> On 15 Sep 2016, at 13:36, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote: >> At an implementation level that is going to be a little difficult >> for some NBD servers, e.g. ones that fork() a different process per >> connection. There is in general no IPC to speak of between server >> instances. Such servers would thus be unsafe with more than one >> connection if FLUSH is in use. >> >> I believe such servers include the reference server where there is >> process per connection (albeit possibly with several threads). >> >> Even single process servers (including mine - gonbdserver) would >> require logic to pair up multiple connections to the same >> device. > > Why? If you only send the completion after your I/O syscall returned > your are fine if fsync comes from a difference process, no matter > if you're using direct or buffered I/O underneath. That's probably right in the case of file-based back ends that are running on a Linux OS. But gonbdserver for instance supports (e.g.) Ceph based backends, where each connection might be talking to a completely separate ceph node, and there may be no cache consistency between connections. -- Alex Bligh -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
