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

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.