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Re: [Qemu-devel] [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension
Date: Thu, 24 Mar 2016 12:37:33 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 24/03/2016 09:26, Wouter Verhelst wrote:
>> > 
>> > No, there is no specific reason. Looks like NBD_CMD_FLAG_ZEROES fits the
>> > spec and implementations nicely. So I'll rewrite the extension and add
>> > the flag instead of the whole command.
> Actually, having given this some more thought...
> There is at least one server-side implementation of nbd (mine) which
> silently ignores flags it doesn't know about. This isn't a problem for
> non-critical flags, but it could be a problem for a flag like this. Of
> course, a client shouldn't send a flag to a server which that server
> hasn't heard of, but mistakes do happen.
> Do we want to keep that in mind? If so, we might want to keep it as a
> separate command after all.
> OTOH, it could be said that silently ignoring unknown messages is a bug.
> I should probably just fix my implementation instead.

Even if it is a bug, it does suggest that the payload format should not
be changed by flags.  For example ignoring flags is a bug for an NBD
server, but not for a Wireshark protocol dissector.


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