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From:   Naohiro Aota <Naohiro.Aota@wdc.com>
To:     "dsterba@suse.cz" <dsterba@suse.cz>
CC:     "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
        David Sterba <dsterba@suse.com>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>, Qu Wenruo <wqu@suse.com>,
        Nikolay Borisov <nborisov@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Hannes Reinecke <hare@suse.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        =?iso-8859-1?Q?Matias_Bj=F8rling?= <mb@lightnvm.io>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH 09/19] btrfs: limit super block locations in HMZONED mode
Thread-Topic: [PATCH 09/19] btrfs: limit super block locations in HMZONED mode
Thread-Index: AQHVHTKI41mlZbldhUiKC20uoMUKKg==
Date:   Tue, 18 Jun 2019 09:01:35 +0000
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On 2019/06/18 7:53, David Sterba wrote:=0A=
> On Fri, Jun 07, 2019 at 10:10:15PM +0900, Naohiro Aota wrote:=0A=
>> When in HMZONED mode, make sure that device super blocks are located in=
=0A=
>> randomly writable zones of zoned block devices. That is, do not write su=
per=0A=
>> blocks in sequential write required zones of host-managed zoned block=0A=
>> devices as update would not be possible.=0A=
> =0A=
> This could be explained in more detail. My understanding is that the 1st=
=0A=
> and 2nd copy superblocks is skipped at write time but the zone=0A=
> containing the superblocks is not excluded from allocations. Ie. regular=
=0A=
> data can appear in place where the superblocks would exist on=0A=
> non-hmzoned filesystem. Is that correct?=0A=
=0A=
Correct. You can see regular data stored at usually SB location on HMZONED =
fs.=0A=
=0A=
> The other option is to completely exclude the zone that contains the=0A=
> superblock copies.=0A=
> =0A=
> primary sb			 64K=0A=
> 1st copy			 64M=0A=
> 2nd copy			256G=0A=
> =0A=
> Depends on the drives, but I think the size of the random write zone=0A=
> will very often cover primary and 1st copy. So there's at least some=0A=
> backup copy.=0A=
> =0A=
> The 2nd copy will be in the sequential-only zone, so the whole zone=0A=
> needs to be excluded in exclude_super_stripes. But it's not, so this=0A=
> means data can go there.  I think the zone should be left empty.=0A=
> =0A=
=0A=
I see. That's more safe for the older kernel/userland, right? By keeping th=
at zone empty,=0A=
we can avoid old ones to mis-interpret data to be SB.=0A=
=0A=
Alright, I will change the code to do so.=0A=