Re: [Qemu-devel] [Nbd] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?

[Paolo Bonzini <pbonzini@redhat.com>]

On 31/03/2016 22:34, Eric Blake wrote:
> give me possibly stale data because I accessed
> the underlying storage rather than paying attention to in-flight writes
> that would change what I read".

Overlapping I/O is always unspecified, so you should expect stale data
if a read overlaps an in-flight write.  Not only there's no guarantee of
atomicity; the disk is a "safe register" rather than a "regular
register"
(http://stackoverflow.com/questions/8871633/whats-the-difference-between-safe-regular-and-atomic-registers)

The way QEMU does "flush first" for FUA on reads provides the guarantee
that "no one else can read something older than what I read", but that
assumes that no one else is doing an overlapping write.

Paolo