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

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/04/2016 14:07, Alex Bligh wrote:
> Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA. Specifically
> the latter may be set on any command, and its semantics on commands other
> than NBD_CMD_WRITE need explaining. Further, explain how these relate to
> reordering of commands.
> 
> Signed-off-by: Alex Bligh <alex-rWA27mgs/Jz10XsdtD+oqA@public.gmane.org>
> ---
>  doc/proto.md | 51 +++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 41 insertions(+), 10 deletions(-)
> 
> ARRRGH - this time with a proper commit header.
> 
> Changes since version 2:
> 
> * Rebase on master
> 
> * Remove bogus 'SHOULD' for FLUSH in relation to writes that are in flight
>   but not yet completed
> 
> * After consultation with lkml etc., document that FUA on things that do not
>   write does nothing
> 
> * 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).

Paolo

> * Document that FUA on TRIM should do something after all, per Kevin Wolf's
>   comment