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
