Re: [Qemu-devel] [PATCHv3] Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA.

[Paolo Bonzini]

On 05/04/2016 17:16, Alex Bligh wrote:
> > > * Document that sending FUA for commands that do nothing is permissible, 
> > > but
> > >  'SHOULD NOT' be done; an existing client does this.
> > 
> > Can you send a pointer to the discussion?  FUA on reads definitely does
> > *something* in SCSI (it ensures that the data is moved out of the
> > volatile cache prior to the read, similar to what QEMU implements).
> 
> Sure. I got a solitary one reply that referenced the kernel, which
> was copied to this list - see below.

Ok, thanks.

> I don't have strong feelings either way, but the safer option would
> be to not REQUIRE the server to rely on any FUA read behaviour (I'm
> already saying they should ignore the bit on reads). Qemu can't
> currently RELY on FUA on reads, as it's not documented anywhere
> and with the reference NBD server did nothing until recently; recently
> it started errorring the read! See below.

Right.  However, bugs get fixed...

Paolo

> On that basis I chose to go with 'FUA on read does nothing'.
> 
> If the kernel actually does something on read perhaps we should
> reconsider.