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

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 03, 2016 at 09:51:49AM +0200, Wouter Verhelst wrote:
> Actually, I was pointing out the TCP head-of-line issue, where a delay
> on the socket that contains the flush reply would result in the arrival
> in the kernel block layer of a write reply before the said flush reply,
> resulting in a write being considered part of the flush when in fact it
> was not.

The kernel (or any other user of SCSI/ATA/NVMe-like cache flushes)
will wait for all I/O that needs to be in the cache for explicitly,
so this is not a problem.

> Can you clarify what you mean by that? Why is it an "odd flush
> definition", and how would you "properly" define it?

E.g. take the defintion from NVMe which also supports multiple queues:

"The Flush command shall commit data and metadata associated with the
specified namespace(s) to non-volatile media. The flush applies to all
commands completed prior to the submission of the Flush command.
The controller may also flush additional data and/or metadata from any
namespace."

The focus is completed - we need to get a reply to the host first
before we can send the flush command, so anything that we require
to be flushed needs to explicitly be completed first.