On 29.06.2011, at 15:57, Stefan Hajnoczi wrote: > On Wed, Jun 29, 2011 at 1:21 PM, Alexander Graf <ag...@suse.de> wrote: >> On 29.06.2011, at 13:59, Kevin Wolf wrote: >>> I'm not entirely sure if I should suggest writeback or none as the new >>> default, but I think it could make sense to change it. >> >> None as default would be a bad choice, as not all underlying file systems >> support it. Try cache=none on tmpfs or nfs. It will just fail on you. >> What I'd personally like to see is some sort of detection that can determine >> which caching mode would perform best. So it would do cache=none when it >> makes sense (raw block device), otherwise cache=writeback. > > O_DIRECT works on the Linux NFS client. The performance currently > isn't great with vectored I/O (which QEMU does) but there is work > ongoing to improve it.
It does? One of the things I stumbled over when doing autotest was that cache=none was the default - and that simply failed on my nfs-root system. > Your point makes sense though. The default (no explicit cache=) > should mean "the most appropriate caching mode". Yeah, that's what defaults should be like in most cases. The same reason people are for going with -cpu host as default :). Alex
