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

[Kevin Wolf]

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.

Kevin