Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache

[Stefan Hajnoczi]

On Tue, Apr 14, 2015 at 7:49 AM, Peter Lieven <address@hidden> wrote:
> Ronnie came up with an idea to reduce latency if !bs->enable_write_cache for
> an iSCSI device.
>
> If !bs->enable_write_cache Qemu sends a flush after every single write. What
> could be done is
> the following:
>
> if (!bs->enable_write_cache)
>  set FUA (force unit access) and DPO (disable page out) bits in every write
> cmd
>  make iscsi_co_flush a NOOP in this case.
>
> Your thoughts?

Do other commands besides writes rely on iscsi_co_flush()?

Paolo: I checked NBD and noticed there is an inconsistency there.
nbd_co_writev_1() uses FUA when bs->enable_write_cache == true but it
also sends flushes.  Does that mean it's doing unnecessary work?

Stefan