Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Message-ID: <c712f0d9-c7f8-172d-8bba-ca6d639bd7c0@samsung.com>
Date:   Mon, 9 May 2022 13:02:23 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
        Thunderbird/91.8.1
Subject: Re: [PATCH v3 00/11] support non power of 2 zoned devices
Content-Language: en-US
To:     <dsterba@suse.cz>, <jaegeuk@kernel.org>, <hare@suse.de>,
        <dsterba@suse.com>, <axboe@kernel.dk>, <hch@lst.de>,
        <damien.lemoal@opensource.wdc.com>, <snitzer@kernel.org>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        <bvanassche@acm.org>, <linux-fsdevel@vger.kernel.org>,
        <matias.bjorling@wdc.com>, Jens Axboe <axboe@fb.com>,
        <gost.dev@samsung.com>, <jonathan.derrick@linux.dev>,
        <jiangbo.365@bytedance.com>, <linux-nvme@lists.infradead.org>,
        <dm-devel@redhat.com>, Naohiro Aota <naohiro.aota@wdc.com>,
        <linux-kernel@vger.kernel.org>,
        Johannes Thumshirn <jth@kernel.org>,
        "Sagi Grimberg" <sagi@grimberg.me>,
        Alasdair Kergon <agk@redhat.com>,
        <linux-block@vger.kernel.org>, Chaitanya Kulkarni <kch@nvidia.com>,
        "Keith Busch" <kbusch@kernel.org>, <linux-btrfs@vger.kernel.org>
From:   Pankaj Raghav <p.raghav@samsung.com>
In-Reply-To: <20220506100054.GZ18596@suse.cz>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
CMS-TYPE: 201P
References: <CGME20220506081106eucas1p181e83ef352eb8bfb1752bee0cf84020f@eucas1p1.samsung.com>
        <20220506081105.29134-1-p.raghav@samsung.com>
        <20220506100054.GZ18596@suse.cz>
Precedence: bulk

On 2022-05-06 12:00, David Sterba wrote:
>>   The current approach for npo2 devices is to place the superblock mirror
>>   zones near   512GB and 4TB that is **aligned to the zone size**.
> 
> I don't like that, the offsets have been chosen so the values are fixed
> and also future proof in case the zone size increases significantly. The
> natural alignment of the pow2 zones makes it fairly trivial.
> 
> If I understand correctly what you suggest, it would mean that if zone
> is eg. 5G and starts at 510G then the superblock should start at 510G,
> right? And with another device that has 7G zone size the nearest
> multiple is 511G. And so on.
> 
> That makes it all less predictable, depending on the physical device
> constraints that are affecting the logical data structures of the
> filesystem. We tried to avoid that with pow2, the only thing that
> depends on the device is that the range from the super block offsets is
> always 2 zones.
> 
> I really want to keep the offsets for all zoned devices the same and
> adapt the code that's handling the writes. This is possible with the
> non-pow2 too, the first write is set to the expected offset, leaving the
> beginning of the zone unused.
> 
I agree. Having a known place for superblocks is important for recovery
tools. We were thinking along the lines of what you have suggested. I
will add this support in the next revision.
>>   This
>>   is of no issue for normal operation as we keep track where the superblock
>>   mirror are placed but this can cause an issue with recovery tools for
>>   zoned devices as they expect mirror superblock to be in 512GB and 4TB.
> 
> Yeah the tools need to be updated, btrfs-progs and suite of blk* in
> util-linux.
> 
>>   Note that ATM, recovery tools such as `btrfs check` does not work for
>>   image dumps for zoned devices even for po2 zone sizes.
> 
> I thought this worked, but if you find something that does not please
> report that to Johannes or Naohiro.
Ok. Thanks.