in that area will result in an error.
> If a read IO span across the emulated area boundary, then the IOs are
> split across them. All other IO operations that span across the emulated
> area boundary will result in an error.
> 
> The target can be easily created as follows:
> dmsetup create <label> --table '0 <size_sects> po2zoned /dev/nvme<id>'
> 
> The target does not support partial mapping of the underlying
> device as there is no use-case for it.
> 
> Note:
> This target is not related to dm-zoned target, which exposes a zoned block
> device as a regular block device without any write constraint.
> 
> This target only exposes a different zone size than the underlying device.
> The underlying device's other constraints will be directly exposed to the
> target.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Suggested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Suggested-by: Damien Le Moal <damien.lemoal@wdc.com>
> Suggested-by: Hannes Reinecke <hare@suse.de>

If/when this series is accepted by Jens and others the DM changes can
just be included along with the rest of the series.

Reviewed-by: Mike Snitzer <snitzer@kernel.org>