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    SubjectRe: [RFC PATCH 00/17] btrfs zoned block device support
    On Fri, Aug 10, 2018 at 03:04:33AM +0900, Naohiro Aota wrote:
    > This series adds zoned block device support to btrfs.

    Yay, thanks!

    As this a RFC, I'll give you some. The code looks ok for what it claims
    to do, I'll skip style and unimportant implementation details for now as
    there are bigger questions.

    The zoned devices bring some constraints so not all filesystem features
    cannot be expected to work, so this rules out any form of in-place
    updates like NODATACOW.

    Then there's list of 'how will zoned device work with feature X'?

    You disable fallocate and DIO. I haven't looked closer at the fallocate
    case, but DIO could work in the sense that open() will open the file but
    any write will fallback to buffered writes. This is implemented so it
    would need to be wired together.

    Mixed device types are not allowed, and I tend to agree with that,
    though this could work in principle. Just that the chunk allocator
    would have to be aware of the device types and tweaked to allocate from
    the same group. The btrfs code is not ready for that in terms of the
    allocator capabilities and configuration options.

    Device replace is disabled, but the changlog suggests there's a way to
    make it work, so it's a matter of implementation. And this should be
    implemented at the time of merge.

    RAID5/6 + zoned support is highly desired and lack of it could be
    considered a NAK for the whole series. The drive sizes are expected to
    be several terabytes, that sounds be too risky to lack the redundancy
    options (RAID1 is not sufficient here).

    The changelog does not explain why this does not or cannot work, so I
    cannot reason about that or possibly suggest workarounds or solutions.
    But I think it should work in principle.

    As this is first post and RFC I don't expect that everything is
    implemented, but at least the known missing points should be documented.
    You've implemented lots of the low-level zoned support and extent
    allocation, so even if the raid56 might be difficult, it should be the
    smaller part.

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