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From:   Roman Stratiienko <roman.stratiienko@globallogic.com>
Date:   Thu, 13 Jun 2019 19:05:38 +0300
Message-ID: <CAODwZ7u+f9vco8h1ZAwwoCefB6kM9gi4L_Mc7muLXYkwHRVc8Q@mail.gmail.com>
Subject: Re: [PATCH 2/2] nbd: add support for nbd as root device
To:     Eric Blake <eblake@redhat.com>
Cc:     Josef Bacik <josef@toxicpanda.com>, linux-kernel@vger.kernel.org,
        nbd@other.debian.org,
        Aleksandr Bulyshchenko <A.Bulyshchenko@globallogic.com>,
        linux-block@vger.kernel.org, axboe@kernel.dkn.org
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On Thu, Jun 13, 2019 at 6:14 PM Eric Blake <eblake@redhat.com> wrote:
>
> On 6/13/19 9:45 AM, Roman Stratiienko wrote:
>
> >>
> >> Just throw nbd-client in your initramfs.  Every nbd server has it's own
> >> handshake protocol, embedding one particular servers handshake protocol into the
> >> kernel isn't the answer here.  Thanks,
>
> The handshake protocol is well-specified:
> https://github.com/NetworkBlockDevice/nbd/blob/cdb0bc57f3faefd7a5562d57ad57cd990781c415/doc/proto.md
>
> All servers implement various subsets of that document for the handshake.
>
> > Also, as far as I know mainline nbd-server daemon have only 2
> > handshake protocols. So called OLD-STYLE and NEW-STYLE. And OLD-STYLE
> > is no longer supported. So it should not be a problem, or please fix
> > me if I'm wrong.
>
> You are correct that oldstyle is no longer recommended. However, the
> current NBD specification states that newstyle has two different
> flavors, NBD_OPT_EXPORT_NAME (which you used, but is also old) and
> NBD_OPT_GO (which is newer, but is more likely to encounter differences
> where not all servers support it).
>
> The NBD specification includes a compatibility baseline:
> https://github.com/NetworkBlockDevice/nbd/blob/cdb0bc57f3faefd7a5562d57ad57cd990781c415/doc/proto.md#compatibility-and-interoperability
>
> and right now, NBD_OPT_GO (and _not_ NBD_OPT_EXPORT_NAME) is the
> preferred way forward.  As long as your handshake implementation
> complies with the baseline documented there, you'll have maximum
> portability to the largest number of servers that also support the
> baseline - but not all servers are up to that baseline yet.
>
> So, this becomes a question of how much are you reinventing baseline
> portability handshake concerns in the kernel, vs. in initramfs.
>
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
>

Thank you for the review comments, I will address them in v2.

--
Regards,
Roman