Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

From: Paolo Bonzini
Date: Thu Sep 15 2016 - 10:08:20 EST

On 15/09/2016 15:34, Eric Blake wrote:
> On 09/15/2016 06:09 AM, Alex Bligh wrote:
>> I also wonder whether any servers that can do caching per
>> connection will always share a consistent cache between
>> connections. The one I'm worried about in particular here
>> is qemu-nbd - Eric Blake CC'd.
> I doubt that qemu-nbd would ever want to support the situation with more
> than one client connection writing to the same image at the same time;
> the implications of sorting out data consistency between multiple
> writers is rather complex and not worth coding into qemu. So I think
> qemu would probably prefer to just prohibit the multiple writer
> situation. And while multiple readers with no writer should be fine,
> I'm not even sure if multiple readers plus one writer can always be made
> to appear sane (if there is no coordination between the different
> connections, on an image where the writer changes AA to BA then flushes
> then changes to BB, it is still feasible that a reader could see AB
> (pre-flush state of the first sector, post-flush changes to the second
> sector, even though the writer never flushed that particular content to
> disk).
> But Paolo Bonzini (cc'd) may have more insight on qemu's NBD server and
> what it supports (or forbids) in the way of multiple clients to a single
> server.

I don't think QEMU forbids multiple clients to the single server, and
guarantees consistency as long as there is no overlap between writes and
reads. These are the same guarantees you have for multiple commands on
a single connection.

In other words, from the POV of QEMU there's no difference whether
multiple commands come from one or more connections.


>> A more general point is that with multiple queues requests
>> may be processed in a different order even by those servers that
>> currently process the requests in strict order, or in something
>> similar to strict order. The server is permitted by the spec
>> (save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
>> process commands out of order anyway, but I suspect this has
>> to date been little tested.
> qemu-nbd is definitely capable of serving reads and writes out-of-order
> to a single connection client; but that's different than the case with
> multiple connections.

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