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

[Paolo Bonzini]

On 14/04/2015 08:49, Peter Lieven wrote:
> Hi,
> 
> 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?

Yes, that would work.  In fact I'm not even sure you need DPO.

speed of cache=writethrough in general doesn't matter much, except if
whoever runs the guest knows that the host has battery-backed cache.  In
that case this trick would improve latency.  You could get the same with
-drive file.cache.no-flush=on but this would just work.

Paolo