Re: [Qemu-devel] [PATCH 1/3] nbd: Only try to send flush/discard commands if connected to the NBD server

[Jamie Lokier]

Kevin Wolf wrote:
> Am 24.10.2012 14:16, schrieb Nicholas Thomas:
> > On Tue, 2012-10-23 at 16:02 +0100, Jamie Lokier wrote:
> >> Since the I/O _order_ before, and sometimes after, flush, is important
> >> for data integrity, this needs to be maintained when I/Os are queued in
> >> the disconnected state -- including those which were inflight at the
> >> time disconnect was detected and then retried on reconnect.
> > 
> > Hmm, discussing this on IRC I was told that it wasn't necessary to
> > preserve order - although I forget the fine detail. Depending on the
> > implementation of qemu's coroutine mutexes, operations may not actually
> > be performed in order right now - it's not too easy to work out what's
> > happening.
> 
> It's possible to reorder, but it must be consistent with the order in
> which completion is signalled to the guest. The semantics of flush is
> that at the point that the flush completes, all writes to the disk that
> already have completed successfully are stable. It doesn't say anything
> about writes that are still in flight, they may or may not be flushed to
> disk.

I admit I wasn't thinking clearly how much ordering NBD actually
guarantees (or if there's ordering the guest depends on implicitly
even if it's not guaranteed in specification), and how that is related
within QEMU to virtio/FUA/NCQ/TCQ/SCSI-ORDERED ordering guarantees
that the guest expects for various emulated devices and their settings.

The ordering (if any) needed from the NBD driver (or any backend) is
going to depend on the assumptions baked into the interface between
QEMU device emulation <-> backend.

E.g. if every device emulation waited for all outstanding writes to
complete before sending a flush, then it wouldn't matter how the
backend reordered its requests, even getting the completions out of
order.

Is that relationship documented (and conformed to)?

-- Jamie