Re: [Nbd] [PATCH][V3] nbd: add multi-connection support

On 29 Sep 2016, at 17:59, Josef Bacik [off-list ref] wrote:
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On 09/29/2016 12:41 PM, Wouter Verhelst wrote:

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On Thu, Sep 29, 2016 at 10:03:50AM -0400, Josef Bacik wrote:

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So think of it like normal disks with multiple channels.  We don't =

send flushes

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down all the hwq's to make sure they are clear, we leave that =

decision up to the

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application (usually a FS of course).

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Well, when I asked earlier, Christoph said[1] that blk-mq assumes =

that

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when a FLUSH is sent over one channel, and the reply comes in, that =

all

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commands which have been received, regardless of which channel they =

were

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received over, have reched disk.
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[1] Message-ID: [ref]
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It is impossible for nbd to make such a guarantee, due to =

head-of-line

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blocking on TCP.
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Huh I missed that.  Yeah that's not possible for us for sure, I think =

my option=20

idea is the less awful way forward if we want to address that =

limitation.  Thanks,

I think if the server supports flush (which you can tell), sending flush =
on
all channels is the only safe thing to do, without substantial protocol
changes (which I'm not sure how one would do given flush is in a sense
a synchronisation point). I think it's thus imperative this gets fixed
before the change gets merged.

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Alex Bligh