On 5 September 2014 14:28, Peter Ross <Petros.Listig@fdrive.com.au> wrote:
But here’s the big surprise – if we set up a ZVOL, then format it with ext4 and put a .qcow2 on top of that… it performs as well, and in some cases better than, the raw zvol itself did! As odd as it sounds, this leaves qcow2-on-ext4-on-zvol as one of our best performing overall storage methods, with the most convenient options for management. It sounds like it’d be a horrible Rube Goldberg, but it performs like best-in-breed. Who’d’a thunk it?
According to the benchmark you have this layering: - qcow2 (virtual disk) as a file on an - ext4 filesystem formatted on a - zvol created on a - raw partition
performing better than a raw partition!
Have you checked whether important things like write barriers and synchronisation are being honoured all the way down the stack? I suspect you'll find they're not, and that is where the performance is coming from.. but will be interested to hear if you can confirm or deny that after investigation. -Toby