Re: [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension
- To: Wouter Verhelst <w@...112...>, Pavel Borzenkov <pborzenkov@...2319...>
- Cc: nbd-general@lists.sourceforge.net, "Denis V. Lunev" <den@...2317...>, Stefan Hajnoczi <stefanha@...696...>, qemu-devel@...530..., Kevin Wolf <kwolf@...696...>
- Subject: Re: [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension
- From: Paolo Bonzini <pbonzini@...696...>
- Date: Thu, 24 Mar 2016 12:37:33 +0100
- Message-id: <56F3D17D.7040100@...696...>
- In-reply-to: <20160324082641.GF1590@...3...>
- References: <1458742562-30624-1-git-send-email-den@...2317...> <1458742562-30624-2-git-send-email-den@...2317...> <20160323172116.GA2467@...3...> <20160324075706.GA24831@...2346...> <20160324082641.GF1590@...3...>
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.
Paolo
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