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Subject: Re: [PATCH v2 0/3] md superblock write alignment on 512e devices
To:     Chris Unkel <cunkel@drivescale.com>
Cc:     linux-raid <linux-raid@vger.kernel.org>,
        Song Liu <song@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20201029201358.29181-1-cunkel@drivescale.com>
 <265efd48-b0c6-cba5-c77e-5efb0e6b9e00@redhat.com>
 <CAHFUYDo23BBq0R5mZBZgcCEzE=rN_ZYHCZp5WEs-nBZwYeyEnA@mail.gmail.com>
From:   Xiao Ni <xni@redhat.com>
Message-ID: <df5a45c9-d286-11af-c206-b0ef5bff79ea@redhat.com>
Date:   Thu, 5 Nov 2020 11:29:58 +0800
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On 11/04/2020 04:12 AM, Chris Unkel wrote:
> Hi Xiao,
>
> Thanks for the excellent feedback.  Since bitmap_offset appears to be
> a free-form field, it wasn't apparent to me that the bitmap never
> starts within 4K of the bitmap.
>
> I don't think it's worth worrying about a logical block size that's
> more than 4K here--from what I can see logical block size larger than
> the usual 4K page isn't going to happen.
>
> I do think that it makes sense to handle the case where the physical
> block size is more than 4K.  I think what you propose works, but I
> think in the physical block > MAX_SB_SIZE case it makes more sense to
> align the superblock writes to the physical block size (as now) rather
Is it a typo error? You want to say if physical block > MAX_SB_SIZE, it 
should align the
superblock writes to logical block size? Because I see the comments 
below, your solution
is to align to logical block size when physical block > MAX_SB_SIZE.
> than rejecting the create/assemble.  Mounting with the possible
> performance hit seems like a better outcome for the user in that case
> than refusing to assemble.
> It's the same check that would have to be written to reject the
> assembly in that case and so the code shouldn't really be any more
> complex.
>
> So basically what I propose is:  if the physical block size is no
> larger than MAX_SB_SIZE, pad to that; otherwise pad to to
> logical_block_size, that is, replace queue_logical_block_size()
> with something equivalent to:
>
>      queue_physical_block_size(...) > MAX_SB_SIZE ?
> queue_logical_block_size(...) : queue_physical_block_size(...)
>
> which is simple, safe in all cases, doesn't reject any feasible
> assembly, and generates aligned sb writes on all common current
> devices (512n,4kn,512e.)
>
> What do you think?
Yes, It's a nice solution :)

Regards
Xiao