linux-kernel - Re: [PATCH v4 08/13] btrfs:zoned: make sb for npo2 zone devices align with sb log offsets

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Date:   Tue, 17 May 2022 06:50:53 +0000
From:   Johannes Thumshirn <Johannes.Thumshirn@....com>
To:     Pankaj Raghav <p.raghav@...sung.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "damien.lemoal@...nsource.wdc.com" <damien.lemoal@...nsource.wdc.com>,
        "pankydev8@...il.com" <pankydev8@...il.com>,
        "dsterba@...e.com" <dsterba@...e.com>, "hch@....de" <hch@....de>
CC:     "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "jiangbo.365@...edance.com" <jiangbo.365@...edance.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "gost.dev@...sung.com" <gost.dev@...sung.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: Re: [PATCH v4 08/13] btrfs:zoned: make sb for npo2 zone devices align
 with sb log offsets

On 16/05/2022 18:55, Pankaj Raghav wrote:
> Superblocks for zoned devices are fixed as 2 zones at 0, 512GB and 4TB.
> These are fixed at these locations so that recovery tools can reliably
> retrieve the superblocks even if one of the mirror gets corrupted.
> 
> power of 2 zone sizes align at these offsets irrespective of their
> value but non power of 2 zone sizes will not align.
> 
> To make sure the first zone at mirror 1 and mirror 2 align, write zero
> operation is performed to move the write pointer of the first zone to
> the expected offset. This operation is performed only after a zone reset
> of the first zone, i.e., when the second zone that contains the sb is FULL.

Hi Pankaj, stupid question. Npo2 devices still have a zone size being a 
multiple of 4k don't they?

If not, we'd need to also have a tail padding of the superblock zones, in order
to move the WP of these zones to the end, so the sb-log states match up.